Individuals may improve their fitness by using information provided by other species. Such information may be exchanged in predictable behavioral patterns: (1) Generalists living in large groups should produce information that is broadly useful and conspicuous. (2) Receivers of information should dominate transmitters because a dominant species can exploit a resource revealed by a subordinate, but not vice-versa. (3) Information should be transmitted intentionally only when the sender benefits. We tested for these patterns in an avian scavenger guild in the Pacific Northwest of North America using deer carrion monitored with motion-activated game cameras (N = 90 different bait stations), and artificial carcasses (a tanned deer hide plus antlers) populated by raven (Corvus corax) decoys (N =15 locations with simultaneous experimental and control treatments, monitored visually). In the guild, corvids (ravens and magpies, Pica hudsonia) are social, conspicuous generalist foragers, whereas raptors (hawks and eagles) are solitary and competitively dominant. At the camera traps, roughly half of the deer carcasses were fully intact, whereas others were experimentally pre-opened to allow corvids access to flesh. This was done to test whether corvids benefit from the arrival of hide-cutting raptors, and therefore may be incentivized to recruit them.
Results/Conclusions
As predicted, raptors used corvids to locate carrion more often than vice-versa. Corvids appeared first at 81% of camera traps, whereas raptors appeared first at only 7%. Appearance times at camera traps were highly correlated between raptors and corvids in cases where they were the first two taxa to appear (r = 0.94, P<0.001). Raptors approached 6 of 15 carcasses populated by raven decoys, and 0 of 15 simultaneous controls (P < 0.05), which experimentally confirmed causation in the arrival correlations at camera traps. Corvids overwhelmingly changed from feeding at the eyes and anus to feeding on flesh after raptors cut open a carcass. However, we did not find evidence that corvids intentionally recruit raptors to open carcasses for them: the time from corvid to raptor arrival did not differ between intact and pre-opened carcasses. A scavenging mutualism built on carrion discovery and access may stabilize this food web and facilitate the overwinter survival of charismatic Holarctic raptors.