| Summary: | <p>One goal of community ecology is to examine proximate and ultimate factors driving interactions between species. Part of this work addresses breeding bird aggregations, termed nesting associations. I evaluated costs and benefits of nesting along an association gradient in smaller black-billed magpies (Pica hudsonia) and larger American crows (Corvus brachyrhynchos) over two nesting seasons in Jackson, Wyoming. Specifically, I explored mechanisms of proximate protective benefits gained by magpies nesting nearer crows, along with foraging costs of associating with crow competitors. I also evaluated ultimate influences of association on nesting success for crows and magpies. Protection can operate under two non-exclusive proximate mechanisms, termed predator exclusion and information transfer. I found both mechanisms influenced nest protection for magpies. Crows performed more defensive behaviors at magpie nests nearer their own nests, and magpies responded to crow defensive signals. However, I did not find fitness benefits for magpies related to nesting nearer or farther from crows. I also explored the competitive foraging costs of heterospecific nesting associations by quantifying resource discovery behaviors and providing novel resource extraction problems to crows and magpies. Crows and magpies behaviors differed related to resource extraction problems such that magpies were less neophobic than crows. However, magpies paid a cost, measured as higher food losses to kleptoparasitism, for nesting nearer to crows. Further, crows came to dominate shared resources initially discovered by magpies. These results highlight potential foraging benefits to crows for associating with magpies. Despite these proximate benefits, crows also did not have greater nesting success when nesting nearer to magpies. Crows also experienced proximate costs to associating, measured as increased defensive behaviors and longer latencies to complete more complex resource extraction problems when nearer to magpies. I found that crow nesting success was related to success of associated conspecifics, but was not related to the number of defenders participating in nest defense. In my study system of crows and magpies, both species experienced benefits and costs to nesting nearer a heterospecific associate. However, costs and benefits may balance each other out such that there is no observable difference in fitness for either species based on association.
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