Scavenging affects persistence of avian carcasses resulting from window collisions in an urban landscape

Abstract

Collisions with windows remain an important human‐related threat to bird survival in urban landscapes. Accurately estimating the magnitude of avian mortality at windows is difficult and may be influenced by many sources of error, such as scavenging of carcasses. Failure to account for removal of carcasses by scavengers can bias estimates of window mortality. We tested the hypothesis that carcass survival depends on local habitat factors known to influence scavenger behavior. Scavenger activity on bird carcasses was documented at 20 buildings in an urban landscape in northwestern Illinois for 1 week during each season of a year. Known‐fate models were used to relate carcass survival to local habitat composition and to evaluate temporal variation in survival. We also documented species of scavengers and the timing of scavenging using motion‐triggered cameras. Daily carcass survival was greater in winter than during spring, summer, and fall. Survival was related negatively to canopy cover (trees and shrubs within a 50‐m buffer) and window area, and positively to pavement cover. Using an exponential model of survival time, estimated mean time of survival of carcasses (t± SE) was 82.9 ± 11.7 d for winter and 11.8 ± 7.2 d for other seasons. Raccoons (Procyon lotor) scavenged more carcasses than other species. Our results suggest that (1) carcass survival times may be short at locations with preferred habitats of known scavengers and predictable sources of food, and (2) knowledge of scavenger distribution and activity can inform predictive models of persistence. In studies of bird‐window collisions, the influence of scavenger bias can be minimized by maintaining short time intervals between carcass searches. Search intervals can be inferred by estimating the number of days that a carcass should persist at a site, which can be calculated using predicted daily survival probabilities of carcasses at study buildings.

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“…Thus, researchers may wish to complete carcass surveys at intervals of three days. However, scavenger communities vary regionally, which may introduce variation in the persistence times of carcasses (Hager et al, 2012). Given this, researchers may wish to customize the survey interval for their site in one of two ways.…”

“…First, they could assume that carcass persistence is less than 3 days and simply opt to conduct daily consecutive surveys. Daily consecutive surveys would reduce the effects of imperfect detection resulting from scavenger removal of carcasses (Hager et al, 2012). It should be noted that if some event, e.g., severe weather, a field worker who is ill, etc., prevents fieldwork for a day during the official survey period, then surveys should be resumed the following day (see "6.…”

“…Data on carcass persistence in the upper Midwest suggests that most scavenging occurs at night (Hager et al, 2012). Given these patterns, carcass surveys should be done in the mid-to late-afternoon hours to have the best chances of finding carcasses if they are present.…”

“…Moreover, supplies and equipment are inexpensive. Applicability of the protocol stems from recent research assessing bird carcass detection next to buildings in urban landscapes (Hager et al, 2012(Hager et al, , 2013. Thus, it reduces bias associated with imperfect detection of carcasses due to the effects of scavengers and field worker abilities.…”

“…Previous research in the upper Midwestern United States suggests that bird carcasses remain undisturbed by scavengers for a minimum of three days (Hager et al, 2012). Thus, researchers may wish to complete carcass surveys at intervals of three days.…”