Background

Background on project objectives and questions to be asked

Thank you for agreeing to take part in this online expert elicitation to help inform the development of a national recovery strategy for red squirrels in England. In this exercise, we will seek your expert judgment on a number of parameters that influence red squirrel population dynamics. These parameters span a range of topics, including the life history characteristics of red and grey squirrels, the epidemiology of squirrelpox virus, and the effectiveness of grey squirrel management strategies. Your answers will directly feed into a large-scale population model that aims to predict the recovery of red squirrels under different biological and management scenarios. Essentially, your expertise will help parameterise the model and include uncertainty, ensuring that it reflects the best available knowledge about these species and their interactions.


Admin annotations: Keep in mind the point of this discussion is not to get everyone to agree on a single value, or to convince each other of the values they put in. We want to make sure everyone is a) answering the same questions (i.e. interpreting the questions the same way), b) answering the questions using the same information and insight.

A - Part I. Squirrel life history

Breeding

Question A.2

How many litters do you expect an average female squirrel of each species to have per year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?



Admin annotations: In general, high consensus around 1 to 2 litter per species. Some experts are showing low confidence which increases the tail a lot. Also a slight signal of greys reproducing slightly more often than reds. One expert had a pretty high estimate for greys, though. One expert (“H”) points out number is lower because not all females reproduce each year (therefore average number of litters per female can be lower than 0)

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 0 2.0 4 85
D 0 6.0 7 60
E 1 2.0 3 98
H 0 1.0 2 80
I 2 2.5 3 100
J 0 3.0 3 85
K 0 3.0 4 100
L 1 2.0 3 80
O 0 1.2 3 100
Q 1 2.0 3 100
Red B 0 2.0 2 100
C 0 1.3 2 90
D 0 1.5 3 70
E 1 2.0 2 95
G 0 2.0 2 100
H 0 0.0 1 100
I 1 2.0 3 100
J 0 2.0 2 100
K 0 2.0 3 100
L 1 2.0 2 90
N 2 2.0 4 100
O 0 1.3 3 100
Q 1 1.0 2 80

Comments

Grey
  • H: Based on 7 years of trap and sample records from free-living greys in South Cumbria, Yorkshire and North Wales. Published research and our own findings with captive squirrels suggests that, if there are sufficient food resources, some females will breed twice per year. This will not happen every year not all females in a population will breed in the same year, so overall I believe the average will be around 1. In some environments this may be higher due to increased resources.
  • K: Personal experience and records from others of catching lactating and pregnant females throughout the year.
  • O: On average, about 205 of females will produce 2 litters
  • I: Because we are always actively culling Grey Squirrels at every opportunity it is not possible to know precisely how many broods each female has in a 12 month period, but the fact that we have culled wet females every month of the year it seems that they must be able to have at least 3 broods if availability of food and climatic conditions are suitable in a good year.
Red
  • H: From the literature I have read, it is likely that red squirrel breeding is limited to some females in a population, not all
  • E: Typically two litters per year, whilst grey can have up to three if conditions permit.
  • K: personal experience and records from Yorkshire Arboretum of captive red having 3 litters in one year.
  • O: Most females will produce 1 litter, some 2 litters - say 20-40%
  • I: In a particular woodland which is monitored on a daily basis and where we are confident that Grey Squirrels are agressively and selectively removed by shooting, rather than trapping we have seen three Red females haveing two broods in a year. Last year at York Arboretum one of the female Red Squirrels is recorded to hav had three broods.
  • G: Be aware that this is influenced by food supply as well as the age of the female with young females likely only having one.

Question A.3

How many individuals do you expect to survive after weaning from a single litter produced by an average female of each species of squirrel, under ideal conditions (i.e., in a woodland where inter- and intra-specific competition is negligible)?



Admin annotations: Similar to above, high consensus. Confidence lower (it is a harder process to observe directly), and slight advantage for greys, again.

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 0 4 8 80
D 1 4 5 55
E 2 3 8 80
H 1 2 3 80
I 2 3 5 100
J 0 2 3 96
K 0 5 6 100
L 0 2 4 61
N 1 6 6 100
O 0 3 9 100
Red B 1 2 4 100
C 0 3 6 90
D 0 3 4 60
E 1 3 6 80
G 1 3 4 100
I 1 2 3 87
J 0 2 3 98
K 0 3 5 100
L 0 1 4 80
N 1 2 3 100
O 1 2 6 100
Q 0 0 0 50

Comments

Grey
  • H: Based on published literature - probability of juvenile survival vs average litter
  • K: Personal experience of clearing a small woodland of a ‘family’ of greys.
  • I: Again because I only have experiece in woodlands where we are acitively controlling grey squirrels my experience is distorted as I do not have any evidence of where active culling of greys doesn’t take place. I think natural predation of Grey Squirel kits is likely to be similar to that which we have witnessed with Red Squirrels. We have several records of removing all Grey Squirrels from individual woodlands and we have shot entire broods of kitts out of maternal dreys, including the adult female, so my evidence is very distorted from what would happen without lethal control being undertaken.
Red
  • E: Average no. kittens per litter is 3-4 but can have up to 6 (for reds). From my understanding, most of the litter seems to survive to weaning, unless something affects the drey (e.g. storm).
  • G: Adult survival based on collars is very high and can be over 90% the critical period is the first year and here there are few data as most studies have some indication of litter size but there is very little data on juvenile dispeersers and their survival. Estimates range as high as 75% mortality but this will be dependent on food availability as well as local density
  • K: Reports of captive breeding.
  • C: Survive to what point?, I’m asuming into the next year and adulthood
  • I: In reality there are so many vaariables it is difficult if not impossible to be sure of the outcome. We have witnessed a fox taking a young Red kit which was foraging on the ground. We have found a badly damaged Red kit, which appeared to have puncture injuries, probably from a buzzard or sparrow hawk. We have watched 4-5 kits running playfully in the canopy as they have literally just come out of the maternal drey for the first time. After a month or two then they disperse so it is very difficult to be sure which are young and which are older, although some older Reds have distinctive colouration of patterns of coat which enables you to visually idetify some individuals.

C. Squirrelpox

Squirrelpox virus (SQPV) plays a crucial role in shaping the population dynamics of red and grey squirrels in the UK. The impact of the disease differs significantly between the two species. For red squirrels, SQPV infection is invariably fatal, with no assumed recovery under any circumstances. Grey squirrels, on the other hand, are assumed to survive SQPV infection without mortality, acting primarily as asymptomatic carriers. Additionally, we recognize that both the likelihood of mortality (in red squirrels) and the recovery process (in grey squirrels) may vary between juveniles and adults of each species. In this section, we ask for your expert judgment on key parameters of SQPV epidemiology, including the time it takes for red squirrels to succumb to the virus, the recovery time for grey squirrels, and the likelihood of reinfection in previously exposed grey squirrels.

Disease progression

Question C.1

Given that either a juvenile or adult red squirrel becomes infected with squirrelpox virus (SQPV), what is the expected time it will take until the individual dies, in days?


Admin annotations: This question is not very controversial either. There is extensive information for disease progression adults from experimental infections and field observations. That is reflected in the comments. Because less information is available for infected juveniles, intervals were wider.

