Citizen Science Reveals How Brown-headed Cowbird Host use is Linked with Individual Host Abundance

Robert Ernst1,

1 Department of Biological Science, Georgia Institute of Technology

Introduction

Molothrus ater (Brown-headed Cowbirds) are avian nest parasites. Females lay their eggs in a victim’s nest; the host then serves as the “foster parent” and raise the cowbird chicks as their own. This project used eBird citizen science observations to extract a map of hosts raising cowbird chicks across the Brown-headed cowbird range.

Hypotheses

Female egg laying is hypothesized to occur by “Random selection”, where females indiscriminately lay eggs at their chosen breeding site, and the rate at which a species is used as a host is related to its abundance. We tested this by comparing the proportion of Song Sparrows raising cowbird chicks to the relative abundance of cowbirds in eight different regions.

H0: The proportion of song sparrows parasitized has a strong positive correlation with song sparrow abundance.

H1:The proportion of song sparrows parasitized has little to no correlation with song sparrow abundance.

Methods

  1. Investigated free text “Species Comments” field in eBird observations of Cowbirds.
  2. Filtered keywords “fed”, “tend”, “feed”, “raise”, “fledg”, “juv”, “beg”, “food”, and ”follow”.
  3. Manually read over 60,000 observations
  4. Final data set contains ~16,500 observation of Brown-headed cowbird hosts

Figure 1. Map of North America displaying the regions containing observations of hosts used by Brown-headed Cowbirds. Dark purple indicates many observations, while yellow indicates less observations. Regions that contain more observations are generally centered around areas that have high amounts of eBird users.

Results

Figure 2. Displays the relationship between song sparrow relative abundance in region compared to the proportion of host observations that were song sparrows in that region.

The linear regression had an adjusted R-squared value of 0.222, and a p-value of 0.134 that was not statistically significant (p > 0.05). There may be a slight positive correlation between song sparrow abundance, but this relationship is weak and not statistically significant. This provides support for the alternative hypothesis that song sparrow parasitism is high regardless of song sparrow abundance in the environment. Female Brown-headed cowbirds may have a strong preference for song sparrows, and may choose to seek them out as hosts, whether or not they are common at that female’s breeding ground.

Next Steps

  1. We want to use a similar approach to collect host data for the less studied cowbird species of M. aeneus (Bronzed Cowbird) and M. bonariensis (Shiny Cowbird)
  2. We are interested in using this abundance comparison approach for more hosts other than song sparrows with chipping sparrows, northern cardinals, and common yellow-throats being of most interest.
  3. We would like to create plots showing how the families of hosts observed raising cowbirds changes across geographic space.

References

Sullivan, B.L., C.L. Wood, M.J. Iliff, R.E. Bonney, D. Fink, and S. Kelling. 2009. eBird: a citizen-based bird observation network in the biological sciences. Biological Conservation 142: 2282-2292.