Reproductive success and survival of the Common Grackle in central Illinois
Horsley, Noah Prescott
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https://hdl.handle.net/2142/107974
Description
Title
Reproductive success and survival of the Common Grackle in central Illinois
Author(s)
Horsley, Noah Prescott
Issue Date
2020-05-05
Director of Research (if dissertation) or Advisor (if thesis)
Ward, Michael P
Department of Study
Natural Res & Env Sci
Discipline
Natural Res & Env Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
M.S.
Degree Level
Thesis
Keyword(s)
Common Grackle
population decline
reproductive success
Abstract
The Common Grackle (Quiscalus quiscula) has historically been one of the most abundant bird species in North America. However, based on North American Breeding Bird Survey data, the global population has exhibited a steady decline of -1.75% per year over the past 50 years and has suffered a cumulative loss of 58%. Understanding why the Common Grackle - a habitat and dietary generalist - is declining is vitally important for assessing the risk to other farmland birds and the health of the environment. I collected demographic data from Common Grackle colonies at two rural study sites in Champaign County, Illinois in 2018 and 2019. I monitored 188 nests and tracked the survival of 53 fledglings for the 20-day post-fledging period. I found no evidence that demographic rates during the breeding season were driving the observed decline in Common Grackle populations in Illinois. Average clutch size was 4.26 (SE = 0.07), slightly below the average of 4.35 (N = 14, p = 0.09) for published studies. My estimate of nest success was 62.2% (SE = 3.7%), far above the average of 26.7% (N = 6, p = 0.01) for other studies. Post-fledging survival for Common Grackles had not been estimated before, but the estimate of 61.7% (SE = 7.5%) was comparable to post-fledging survival in other songbirds (range 23-87%). Collectively, these results imply that population decline may be due to low juvenile or adult survival during the non-breeding season. Using population models, I was able to estimate adult survival for a stable population, as well as investigate the relative influence of fecundity and adult survival on population growth. I found that annual adult survival must be at least 65% for a stable population, and that changes to adult survival impact population growth ~2 times more than changes to fecundity. Future research on the decline of this common species should be directed towards quantifying adult survival in the non-breeding season, particularly on the wintering grounds, and investigating potential sources of mortality.
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