Age Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Adult A 12 18 35 75
D 2 14 15 50
E 7 10 15 85
I 8 10 14 100
K 0 10 14 72
L 7 10 14 81
M 3 7 21 90
O 7 14 21 100
P 7 14 28 81
Q 1 1 14 80
B 0 0 0 100
J 0 14 0 100
N 0 0 0 100
Juvenile A 12 18 35 75
B 3 12 15 95
D 2 14 15 50
E 7 10 15 85
I 5 8 14 100
K 0 10 14 69
L 5 10 14 70
M 2 4 15 75
N 0 10 21 100
O 5 10 14 100
Q 1 14 14 90
P 7 28 14 80

Comments

Adult
  • A: Comment as for juveniles
  • P: This has only been experimentally conducted in adults. i would expect juveniles to be more sensitive.
  • K: No evidence.
  • I: I have experience of Squirrel Pox affecting Reds, where there is insufficient Grey control. I one particular wood where we have been agressively controlling Grey Squirrels we haven’t seen a single case of Squirrel Pox affecting Red Squirrels, the populations of Reds have remained healthy but this has required a huge effort to kill all greys as soon as they are identified, This has also required vigilance on the part of the woodland owner and good communications and good hygiene of squirrel feeders. We also use a specific feeder that is designed to be capable of being sanitised and cleansed frequently and effectively..
Juvenile
  • A: Tompkins et al 2002 found that the time taken for development of severe lesions and wt loss etc. (at an extent considered likely to cause mortality in wild individuals) was 15-25 days. In 4 captive squirrels of unspecified age (v. 4 unchallenged, control indivs)
  • E: Although juveniles seem to be more susceptible to catching the disease.
  • B: this is an inexact science - experience suggests around 12 days, but depends on food availability and the ability to eat, alongside secondary infections such as FED. The increased susceptibility to predation will also lower the period.
  • L: I’m far less familiar with juvenile survival from this disease than adult survival.
  • P: There is experimental data from A Sainsbury who conducted infection studies which should give an approximate range and average for adults
  • K: No evidence.
  • M: As far as I’m aware, we don’t have definitive data for the difference in survivability of juvenile reds vs adults following exposure to SQPV. However, inferring from other species and other disease processes, it is likely that juveniles are more readily affected and succumb to death more rapidly than adults which I’ve attempted to represent in this answer.

Question C.2

Given that either a juvenile or adult grey squirrel becomes infected with squirrelpox virus (SQPV), what is the expected time it will take until the individual recovers, in days?


Admin annotations: This one shows why we need a round of discussions! The question was underspecified and too jargony. We have two groups of estimates. The first one interpreted recovery as going from infected (therefore infectious) to not infected anymore (due to seroconversion or other mechanism). The other interpreted recovery as going from being diseased (or experiencing symptoms) to not experiencing them anymore. What the we wanted was the first one, but the question was indeed unclear, as people pointed out in the comments. Also, one expert had a high recovery time (i.e. infectious period), which might reflect a different understanding of the epidemiological dynamics (see question below).


If we truncated the plot to show a greater resolutions on the lower estimates:

Age Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Adult A 0 0 12 80
O 0 2 7 100
P 7 30 200 100
B 0 0 0 100
I 0 0 0 100
K 0 0 0 89
L 0 0 0 100
M 0 0 0 100
N 0 0 0 100
Q 0 0 0 100
Juvenile A 0 0 12 80
M 1 3 14 65
O 0 3 7 100
P 10 30 180 51
I 0 0 0 100
K 0 0 0 87
L 0 0 0 100
N 0 0 0 100
Q 0 0 0 100

Comments

Adult
  • A: Same comment as for juveniles
  • P: ### Really important fact: grey adults can become reinfected with Sqpox (or it goes latent) so they can have an intermittent viraemia with periodic shedding for several months. The infection intensity is less than with a primary infection. This is comparatively uncommon ###
  • K: They do not get ill and do not have to ‘recover.’
Juvenile
  • A: Can you clarify what ‘recovery’ means in this context? Below you refer to it as the time for development of a sufficient antibody response? For now I’ve taken it to mean clinical recovery - although it’s assumed that, as per published literature (e.g. Tompkins et al 2002), grey squirrels are not clinically affected at all so there is no ‘recovery period’ per se….
  • B: greys typically are asymptomatic. I have never seen one with any symptoms of SQPV
  • P: The infectious period in greys can last as long as 6 months.
  • K: I understood that only one grey has ever been found to have symptoms of pox so it seems that they do not get ill with the disease and therefore do not have a recovery time.
  • M: Given that greys typically do not develop disease, it is difficult to state what is meant by ‘recovery’? Is this time to seroconversion or something else?
  • I: I have not seen any adult of juvenile grey displayong SQPV virius syptoms in the wild

Immune response

Howell et al. 2004 has proposed a new mechanism for squirrepolx epidemiology, where grey squirrels infected with squirrelpox virus (SQPV) can become reinfected even after developing antibodies, indicating that immunity is partial or waning rather than lifelong. Field data showed that seropositive grey squirrels often experience repeated cycles of infection and recovery, with a high prevalence of viraemic individuals in the population. Experimental findings further suggest that seroconversion may require multiple infection challenges, and once antibodies are developed, they persist but do not fully prevent reinfection. These results underscore the complexity of immune responses in grey squirrels and the role of partial immunity in sustaining SQPV dynamics within their populations. For the purposes of this exercise, we assume that grey squirrels develop partial immunity from SQPV.

Question C.3

If a grey squirrel has previously been infected with SQPV and has subsequently recovered (i.e., it has developed an immune response to the disease), what is the relative likelihood that it will become reinfected if exposed under the same epidemiological conditions as its previous initial infection? Assume all other factors remain constant, including habitat, the density of infected individuals in the area, and environmental conditions. NOTE: This is a relative comparison. If you believe the likelihood of reinfection for a seropositive grey squirrel is identical to its initial infection risk, your answer should be 100%. If you believe that a recovered grey squirrel would never be reinfected, your answer should be 0%.

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
A 60 90 100 75
M 50 60 90 60
O 20 50 90 100
P 40 60 80 80
I 0 0 0 100
N 100 100 100 100
Q 0 0 0 100

Comments

Relative susceptibility
  • A: Inferred from Tompkins et al 2002 & Chantrey et al 2019
  • P: We showed greys in the wild get reinfected or are latently infected. Recurrence was frequent. Immunity seemed to be transient. Epidemics. 2019 Sep;28:100352. doi: 10.1016/j.epidem.2019.100352.
  • M: Antibody responses are generally thought to be incompletely effective in greys and it is likely that some degree of reinfection potential persists. There is also some suggestion of recrudescence, which is unusual for a pox virus infection. The exact nature of this reinfection probability is unfortunately difficult to quantify and seems to vary markedly between individuals.
  • I: Having never seen a single case of SQVP in grey squirrels in the wild, i think that this is a hugely acedemic theory without relevance in the real world. I believe that grey squirrels my be A symptomatic in the ame way that some humans may be A symptomatic for Covid and other viruses

Vaccination

Vaccination has the potential to be a transformative tool in mitigating the impacts of squirrelpox virus (SQPV) on red squirrel populations. By conferring immunity to individuals, a successful vaccination program could reduce mortality rates and increase the viability of red squirrel populations. However, the effectiveness of such a program depends on several factors, including the likelihood of vaccine development, the expected timeline for its availability, and its efficacy.

