Heterospecific information supports a foraging mutualism between corvids and raptors

Abstract

Although animal social information is most commonly utilized by conspecifics, some conditions may favour heterospecific transmission. Generalists living in social groups are thought to produce information that is conspicuous and broadly useful to other species. Information may be signalled intentionally to other species when the sender benefits. We examined these predictions in a scavenger guild comprising two generalist social corvids (ravens, Corvus corax; magpies, Pica hudsonia), four raptor species and coyotes, Canis latrans. We used game cameras to monitor scavenging at 89 mule deer, Odocoileus hemionus, carcasses in Oregon and Washington, U.S.A. Corvid appearance times explained variation in golden eagle, Aquila chrysaetos, bald eagle, Haliaeetus leucocephalus, hawk (Buteo jamaicensis and Buteo lagopus) and coyote arrivals, indicating that raptors and canids use corvids to locate food. Larger numbers of magpies, but not ravens, were associated with shorter raptor appearance times, providing partial support for the importance of group size in broadcasting information to heterospecifics. We also directly observed simulated carcasses composed of a deer hide and antler. Seventy one corvids and 13 raptors appeared among 16 treatments accompanied by raven decoys and playbacks, whereas only one corvid and no raptors appeared at paired controls. The time that ravens first flew in circles above a simulated carcass explained 98% of the variation in subsequent raptor arrival time, indicating that a specific raven behaviour provides a cue of carrion. After hide-cutting raptors opened a carcass, corvids benefited by switching from feeding at the eyes and the anus to feeding on flesh. However, raptors did not arrive sooner after corvids at closed carcasses than experimentally pre-opened carcasses, suggesting that corvids do not intentionally transmit more information to raptors when a carcass requires opening, at least under these conditions. A scavenging mutualism with corvids built on carrion discovery and access, but not intentional recruitment, may facilitate winter survival of raptors.