The ecological relationships between scavengers, predators and their prey can substantially influence their behaviour and population dynamics. Still, the impact of scavenging, i.e. the consumption of carrion, has been largely underestimated in ecosystems. In this project we studied one of the Northern hemisphere’s most widespread predator-scavenger systems: wolves (Canis lupus) and ravens (Corvus corax), which likely have a co-evolutionary relationship.
Our research has been carried out in Yellowstone National Park, where wolves have been reintroduced in a rewilding project 25 years ago. Since then, these top-predators have been intensively monitored with numerous studies on behaviour and ecology of this top-predator but also on its effects on other species in the ecosystem. Previous studies on ravens, the most abundant scavengers in this area, were based on the observations and counts of unmarked individuals at carcasses and anthropogenic food sources revealing processes on the population but not on the individual level. However, both, population level effects as well as individual strategies, are important to fully understand the role of each species in an ecosystem. Particularly now, where human pressure threatens all ecosystems around the world, it is highly important to better understand such relationships for nature protection and restoration.
The main aim of our study was to identify which foraging and scrounging strategies ravens follow to locate and exploit wolf kills as well as other food sources. Different hypotheses such as opportunistic foraging, a scout strategy (ravens continuously follow predators), a turn-taking strategy (ravens switch between scout and opportunistic strategies) and effects of experience (mostly by age) have been tested. For this we GPS-tagged more than 70 ravens within or close to Yellowstone National Park since October 2019 and compared their GPS-tracks with the movements of GPS-collared wolf packs but also as comparison with GPS-collared pumas and the respective kill sites of both species. Further, we started an extensive resource mapping to include other food sources such as winter-killed animals and in particular anthropogenic food sources.
Surprisingly, the most important resources were of anthropogenic origin such as landfills, sewage ponds, hunting areas that are often more than 100km distant to the National Park, indicating that these resources also have a strong influence on the protected ecosystem. Analysing the exploitation of natural carcasses indicates age effects with adults using this food more often than juveniles, but we could not confirm any of the other hypotheses. Even more surprising, ravens frequently approached fresh carcasses from distances far beyond their perception range which challenge current hypotheses for the transfer of information about the location of food sources. We suspect that ravens do not only learn and remember the location of predictable anthropogenic food sources, but also the areas where they can frequently find carcasses such as wolf territories.