Question C.4

What is the probability that an effective vaccine for squirrelpox virus (SQPV), specifically for use in red squirrel populations, will be successfully developed and made widely available for use by landowners, government organizations, and conservation groups?



Some experts seem to have expressed the value in a 0%-100% scale instead of 0-1. If we correct for that, the new plot is


Admin annotations: Crazy high uncertainty, with some diverging views on how likely it is to happen. This is expected, as there is no current development in place and there are too many unknowns. We fully understand that, however vaccination is a consistent action being proposed as a possible course of action, therefore we cannot shy away to evaluating it. On the other hand, we also cannot just assume it will happen, hence these uncomfortable questions.

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
C 0.2 0.40 0.5 75
D 0.0 0.20 0.8 50
E 0.5 0.70 0.9 55
J 0.0 0.30 1.0 81
K 0.0 0.50 1.0 88
M 0.0 0.01 0.5 80
N 0.0 1.00 1.0 100
O 0.0 0.50 1.0 100
Q 0.1 0.30 0.5 50
A 20.0 50.00 70.0 75
B 0.0 0.00 0.0 100
H 0.0 0.00 0.0 100
I 1.0 1.00 3.0 80
P 5.0 20.00 30.0 71

Comments

SQPV Vaccine Development Probability
  • A: The Moredun Institute apparently had a project looking to develop a vaccine and did develop a prototype. But to be licensed, it requires testing on a captive (experimental) red squirrel population. Apparently the funders withdrew their support for the project at this point, and it was a challenge to get formal institutional ethical approval for the work (?). A vaccine (whether injectable or oral) would also require field testing before more extensive roll out. This would be expensive and timeconsuming. E.g. an oral vaccine for TB in badgers (which would be much more practical than the current injectable vaccine which is used for individuals in the field) has taken decades to develop and is still not yet ready for implementation, despite significant funding. Not that vaccination isn’t worth pursuing… But I wonder if managing habitat/ecosystems for red recovery would perhaps be the better focus and address one the key underlying drivers of disease
  • B: Given the progress of the contraceptive is measured on a geological time scale and there is no strategy to deploy it, the chances of a vaccine ever being developed and deployed are effectively zero. The creation of a vaccine could become counter productive and SQPV may not be the major disease risk.
  • K: A vaccine would be useful for an island population such as Anglesey but in the rest of the country, even if the pox issue is removed, there is still a problem that reds cannot live with greys which outcompete them for food and habitat. Put the money into grey eradication throughout the UK - immediately, not in five-ten years when new tech is available. There will be no reds by then.
  • M: The main factors limiting vaccine delivery, viability and availability would be an effective delivery method, which is not currently achieved. Furthermore, the short generation time of red squirrels and the potential incomplete and waning protection provided by SQPV antibodies in greys suggests that a vaccine may only remain effective for a short period of time, necessitating repeated extensive vaccination of a large proportion of the red population to retain herd immunity, which is unlikely to be feasible.
  • I: For small island populations this my be a possible solution if a vaccine can be produced, but this is not proven. I cannot see how it would be possible to trap and vaccinate every wild squirrel in the UK, we do not even know where they all are!! Also there are recorded fatalities of Red Squirrels being handled in captive populations, that will also be a real risk if Red Squirrels are captured for vaccination purposes.
  • C: Vaccine progress is being blocked, ‘hand in the rugby ruck slowing the ball’ is a good analogy. Until the political interference stops, those medelling are hampering progress.

Question C.5

If an SQPV vaccine is developed and approved as a management tool for red squirrel populations, by what year is it expected to become widely available?



Some experts seem to have expressed the value in “years from now” instead of calendar years. If we correct for that, the new plot is


Admin annotations: Same as above.

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
A 2028 2034 2050 75
B 2030 2050 2099 100
C 2030 2034 2040 80
E 2025 2030 2035 55
M 2030 2045 2060 50
O 2025 2033 2040 100
K 0 5 10 88
N 0 1 1 100
P 5 15 30 65
Q 0 0 0 100

Comments

SQPV Vaccine Rollout Year
  • B: This is many many years in the future and the issues surrounding the development of a vaccine have not yet been addressed
  • K: Good luck!
  • M: This is an incredibly difficult question to answer with any certainty. Given the potential hurdles of red squirrel SQPV vaccination which have not been overcome, such as an effective delivery method and maintenance of immunity across progressive generations, vaccine development is likely to take a number of years. This is assuming that these hurdles can be overcome at all, and there is a distinct possibility that effective SQPV vaccination may never be an option.

Question C.6

For an individual red squirrel vaccinated against SQPV, what efficacy would you expect such a vaccine to have (i.e., the probability that the vaccinated individual will successfully seroconvert and develop immunity to the virus)?



Some experts seem to have expressed the value in a 0%-100% scale instead of 0-1. If we correct for that, the new plot is


Admin annotations: Same as above, with the added difficulty that there is no current vaccine model (type, mode of delivery) to specify against. One expert assumed the most likely mode of delivery (oral bait through feeding stations) given it is the most (if not only) practical one. Another expert assumed MRNA vaccines.

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
C 0.70 0.85 0.95 55
E 0.50 0.80 0.90 55
M 0.20 0.50 0.80 50
O 0.40 0.80 1.00 100
P 0.01 0.40 0.70 50
A 15.00 75.00 85.00 60
N 1.00 1.00 1.00 100

Comments

SQPV Vaccine Efficacy
  • A: It depends how the vaccine is administered. An injectable vaccine would be more likely to elicit immunity than an oral bait, although the latter would be much more practical. For now, I’ve assumed oral bait since this would probably be the only practical option a geographically extensive vaccination campaign, and have assumed it would be administered at feeding stations over multiple days (squirrels would need to get acclimatised to feeders first)
  • M: Again, this is a very difficult question to answer, as the vaccine which would be used in the field is not currently available or undergone sufficient trials to allow estimation of potential protective immunity in reds. However, given the push towards a feed delivered vaccine, this is unlikely to reach 100% protection within a red population.
  • C: assuming mrna - biggest risk is virus mutates and mrna becomes less effective without redesign.

A - Part II. Squirrel life history

Understanding the life history traits of red and grey squirrels is fundamental to predicting their population dynamics and informing management strategies. These traits shape how these species respond to environmental conditions and management interventions. In this section, we will ask for your expert judgment on parameters such as breeding season length, litter size, annual survival probabilities, and home range sizes.

Breeding

Question A.1

Under current conditions, what is the average length, in months, of the breeding season of each species of squirrel (i.e. the window during the year when each species engages in reproductive activities, including mating, gestation, and giving birth to offspring)?


Admin annotations: Here we get to the first issue with averages across habitats and enviromental conditions. First point, the current models I am basing this one use one fixed breeding period, across species and habitats (6 months). This is an effort to more accurately express the dynamics of different “pulse dynamics” between species and the trapping periods as well. We understand an average value will not represent any particular woodland, but realized across the woodlands in a tetrad, and across years, it should be representative. That being said, the general pattern is that greys are able to reproduce for longer periods throughout the year (maybe the whole year). One note is that two experts had markedly lower values for red breeding window than the others. This is likely because they were working under a different definition of breeding period.

