PS 36-150
Defender or sentinel? Evaluating mechanisms of predator protection in corvid nesting associations
The predator protection hypothesis posits that less aggressive or smaller species form nesting associations with more aggressive or larger species for nest protection. In Jackson Hole, WY, smaller black-billed magpies (Pica hudsonia) are nesting in association with larger American crows (Corvus brachyrhynchos). There are two non-exclusive mechanisms under which predator protection can operate; magpies may be nesting inside the defended area around crow nests (the protective umbrella) and benefiting directly from crow defensive behavior, or magpies may be gathering information about predation from nearby crows and increasing their own defense behaviors accordingly, called information parasitism. We studied the relative importance of these mechanisms to nest defense and nesting success in magpies, testing whether either or both influence nest defense and success. We determined the mean size of the protective umbrella around crow nests, and performed predator intrusion tests at magpie nests to evaluate crow defensive responses. We also performed playback experiments at magpie nests using crow predator calls to quantify the importance of information parasitism.
Results/Conclusions
During predator intrusion tests, we found that magpies recruit more crows to defend their nests when nesting nearer to crows, and that crows also show increased defensive behaviors at magpie nests closer to their own nests. Additionally, magpies respond to crow defensive signals during playback experiments, regardless of the distance to a crow nest. Thus, direct protection and information parasitism may be important to nest protection in nesting associations between magpies and crows. However, there was no relationship between hetero-specific nest distance or presence inside the crow protective umbrella and nesting success for magpies, indicating that the ultimate benefits may be environmentally mediated or counteracted by competition.