The evolution of using shed snake skin in bird nests

Many species of birds use shed snake skin in nest construction, but this behavior remains poorly understood. Ecological context is likely key for understanding how this unusual, but widespread, behavior evolved. We use comparative and experimental approaches to suggest that the evolution of this behavior is mediated by nest morphology and predator communities. First, we reviewed the literature and found that 78 species from 22 families have been reported to use shed snake skin in nest construction. All but one of these species are passerines and, using comparative analyses, we show that this behavior is disproportionately observed in cavity-nesting species. Second, we examined a subsample of North American species, all of which are reported to use snake skin in nest construction, to see if the proportion of nests with snake skin differs between cavity and open cup-nesting species. This analysis suggested that the proportion of nests with snake skin is roughly 6.5 times higher in cavity than in open cup-nesting species. Finally, we used a series of experiments and comparisons to test four hypotheses whereby snake skin could award fitness benefits (nest predation, nest microbiotas, nest ectoparasites, social signaling) and found support for the predation hypothesis. Snake skin reduced nest predation in cavity, but not open 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 compared to open cup-nesting birds.

Methods
We provide detailed methods, statistical approaches, and results for our three analyses of snake skin use in bird nests: (i) comparative data, (ii) frequency of use of snake skins in nests, and (iii) experimental approaches testing microbiota, ectoparasite, social signaling, and predation hypotheses. See Supplement for full descriptions of methods.