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 1 6 12 100
D 4 10 12 55
E 5 6 9 99
H 6 7 9 90
I 9 10 12 100
J 6 9 12 92
K 6 12 12 100
L 8 11 12 80
O 5 9 10 100
Q 8 9 9 100
Red B 0 1 2 100
C 10 10 12 96
D 6 8 10 70
E 5 6 9 99
I 6 8 9 76
J 4 5 6 100
K 3 9 10 100
L 6 6 7 81
N 0 1 1 100
O 5 9 10 100
Q 7 8 9 100
G 10 10 10 100

Comments

Grey
  • H: This is very much an average based on our breeding records from trapped free-living squirrels over the past 7 years and our captive squirrel behaviour. I have heard that females have been found breeding outside of these months, however, I have not personally seen any evidence for this. My knowledge is specific to North Yorkshire, South Cumbria and North Wales
  • B: Again, difficult to answer as greys seem to be breeding all year round and producing several litters, but this is largely anecdotal and may well be regional
  • K: Personal observations of catching pregnant and lactating female greys throughout the year as well as very young adolescents.
  • O: Length of season depends on tree seed availability
  • I: I have personally culled pregnant females and young kits every single month of the year. I have been undertaking active grey control for the last 20+ years. I do find that December breeding is dependant on the availability of food and to a lesser extent climatic conditions. I hav also witnessed mating of squirrels in every month of the year.
Red
  • E: Two breeding seasons, approx. Feb-Apr and June-Aug but can start in Dec.
  • B: this is difficult to answer, as the actual mating period only lasts days, but can occur over several months
  • G: This is a poorly structured question. Define what you mean. Each of the species has two breeding periods in a year - depending on location start can be late Dec to October. With a break for individuals inbetween the two but not for the population as breeding is not synchronised.
  • K: Personal observations
  • O: Length of season depends on tree seed food availability
  • I: The breeding season varies year on year and seems to be dependant upon climate and availability of food, both natural and supplementry.

Survival

Question A.4

What is the average probability that a given juvenile squirrel (of both species) will survive until adulthood, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible, there is no squirrelpox virus, no pine martens in the landscape and no lethal control being conducted)? Assume a juvenile period of 9 months.


Two experts expressed values above 1 (but too low to be reasonably interpreted as a percentage), so if we remove those:


Admin annotations: I think this is the first probability question. So good to point out somethings. Here and throughout, there are answers in probability that have the most likely as 0 or 1. This is unlikely to be true, as a proportion of individuals invariably dies over a given period. What we are asking is this proportion, not the realized outcome for a single individual. Also, we are asking for the average of a population (therefore unlikely average is 0 or 1 ). In general, people assigned higher survival probabilities for greys. Some comments pointed out no natural predators for greys as a source of that increased survival

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 0.00 0.70 1.0 90
D 0.00 0.60 1.0 60
E 0.15 0.25 0.6 70
H 0.20 0.40 0.6 75
K 0.00 0.90 1.0 100
L 0.00 1.00 1.0 50
O 0.20 0.70 0.8 100
Q 0.30 0.40 0.5 60
N 0.00 6.00 6.0 100
Red B 0.00 0.50 1.0 100
C 0.20 0.40 0.7 72
D 0.00 0.30 1.0 60
E 0.15 0.25 0.6 70
K 0.00 0.80 1.0 100
L 0.00 1.00 1.0 50
O 0.20 0.70 0.8 100
Q 0.30 0.40 0.5 100
I 1.00 2.00 3.0 97
N 0.00 1.00 3.0 100

Comments

Grey
  • H: Based on published literature that is not recent, so these values may have changed
  • L: Again, highly speculative, if conditions are absolutely ideal, they aren’t being culled or are going to be a casualty as a result of a road death in theory all juveniles should survive their first winter, however, this is highly unlikely.
  • K: No natural predators.
Red
  • E: Published literature suggest fewer than 25% of juveniles survive first winter and reach adulthood.
  • L: This is highly speculative. If all conditions environmental, predatory, no disease, in theory any juvenile should survive, even though survival rates for this species through their first winter ae generally extremely poor.
  • K: 30+ years of experience! They can thrive well naturally in the UK - all they need is to be free of greys.
  • D: Survival until adulthood is effected by whether the juvenile comes from a first litter or a subsequent litter in the breeding season. Survival rates decrease for second and subsequent litters

Question A.5

What is the average probability that a given adult squirrel (of both species) will survive a full calendar year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible, there is no squirrelpox virus, no pine martens in the landscape and no lethal control being conducted)?


Two experts expressed values above 1 (but too low to be reasonably interpreted as a percentage), so if we remove those:


Admin annotations: Same as above for probabilities. Same general pattern. Same concerns about average, and how “ideal” conditions mean the individuals do not die

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey D 0.00 0.70 1.000 50
E 0.50 0.70 0.800 80
H 0.40 0.70 0.800 70
K 0.00 0.80 1.000 100
L 0.00 1.00 1.000 50
O 0.20 0.70 0.800 100
Q 0.30 0.40 0.500 50
Red B 0.00 0.80 1.000 100
C 0.30 0.60 0.900 73
D 0.00 0.50 1.000 60
E 0.50 0.70 0.800 80
G 0.31 0.77 0.959 100
L 0.00 1.00 1.000 50
O 0.20 0.70 0.800 100
Q 0.30 0.50 0.500 60
K 0.00 7.00 1.000 100
N 0.00 1.00 3.000 100

Comments

Grey
  • H: Interpretation from published literature, but may be out of date
  • L: As per my answer for red squirrel. If conditions are ideal in theory the squirrel should survive, however, in reality this is unlikely to be true. However, I believe that the grey squirrel is more likely to survive than the red squirrel but conditions are ideal …..
  • K: Not allowing for old age.
Red
  • B: always other dangers and this is a best case scenario
  • L: As per the question for juveniles, this is highly theoretical. If the conditions for survival are ideal, in theory the squirrel should survive. In reality this may not be true for a variety of reasons.
  • G: These are data from spruce forests from our study see Petty et al 2003 Biological Conservation 111 (2003) 105–114
  • K: Not allowing for old age.

Movement

Question A.6

What is the average home range size of each species of squirrels (in hectares), under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).


Admin annotations: Issues with home ranges varying widely between states of the individuals of the population. Again, we are looking for the realized average for the population-level (this will define how many squirrels are in contact with a given trap.

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey B 1.0 10 50 100
E 0.5 5 20 80
F 0.1 10 12 90
H 2.0 5 10 90
K 1.0 5 10 88
O 3.0 5 20 100
Red B 0.0 10 50 93
C 3.0 4 7 69
E 2.0 7 20 90
K 1.0 2 10 91
N 2.0 7 23 100
O 3.0 8 20 100
Q 1.0 1 2 50

Comments

Grey
  • H: Based on short-term movement data gathered in the field and from movement data in published literature

  • F: GPS tracking data from 29 grey squirrels in broadleaf woodland, mid-Wales. “29 squirrels were tracked for a mean duration of 16 days (SE 0.9 days, range 5-23) and the mean number of locations per individual was 265 (SE 17, range 82-437). Mean home range (90% KDE) was 10.4ha (SE 1.1ha) and the mean core range (50% KDE) was 2.0ha (SE 0.2ha)”

