From pattern to process: the evolution of using shed snake skin in bird nests

Ecological context is central for understanding the evolution of traits. We explore a unique and poorly studied avian behavior: the use of shed snake skins in nest construction and suggest that the evolution of this behavior is context dependent upon, at least, nest morphology and predator communities. Of the 78 species from 22 families reported to use shed snake skin, all but one are passerines, and this behavior is disproportionately observed in cavity-nesting species. Next, we tested three hypotheses whereby snake skin could award fitness benefits (nest predation, nest microbiotas, and nest ectoparasites) and found support for the predation hypothesis. Snake skin reduced nest predation in cavity, but not cup, nests. These unequal fitness benefits highlight different ecological conditions between nest morphologies and likely explains why, across species, cavity-nesting birds show this behavior more frequently and use snake skin more consistently in nest construction compared to cup-nesting birds.