Common ravens (Corvus corax) following gray wolves (Canis lupus) as a foraging strategy in Yellowstone National Park, WY

Daniel R. Stahler, Bernd Heinrich, Department of Biology, University of Vermont, Burlington, VT 05405, USA; Douglas W. Smith, Kerry M. Murphy, Yellowstone Center for Resources, PO Box 621, Yellowstone National Park, WY 82190, USA

Few studies have reported on the impact that wolves have on the scavenging complex of an ecosystem or the strategies that scavengers employ in discovering and utilizing a food source such as a wolf-killed carcass. Researchers have identified the raven as one of the most important scavengers present at wolf-killed carcasses and have commented on the unique relationship that exists between these two species. Although this relationship is well recognized in the realm of wolf biology, natural history, and mythology, no study has collected quantitative behavioral data on this association. This study was designed to test the hypothesis that ravens follow wolves as a foraging strategy. The presence, absence, and behavior of ravens was documented during visual observations of wolves in Yellowstone National Park (YNP) to determine the extent to which ravens associate with these predators during various activities not associated with an available carcass (traveling, resting, chasing prey, etc.). In the majority of wolf observations, ravens were present throughout their activity bouts, suggesting that ravens have learned to follow wolves despite the absence of a food source, with the foresight that food will be made available by the predator in the future.

In addition, discovery success by ravens of wolf-killed carcasses and experimentally placed carcasses, as well as subsequent behavior upon discovery, was compared to support the hypothesis that ravens benefit by associating with wolves versus other means of locating and utilizing a food source. In YNP, ravens discovered 100% of the observed winter wolf predation sequences on elk (Cervus elaphus). In the majority of these observed wolf kills, ravens were present during the chase sequence and remained at the carcass to feed alongside wolves after the death of the prey. In the remaining kills, ravens were present within four minutes after the time of death, and subsequently remained to feed. In contrast, ravens discovered only 36% of the experimentally placed carcasses in the same study region, and did not land or feed despite the availability of unattended exposed meat. It has been shown that ravens are initially fearful of large carcasses when first encountered, gradually losing their caution through experience gained either individually or by observing both conspecific and/or non-conspecific carcass feeders. We show that ravens preferentially associate with wolves, suggesting that in addition to making food available through their kills, the presence of wolves allow ravens to overcome their innate fear of carcasses through socially facilitated learning.