    Important to note that this tracking was undertaken during a period where pine martens had been released into the area. Conclusions of the study were that martens may INCREASE space use by GS therefore home ranges may be greater than normally expected. Also important to consider when reviewing other answers is the difference between ‘home’ and ‘core’ range. Reference: McNicol 2019 (PhD thesis)/McNicol et al 2020 Translocated native pine martens Martes martes alter short‐term space use by invasive non‐native grey squirrels Sciurus carolinensis
  • B: Same for both species, experience suggests the 10Ha figure.
  • K: This question does not take into account if the squirrel is: - a breeding female with young - an adult male with a ‘territory’ - an adolescent male dispersing from birth area to new territory - a young female looking for her own breeding area
Red
  • F: Unknown - not area of expertise. Answer provided for grey only.
  • E: Very dependant on other factors such as sex (males have larger HRs than females), habitat type (coniferous vs deciduous vs urban), habitat quality, etc.
  • B: This is very dependent on the geography and type of woodland. Mixed woodland will have a smaller range than coniferous monoculture, and squirrel density will also be relevant.
  • G: see email I sent you
  • K: This question does not take into account if the squirrel is: - a breeding female with young - an adult male with a ‘territory’ - an adolescent male dispersing from birth area to new territory - an young female looking for her own breeding area
  • C: What do you mean by ideal conditions? I’m assuming typical rather than an atypical

B. Red squirrel translocation

Post-release effects are a critical consideration in translocation efforts, as they can significantly influence the survival and breeding success of released squirrels. After being released into the wild, squirrels may experience a period of acclimation during which their survival and reproduction rates differ from those of established wild populations. These effects may vary depending on whether the squirrels were sourced from captivity or the wild. In this section, we seek your expert judgment on key aspects of post-release effects.

Question B.1

How many months do you believe it will take for released squirrels (from captive or wild sources) to become acclimated to the release site (i.e. have survival and breeding rates expected from the release habitat and not influenced by the translocation itself)?


Admin annotations: A recurring issue here is that the conditions of release were underspecified. Work together to figure out parameters for release? (Time of year and delayed/immediate/age?). In addition, some values of absolute zero - this might be empty questions but can be no acclimation period.

Source Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Captivity A 3 14 24 65
B 1 3 12 76
C 2 3 4 64
D 2 3 6 50
E 2 5 8 60
K 0 6 12 100
O 1 1 2 100
N 1 1 1 100
Q 0 0 0 100
Wild D 2 3 6 50
E 1 4 6 60
K 0 4 12 100
O 1 1 2 100
A 1 12 4 65
B 0 0 0 100
C 0 0 0 100
N 0 0 0 100
Q 0 0 0 100

Comments

Captivity
  • B: The difficulty in monitoring an individual makes this guesswork and the added factor of habitat/existing populations compounds the issue
  • K: Impossible to answer without knowing time of year for translocation, conditions of new habitat, age of animals, sex of animal, etc
  • C: As most are released in autumn winter they are entering the forest at a time of potentially very low / no reproduction and in the coming months (late winter) high natural mortality, and yet they are supplementary fed so buffered. What Ive given is a guess to reflect settlement and range establishment
Wild
  • K: Impossible to answer without more knowledge of time of year for release, age and sex of animal, conditions of new habitat, etc

Question B.2

After a red squirrel is released into the wild (from captive or wild sources), how many litters do you expect it to produce, while it is still acclimating to its new environment?
NOTE: Take note that this question asks for the total number of litters over the acclimation period. This is dependent on the previous question you answered, not a fixed yearly rate.


Admin annotations: Same as above for underspecification. People assumed autumun release so no breeding on that period. Low overlap of captivity and wild answers.

Source Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Captivity B 0 1.0 2 76
C 0 0.1 1 60
D 0 1.0 1 50
E 0 0.0 1 60
O 0 1.0 2 100
K 0 0.0 0 100
N 0 0.0 0 100
Q 0 0.0 0 100
Wild D 0 1.0 1 50
E 0 1.0 1 60
B 0 0.0 0 100
C 0 0.0 0 100
K 0 0.0 0 100
N 0 0.0 0 100
O 0 0.0 0 100
Q 0 0.0 0 100

Comments

Captivity
  • B: As previously, this is just guesswork and without knowing the actual conditions it is impossible to say with any certainty.
  • K: A female would probably not breed while it is still acclimating, a male could breed with any female it finds.
  • D: Releases normally occur in the autumn when the tree crop food sources are highest to give best chance of survival. This will be outside of the breeding season.
  • C: Again this reflects autumn release
Wild
  • K: A female would probably not breed while it is still acclimating, a male could breed with any female it finds.

Question B.3

After a red squirrel is released into the wild (from captive or wild sources), what is the probability it will survive until the next month, while it is still acclimating to its new environment?


Admin annotations: Same as above for underspecification. People assumed autumunal release. Lots of uniformative guesses, reflecting underspecification. On the other hand, people did conduct releases and show how many would die, so there is their anecdotal information to go off from.

Source Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Captivity B 0.0 0.9 1.0 81
C 0.3 0.5 0.8 61
D 0.0 0.5 1.0 100
K 0.0 0.5 1.0 100
N 0.0 1.0 1.0 100
O 0.0 0.6 1.0 100
Wild B 0.0 0.6 1.0 100
D 0.0 0.5 1.0 50
K 0.0 0.5 1.0 100
O 0.0 0.7 1.0 100

Comments

Wild
  • B: Experience suggests that red squirrels have a social dynamic and removal from that is an unknown quantity - this and the possibility that the animal might stray long distances in looking for familiar territory could make survival chances lower than a captive bred animal. The method of release will also be relevant.
  • K: Too many variables for this to be answered.
Captivity
  • B: this will depend on many factors, including presence of avian predators, food availability and condition of the squirrel
  • K: Too many variables for this to be answered.

D. Grey squirrel management

Management of grey squirrel populations is a crucial component in supporting the conservation of red squirrels. Grey squirrels, being an invasive species, have a significant negative impact on red squirrel populations, largely due to competition for resources and the transmission of squirrelpox. To mitigate these effects, various management approaches are utilized, including trapping and shooting. The effectiveness of these approaches are assessed below.

Trapping

The following traps are being evaluated on this elicitation:
- Single-capture live traps: Cage traps designed to capture one grey squirrel at a time without causing harm. Once trapped, the squirrel can be humanely dispatched.
- Multi-capture live traps: Larger cage traps capable of capturing multiple grey squirrels simultaneously, in different subsections of the trap
- Snap traps: Lethal traps that quickly kill grey squirrels using a spring-loaded mechanism. Designed to ensure a swift and humane death.
Goodnature traps: Automated, self-resetting traps powered by CO2 canisters. They humanely kill grey squirrels upon activation and automatically reset after each use, allowing continuous operation with minimal maintenance.

Question D.1

Given a density of 1.25 grey squirrels per hectare in a woodland, what is the average probability that a given trap will catch a squirrel over one day of deployment. Assume all squirrels are within range of the trap.
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).


Admin annotations: Loads of variation, reflecting the many different ways these traps can be deployed. Issue with density, explain that this is 3/4 of mixed woodland carrying capacity. These values will not be fixed, but adjusted for higher or lower densities. Goodnature and snap traps deemed worse than live traps. Maybe work further to specify what the “average” deployment looks like?

