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Variation in bird-window collision mortality and scavenging rates within an urban landscape
Citation:
Bracey, A., M. Etterson, G. Niemi, AND R. Green. Variation in bird-window collision mortality and scavenging rates within an urban landscape. The Wilson Journal of Ornithology. Wilson Ornithological Society, Ann Arbor, MI, 128(2):355-367, (2016).
Impact/Purpose:
This manuscript describes the research performed by Annie Bracey for her Master’s degree at the University of Minnesota Duluth. The objective of her research was to estimate avian mortality associated with window kills at residences along Park Point, Duluth, MN, USA during migratory periods. In the manuscript she applies recent estimators, developed by M. Etterson at MED, to the problem of extrapolating from the numbers of carcasses counted to an estimate of the numbers of birds actually killed, using information about carcass scavenging rates and observer detection probabilities. The primary audience for this manuscript will be the ornithological community and wildlife managers.
Description:
Annual avian mortality from collisions with windows and buildings is estimated to range from a million to a billion birds in the United States alone. However, estimates of mortality based on carcass counts suffer from bias due to imperfect detection and carcass scavenging. We studied window collision mortality at 42 homes along a major migratory corridor on the shores of Lake Superior in Duluth, MN from 2006 - 2009 to quantify fatalities during migration. We also conducted carcass distribution trials to estimate scavenging and detection rates and analyzed the resulting data as a hidden Markov model. We used hierarchical models of scavenging probability to compare the relationship between scavenging rates and six measured covariates using Akaike’s Information Criterion (AIC). The adjusted number of birds killed (Nk) on a daily basis each season was estimated using the Horvitz-Thompson estimator. A total of 40 species and 108 individual birds were recorded as window kills. Fatalities increased with distance from the city center and were higher at houses on the lake side of the study site and on windows facing Lake Superior. The average daily scavenging rate over a 6 day exposure period ( ) was ~ 0.229 ± 0.020, giving an expected time to scavenging of 4.37 days. Model selection results suggested the best explanatory variable for the probability of scavenging was carcass size, followed by house location, together accounting for 87% of all model weights. The probability of discovery of a window kill ~ pf = 0.16. From this, we estimated daily mortality at 520 houses on Minnesota Point during migration to range from 13.7 to 48.6.
