The spatial and temporal exploitation of anthropogenic food sources by common ravens (Corvus corax) in the Alps

Abstract

Background Anthropogenic food sources (AFSs) are widespread in human-transformed landscapes and the current scale at which they occur drives ecological change at the individual, population, and community levels. AFSs are exploited extensively by common ravens, Corvus corax. Understanding how raven populations use AFSs can provide insight into their ecological responses to AFSs. Methods We equipped 81 ravens in the Austrian Alps with GPS-transmitters over a period of 2.75 years. Using these tracking data, we investigated how cohort differences (i.e., age, sex, and origin) and seasonal changes influence raven movement patterns (i.e., occurrence distribution and maximum daily displacement) and AFS-use (i.e., number of AFSs visited and probability of being present at any AFS) at 45 extensively exploited sites. Results We found that proxies for experience and dominance, inferred by age (i.e., juvenile versus adult) and origin (i.e., wild-caught versus captive-bred-released) cohorts, influenced movement patterns and the number of AFSs visited. However, all individuals were equally likely to be present at AFSs, highlighting the importance of AFSs for non-breeders in the study population. Seasonal changes in environmental conditions that affect energetic demands, the availability of natural and anthropogenic food, and foraging competition, influenced individuals’ occurrence distributions and AFS-use. We found that under harsher conditions in autumn and winter, individuals ranged wider and depended on AFSs to a larger degree. However, contrary to expectation, they were less likely to be present at AFSs in these seasons compared to spring and summer, suggesting a trade-off between time spent moving and exploiting resources. We attribute the small ranging movements exhibited by non-breeders in spring and summer to the presence of highly territorial and socially dominant breeders. As breeders mostly stay and forage within their territories during these seasons, competition at AFSs decrease, thereby increasing the likelihood of individuals being present at any AFS. Conclusions We emphasize that movement and AFS-use differ according to cohort differences and the seasonality of the environment. Our results highlight that predictable AFSs affect foraging strategies among non-breeding ravens. The extent of AFS-exploitation among non-breeding ravens in our study emphasize the potential of AFSs in shaping raven movement and resource-use.

Warum sind Raben klug? Anwendung der sozialen Intelligenz Hypothese

“…Note that the food at those sites is predominantly of anthropogenic origin like the feeding of animals at wild/game parks and farms or the ‘leftovers’ at garbage dumps, compost sites, skiing huts etc. (Jain et al 2022 ). Our study site at the Cumberland Wildpark thus reflects a typical foraging location for ravens in Middle Europe.…”

“…1 b, c). Part of the variation can be explained by ecological factors like differences in food availability across seasons (Jain et al 2022 ). However, a large part seems to be due to individual preferences: some ravens consistently exploit one or few sources, staying at given sites over years; others exploit a variety of sources, frequently changing between sites and thereby covering a large home range area (Loretto et al 2016 , 2017 ).…”

Scavengers use natural and anthropogenic resources connecting protected areas with surrounding lands

“…They also take advantage of ephemeral anthropogenic resources such as roadkill and offal left by hunters (Heinrich, 1988;Wilmers et al, 2003a). Throughout the year, but especially in winter, ravens are known to feed on large and permanent anthropogenic subsidies frequently, such as landfills, waste treatment centers, and discards in agricultural and urban areas (Webb et al, 2011;Loretto et al, 2017;Harju et al, 2018;Jain et al, 2022). These anthropogenic resources typically lead to high densities of ravens in the surrounding lands and an increasing population overall (Marzluff et al, 1994).…”