Trap Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Goodnature traps B 0.0 0.20 1.0 100
C 0.0 0.05 0.1 69
F 0.0 0.01 0.1 80
H 0.4 0.60 0.8 80
K 0.0 0.20 0.5 100
Q 0.1 0.10 0.3 80
E 0.0 0.00 0.0 100
I 0.0 0.00 0.0 100
N 0.0 0.00 0.0 100
O 0.0 0.00 0.0 100
Single capture live traps B 0.0 0.20 1.0 95
C 0.3 0.50 0.6 60
E 0.5 0.70 0.9 65
F 0.0 0.70 1.0 100
H 0.8 0.90 0.9 95
I 0.0 0.75 1.0 100
K 0.0 1.00 1.0 100
N 0.0 0.00 1.0 100
O 0.1 0.50 0.8 100
Q 0.1 0.10 0.3 80
Snap traps B 0.0 0.20 1.0 100
C 0.0 0.30 0.4 65
F 0.0 0.30 1.0 80
K 0.0 0.50 1.0 100
Q 0.1 0.10 0.3 80
E 0.0 0.00 0.0 100
I 0.0 0.00 0.0 100
N 0.0 0.00 0.0 100
O 0.0 0.00 0.0 100

Comments

Goodnature traps
  • H: Based on GN trap data, however, additional data required
  • F: PRACTICAL/EXPERIENCE-BASED ANSWER Colleagues have reported poor returns when testing good nature traps, with high levels of by-catch - especially of wood mice and occasionally woodpeckers. They have been broadly ineffective at trapping grey squirrels and definitely not well targeted to the species.
  • K: Difficult to have success for most people and expensive. Should not be used if reds of Pine Martens could be nearby.
  • C: I had a student compare single catch vs fineren box /magnum 116 trap and live catch is better.
Single capture live traps
  • H: Based on 7 years of trapping data, however, necessary to know size of the wood and trap density to be certain

  • F: PRACTICAL/EXPERIENCE-BASED ANSWER With single-capture traps, the probably can vary greatly based on whether pre-baiting of the trap has been undertaken, the time of year trapping is occurring, previous and current weather, the positioning of the trap and if the site has been trapped previously.

    If there has been 5-7 days of pre-baiting, trapping is occurring during a period of poor natural food availability , dry weather and traps placed at the base of trees with open ground surrounding them, the chance of trapping a squirrel is high.
  • B: This is impossible to answer. Factors such as prebaiting, type of bait, wariness of the squirrels, disturbance, etc are all important.
  • K: Used correctly, i.e. the right trap in the right place, any trap will catch a grey quickly. There is no one trap better than another and many controllers may use several. feeders, traps and/or shooting in the same woodland. There is no single way to do this, each situation is different and experience used with trial and error will always be successful. No kill traps or Good Nature traps should be used when red squirrels or Pine Martens could be in the area. There are usually many more than 1.25 greys in a hectare of previously uncontrolled woodland!!
  • O: Many factors affect catchability, especially type of forest and natural food availability.
  • I: We regularly trap grey squirrels in trapping sessions. Your assumption of 1,25 grey squirrels per hectare is a gross under estimate. Two months ago we started lethal grey control in a 4 hectare wood and have to date removed 52 Grey Squirrels with still more grey squirrels present. We expect to cull 100+ from that wood in a year. in another 11 Hectare wood where there has been grey control going on for 5 years we have culled 177 grey squirrels this year to date and expect to cull 200+ by the end of this year.
Snap traps
  • F: PRACTICAL/EXPERIENCE-BASED ANSWER Similar to live capture traps, the efficacy of snap traps is dependent on environmental factors such as food availability, time of year, age of squirrels (i.e. are there dispersing/naïve juveniles?). Snap traps are also depend on a degree of pre-baiting as this will establish scent lines/regular visitation by squirrels which can enhance first day capture rate dramatically and possibly improve humaneness of kill as squirrels are entering the trap fully and confidently and are therefore in the correct position for an efficient lethal kill. However, given that grey squirrels predominantly forage on the ground, the kill traps are estimated to be less effective than single capture traps placed on the ground (however can be less labour intensive for the trapper).
  • K: When located and used correctly, kill traps are very effective. Banned in areas where the may be reds or Pine Martens, eg Cumbria, Northumberland, Devon, Wales, and will possibly be banned in more parts of UK.
  • Q: Presume the area hasn’t been pre-baited. When trapping I’d pre-bait two weeks prior to activating a trap. Assume also using attractants aniseed oil or know hazelnut oil. N.B. I’ve only ever used live catch traps but would assume the take up to be the same though - could be wrong

Question D.2

Given a density of 1.25 grey squirrels per hectare in a woodland, how many squirrels will one multi-catch live trap catch over one day of deployment. Assume all squirrels are within range of the trap.
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).


Admin annotations: Same as above. Maybe work further to specify what the “average” deployment looks like?

Expert Minimum Plausible Most Likely Maximum Plausible Confidence
B 0 0.2 5.0 95
C 0 0.0 0.1 90
I 0 1.0 1.0 100
K 0 1.0 1.0 100
N 0 0.0 1.0 100
O 0 2.0 6.0 100

Comments

Multi-trap catch rate
  • B: Again, impossible to answer as it is dependant on numerous factors.
  • K: Used correctly, i.e. the right trap in the right place, any trap will catch a grey quickly. There is no one trap better than another and many controllers may use several. feeders, traps and/or shooting in the same woodland. There is no single way to do this, each situation is different and experience used with trial and error will always be successful. No kill traps or Good Nature traps should be used when red squirrels or Pine Martens could be in the area. There are usually many more than 1.25 greys in a hectare of previously uncontrolled woodland!!
  • I: This is an extremely low densiity of squirrels, the incidence of multiple squirrel catches in a day with such a low density of squirrels is very unlikely. I use both single catch and multicatch traps and find multicatch are only beneficial when the burden of squirrels in a wood is above 5 per hectare.
  • C: These are garbage

Shooting

The following shooting strategies are being evaluated on this elicitation:
- Free shooting: This method involves tracking and shooting individual grey squirrels as they are spotted in the field.
- Drey-poking: This technique targets dreys (nests) by disturbing them, causing the squirrels to flee, making them easier to shoot.
- Shooting over bait stations: In this approach, bait stations are used to attract squirrels to a specific location, where they can be shot.

Question D.3

How much ground does an average shooter cover one day’s fieldwork, in hectares?
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).


Admin annotations: Loads of variation, reflecting the many different ways shooters move. Maybe work further to specify what the “average” shooter looks like?

