Abstract
Bird-window collisions are the second leading cause of human-related avian mortality for songbirds in Canada. Our ability to accurately estimate the number of fatalities caused by window collisions is affected by several biases, including the removal of carcasses by scavengers prior to those carcasses being detected during surveys. We investigated the role of scavenger behavior in modifying perceived carcass removal rate while describing habitat-specific differences for the scavengers present in a relatively scavenger-depauperate island ecosystem. We used motion activated cameras to monitor the fate of hatchling chicken carcasses placed at building (under both windows and windowless walls) and forest (open and closed canopy) sites in western Newfoundland, Canada. We recorded the identity of scavengers, timing of events, and frequency of repeat scavenging at sites. Using 2 treatments, we also assessed how scavenging varied with 2 levels of carcass availability (daily versus every third day). Scavenger activities differed substantially between forest and building sites. Only common ravens (Corvus corax) removed carcasses at building sites, with 25 of 26 removals occurring under windows. Burying beetles (Nicrophorus spp.) dominated scavenging at forest sites (14 of 18 removals), completely removing carcasses from sight in under 24 hours. Availability had no effect on removal rate. These findings suggest that ravens look for carcasses near building windows, where bird-window collision fatalities create predictable food sources, but that this learning preceded the study. Such behavior resulted in highly heterogeneous scavenging rates at fine spatial scales indicating the need for careful consideration of carcass and camera placement when monitoring scavenger activity. Our observations of burying beetle activity indicate that future studies investigating bird collision mortality near forested habitats and with infrequent surveys, should consider local invertebrate community composition during survey design. The high incidence of invertebrate scavenging may compensate for the reduced vertebrate scavenger community of insular Newfoundland.
Highlights
Human modified environments can present unfamiliar hazards to local wildlife populations, having potentially detrimental impacts on the species involved [1,2,3]
An equal number of native and chicken carcasses (6 each) were scavenged from forest interior locations, and there was no difference in the proportion of time until first scavenging event between carcass types
Ravens were the only animals identified scavenging at building sites (24 of 24 removals; for 2 scavenging events the individual could not be identified) (Fig 2)
Summary
Human modified environments can present unfamiliar hazards to local wildlife populations, having potentially detrimental impacts on the species involved [1,2,3]. Our ability to collect accurate data on human-related mortality within wildlife populations is constrained by several biases in the survey techniques used. One of the most important biases affecting mortality estimates is the removal of carcasses by scavengers or decomposers before a carcass survey is conducted [7]. Given that the scavenging of carcasses can occur via removal by vertebrate scavengers or through decomposition by invertebrate scavengers and microbes [8], there may be high spatial and temporal variability in scavenging rates depending upon differences in the abundance of the species involved, the extent of competition for carcasses among these species, and seasonal variation in activity levels [9,10]. There are currently are no inclusive estimators available to account for carcass scavenging and other biases when examining human-related mortality events [12]
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