Trap Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Drey-poking B 0.25 5.00 10.0 50
E 500.00 700.00 1000.0 55
F 0.00 100.00 120.0 60
I 2.00 3.00 5.0 100
K 0.00 30.00 50.0 100
Q 10.00 15.00 20.0 100
N 0.00 0.00 0.0 100
Free shooting B 0.25 10.00 100.0 75
E 500.00 700.00 1000.0 80
F 20.00 100.00 120.0 70
H 5.00 15.00 25.0 75
I 2.00 6.00 10.0 100
K 0.00 30.00 50.0 100
N 0.00 1.00 1.0 100
Q 10.00 15.00 20.0 90
Shooting over bait-stations B 0.10 0.25 0.5 74
E 200.00 500.00 700.0 55
F 20.00 200.00 240.0 70
H 5.00 15.00 25.0 100
I 2.00 10.00 10.0 100
K 0.00 30.00 50.0 100
Q 1.00 1.00 2.0 100
N 1.00 1.00 1.0 100

Comments

Free shooting
  • H: There are a lot of influencing factors here; size of wood, travel time, terrain etc. Answer is based on a small number of field trials
  • B: Impossible to answer without more information. Type of woodland, density of cover, presence of a dog for retrieves, shotgun or air rifle, number of grey squirrels, weather, time of year and many other factors.
  • K: There is no such thing as an average shooter! Some people shoot from their bathroom window, some go out at the weekend with others for a day shooting and get 100-200greys out of several woodlands. Some will check traps twice a day and take their guns in case they see any greys in the trees, others will only sit and shoot at feeders.
  • I: I personally find free shooting the least successful as just roaming around on the off chance of seeing a squirrel is largely a matter of chance, in reality most squirrels will see you first and beat a retreat of hide. You are also likely to be using a shotgun which has limits due to noise if near residential populations. Shooting skyward with any rifle is potentially dangerous.
Shooting over bait-stations
  • F: Bait station shooting is based on static points for example 4 bait stations within a 240ha site. Therefore if one assumes all squirrels are in reach of a feeder this allows for effective site coverage. However shooting is most effective at set times of day (early morning, late afternoon) and effectiveness/cull rate will decline with effort at the site.
  • B: My experience is that only one location will be used, so the area covered is minimal.
  • K: There is no such thing as an average shooter! Some people shoot from their bathroom window, some go out at the weekend with others for a day shooting and get 100-200 greys out of several woodlands. Some will check traps twice a day and take their guns in case they see any greys in the trees, others will only sit and shoot at feeders.
  • Q: Don’t free shoot / walk up - thinks it’s a waste of time. Though heard it works for some using thermal optics and small calibre rifles or FAC air rifles. Shooting over bait stations from a hide @ 25yds for 3-4 hours has the best results for me but don’t believe this effective on a landscape scale given it’s time / space inefficient once the localised grey population has been reduced. Never drey poked given in an area of mixed populations.
  • I: In my experience shooting over feeders (bait stations) is by far the most effective methos of controlling Grey squirrels. You are bringing the squirrels down to a safe place to shoot them in a controlled manner. The squirrels come to you for food, you don’t have to roam around looking for them. If you are running multiple feeders in a wood you can close the feeders which you are not staking out at a given time. if you then use a live catch trap positioned near the closed feeders the squirrels coming to those locations will then readily go into the live catch trap for their free meal, while you stake out the feeder you have chosen to shoot at that particular day. After shooting greys on the designated shoot feeder for the day you can then go and collect the squirrels from the baited traps at the other feeder locations so that you are removing squirrels from a wide area in a single day.
Drey-poking
  • F: With free shooting, drey poking and bait station shooting, often the area covered is dependent on land ownership and the woodland block that has been selected. These estimates are based on known woodland blocks where grey squirrel control is undertaken. Normally a single area is selected for control that day/week. Drey poking can be undertaken all day so in theory a larger area can be covered compared to bait station shooting and free shooting as it is less dependent on squirrels being active and moving outside of dreys.
  • B: impossible to answer, so best guess only. Number of people involved and degree of competency is important here.
  • K: Drey poking only to be used out of the breeding season of birds and red squirrels which could be nesting in dreys. Must make sure of experienced shooters who can ensure that only greys are escaping the dreys - binoculars?
  • I: Drey poking cannot be undertaken in areas where there are Red and Grey squirrels Drey poking cannot be undertaken in high mature trees, it is simply not feasible to poke dreys 10+ metres high. in woods with old mature trees dreys are often with the trunks of decaying trees and therefor cannot be poked.

Question D.4

Given an average density of 1.25 grey squirrels per hectare in a woodland, how many grey squirrels would a shooter dispatch in a day’s fieldwork, on average?
NOTE: Remember an hectare is equivalent to 100 m x 100 m (or 1/100th of a square kilometer).


Admin annotations: Same as above. Maybe work further to specify what the “average” shooter looks like? Also same comments regarding density - we use a reference one that will then be adjusted.

Type Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Drey-poking F 0 2 8 60
K 0 20 50 100
Q 0 3 6 51
E 0 0 0 100
N 0 0 0 100
Free shooting E 5 15 30 70
F 0 2 4 70
K 0 30 50 100
N 0 1 1 100
Q 0 0 1 100
Shooting over bait-stations F 0 4 10 70
K 0 30 50 100
N 0 1 1 100
Q 0 3 6 100
E 0 0 0 100

Comments

Shooting over bait-stations

  • F: As with single capture traps - pre-baiting can enhance the efficacy of bait station shooting. If the site has been supplied with food for at least a week prior to shooting, and then possibly monitored with trail cameras - the shooter can be confident of visitation rate and times of grey squirrels. This can yield a high kill rate on day one.

    However, due to the static nature of bait stations and territoriality of squirrels, once the regular visitors are shot it may take some time for new squirrels to enter the territory and find the feeder. Therefore a number of bait stations should be rotated through to get maximum cull rate for minimum time input.
  • K: Ditto - no such thing as average shooter. Some people can get 50 in a day, others 1 or 2. Depends how many squirrels there are!! It takes 20% of effort to get rid of 80% greys and 80% effort to get rid of last 20% greys in a woodland. Without consistent control, more will come in within a few months.
  • Q: Don’t free shoot / walk up shoot and can’t drey poke given I’m in a mixed population area. Have shot none and up to 6 from a hide over a bait station. However numbers fluctuate according to the time of year and levels of past control.
Free shooting

  • F: Free shooting is normally undertaken when squirrels are most active i.e. early morning and early evening so the effort and opportunity is not equally distributed across the day. The time of year this is undertaken will impact number shot - this impacts visibility, squirrel activity levels, demographic of squirrels and their cautiousness (juveniles/mating animals etc), as well as the normal activity levels on site - for example remote woodlands where squirrels are ‘shy’ vs dog-walker/busy woodlands where squirrels have become habituated to human presence.

    The type of gun used i.e. shotgun versus rimfire can also impact efficacy due to differing distance to squirrel and location of squirrel for it to be a safe shot.

    Shooting often becomes less effective throughout the session due to the increased disturbance cased by the shooter through shots/movement through the site.
  • E: Dependant on time of year, i.e. natural food sources, canopy cover.
  • K: Ditto - no such thing as average shooter. Some people can get 50 in a day, others 1 or 2. Depends how many squirrels there are!! It takes 20% of effort to get rid of 80% greys and 80% effort to get rid of last 20% greys in a woodland. Without consistent control, more will come in within a few months.
Drey-poking
  • F: The effectiveness of drey poking is dependent on time of year, age and species of tree also impact how hard/easy it is to spot and poke dreys. This is obviously not a suitable method to be used in any red squirrel areas. Drey poking is likely to yield higher kill rates if there are a number of shooters and this requires at least 2 individuals, one to shoot and one to disturb dreys. Drey poking can however be undertaken throughout the day so could yield high numbers of squirrels if guns are accurate shots. However, shooting running squirrels can be more challenging than other methods.
  • K: Ditto - no such thing as average shooter. Some people can get 50 in a day, others 1 or 2. Depends how many squirrels there are!! It takes 20% of effort to get rid of 80% greys and 80% effort to get rid of last 20% greys in a woodland. Without consistent control, more will come in within a few months.

E. Pine marten

Pine martens are an important natural predator influencing squirrel populations in the UK. Their presence can have differential effects on red and grey squirrels, potentially benefiting red squirrel populations indirectly - an effect described by Sheehy et al, 2018 and tested in simulations by Slade et al, 2023. In this section, we seek your expert judgment on the role of pine martens in squirrel predation as well as life history traits of pine martens.

Predation

Question E.1

Given an average density of 2 pine martens per km2 on a woodland, for a given individual red or grey squirrel living in such a woodland, what is the average probability of it being preyed by a pine marten over a given month?

Species Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Grey E 0 0.15 0.2 65
F 0 0.10 0.2 80
K 0 0.80 1.0 100
Q 0 0.20 0.3 50
N 1 1.00 1.0 100
Red E 0 0.05 0.2 65
F 0 0.05 0.1 60
K 0 0.20 1.0 100
Q 0 0.20 0.3 50
N 1 1.00 1.0 100

Comments

Red
  • F: Based on a 10% occurrence of grey squirrel in marten diet - considering the lower density of red squirrels I would predict red squirrel consumption would also be lower. However the same caveats should be considered re: density, habitat, time of year, reproductive status of pred and prey.
  • K: Red squirrels and pine martens have evolved together. There is less threat for a red squirrel from a pine marten than there is from a grey squirrel.
Grey

  • F: Predation is subject to multiple variables such as time of year (do squirrels have young in a drey - potentially easier prey/do female martens have young - squirrel may provide good food source for kits), natural food availability, other prey items in habitat, age/experience of marten, weather conditions. Density of squirrels (and habitat) will also play a role and therefore red squirrel predation is likely to be lower than greys. Broadleaf habitat will have higher densities of greys, red squirrel densities are generally lower but particularly in broadleaf woodland. Coniferous woodland will have relatively low densities of greys. These all impact predation rate.

    EVIDENCE SOURCE: Dietary studies suggest grey squirrels (in non-red area) comprised 10% of marten diet based on hard part analysis of scat - McNicol 2019 PhD Thesis. This is not drastically different from Sheehy studies in different areas ~14% FO in marten diet.
  • K: Evidence from N Ireland and other areas that as PM populations expand, grey populations decline with reports of numerous grey remains in feeding sites. PMs are ‘lazy’ and like to take easy prey. - reds are not ‘easy.’
  • Q: Believe Pine Marten population needs to be up to 3 / km sq to imapact grey squirrels.

Life History

Question E.2

Under current conditions, what is the average length, in months, of the pine marten breeding season (i.e. the window during the year when the species engages in reproductive activities, including mating, gestation, and giving birth to offspring)?


Expert Minimum Plausible Most Likely Maximum Plausible Confidence
D 8 9 11 70
E 2 2 3 75
F 6 8 10 90
L 10 11 11 85
O 7 8 10 100
N 1 1 1 100

Comments

Breeding Season
  • F: Due to mating occurring in summer followed by delayed implantation, this period can be quite long subject to weather conditions and resource availability. For example if martens experience good summer conditions and mate early eg June/July, then experience favourable autumn and winter conditions, females may give birth as early as Jan/Feb - as seen in SW England and occasionally in mid-Wales. However if adverse conditions are experienced over autumn/winter (and as has been reported further north) kits may not be born until March/April. This is more common in N Scotland.
  • L: Mating takes place Jun-July but then there is delayed implantation with births not happening until the following April.
  • D: Pine martens have delayed implantation of blastocyst.

Question E.3

How many litters do you expect an average female pine marten to have per year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?


Expert Minimum Plausible Most Likely Maximum Plausible Confidence
D 0 1.0 1 70
E 0 1.0 1 90
F 0 0.5 1 100
K 0 1.0 2 86
L 0 1.0 1 70
Q 0 1.0 1 75
N 1 1.0 1 100
O 1 1.0 1 100

Comments

No of Litters
  • F: Experience from monitoring release pine martens suggests not all females produce a litter each year. No females were recorded to have produced more than one litter in a single year so the max is estimated to be 1 in ‘average’ broadleaf/mixed woodland.
  • L: The most likely value I’ve given of one litter per year, however, females won’t start to breed until their second year and animals that have been introduced to an area are likely to start to breed later.

Question E.4

How many individuals do you expect to survive after weaning from a single litter produced by an average female pine marten, under ideal conditions (i.e., in a woodland where inter- and intra-specific competition is negligible)?


Expert Minimum Plausible Most Likely Maximum Plausible Confidence
D 0 2.0 4 60
E 1 3.0 5 70
F 0 2.0 4 90
K 0 1.5 2 100
L 0 1.0 6 70
O 1 2.0 4 100
N 1 1.0 1 100

Comments

Surviving kits/litter
  • F: From experience monitoring release pine martens, most females produced 3-4 kits. Rarely were all kits found to survive to independence - through camera trap monitoring of den sites where possible and bait stations near den sites/in the females territory. Most females were observed with 1-2 juveniles out of the den site in the year following birth. We never observed a female with 3+ kits outside of the den when juveniles were more mobile.
  • L: A female could have up to six kits, this is highly unlikely, much more likely to be around two-four, generally three. With perhaps 1-2 surviving their first winter.
  • K: Number provided by ‘experts’

Question E.5

What is the average probability that a given pine marten (either a juvenile or adult) will survive a full calendar year, under ideal conditions (i.e. in a woodland where inter- and intra-specific competition is negligible)?

Age Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Adult D 0.0 0.80 1.0 70
E 0.2 0.50 0.8 65
F 0.5 0.85 1.0 85
K 0.0 0.90 1.0 90
L 0.0 1.00 1.0 50
N 1.0 1.00 1.0 100
Juvenile D 0.0 0.60 1.0 55
F 0.0 0.45 1.0 80
L 0.0 1.00 1.0 50
N 1.0 1.00 1.0 100

Comments

Juvenile
  • F: Based on a small number of translocated martens and known breeding sites. Most females had 2-4 kits, with most females successfully raising 1-2 kits to 1 year. However as kits were not tracked, their fate after dispersing was unknown unless they took up a territory being monitored with trail cameras and could be identified through bib pattern. Therefore lower confidence in this estimate.
  • L: I don’t believe this is likely even under optimal conditions. Juvenile survival rate is quite poor for this species.
Adult
  • F: Based on released adult pine martens (radio tracked) in Wales/Forest of Dean. Survival rate was generally high and competition was very low. Around 80-90%.
  • L: This is of course assuming all conditions are absolutely fine, in theory an adult should survive but it is not likely to be the case. As with my answers for both squirrels on this I am a bit dubious about them.
  • K: Not allowing for old age.

Training question

The question below is a training question to familiarise you with the survey interface. Make sure to play around with the values so you get a sense of how your answers shape the outcome.

What is the probability that, when dropping a piece of toast, said toast will land on the floor with the spread side down, given the following spreads:

  1. Butter
  2. Jam

Spread Expert Minimum Plausible Most Likely Maximum Plausible Confidence
Butter B 0.0 0.5 1.0 100
C 0.0 0.5 1.0 100
D 0.1 0.8 0.9 80
E 0.5 0.7 1.0 95
I 0.0 0.0 1.0 51
J 0.0 0.5 1.0 94
M 0.0 0.5 1.0 66
N 0.0 1.0 1.0 100
O 0.0 1.0 1.0 100
Q 0.1 0.3 0.6 100
Jam B 0.0 0.5 1.0 100
E 0.6 0.9 1.0 95

Comments