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Vanya G. Rohwer, Jennifer L. Houtz, Maren N. Vitousek, Robyn L. Bailey, and Eliot T. Miller: Read the article
Why do some birds incorporate shed snake skin in their nest? Rohwer et al. suggest that the evolution of this unique, but wide spread behavior is dependent upon nest morphology and most commonly observed in cavity nesting species...
While all nests are made to lay and incubate eggs, and shelter fledglings, not all nests are built the same. For a bird, building a nest requires carefully considering factors that will maximize its breeding success. Materials used for nest construction are no exception.
The astonishing diversity in the types of nests found in nature indicates that nests serve more functions than meets the observer’s eye. For example, the morphology of a nest, i.e., shape, size, substrate, and where it is placed in a particular habitat, can influence the interactions of a breeding pair with their surrounding community. These interactions include predation, competition, and food availability (for e.g., insect prey diversity), which can affect nesting behavior, and in turn, breeding success. Therefore, the choice of nest-building material is likely to have evolutionary implications. A striking example comes from shed snake skins, an unusual material found in the nests of several birds. Scientists and naturalists have long wondered whether there are unique benefits to using these peculiar materials. In this study, Rohwer et al. examined the evolutionary advantages of using shed snake skin as a nest-building material. They showed that its benefits vary with nest morphology and predator communities.
While snake skin has been frequently observed to be used in nests, published literature suggests that several long-held ideas on their apparent benefits remain untested. Possible benefits include protection from predators, enhanced defense against various types of bacteria, signaling individual quality, and protection from ectoparasites. At the same time, snake skins have also been suggested to vary with the type of nests, i.e., cavity nests versus other kinds of nests (e.g., open-cup nests). Together, these two ideas suggest that the predominant use of snake skin in certain types of nests (i.e., cavity nests) may have an evolutionary advantage specific to the ecological context of those nests.
The authors used comparative data and controlled experiments to test the role of shed snake skin between cavity vs. non-cavity nesting species and whether they provide any of the benefits listed above. To do this, they added snake skin to cavity and open-cup nest boxes. They recorded changes in predation, nest bacterial communities, parasites, and increased parental care as a proxy for signaling individual quality. Their analyses show that snake skins are indeed used disproportionately by cavity-nesting species relative to open-cup nesters. Further, experimental evidence indicated that adding snake skin reduced predation in cavity nests but did not have any effect on predation in open-cup nests. Interestingly, cavity-nesting birds generally have smaller-bodied predators that are themselves often preyed on by snakes. In contrast, open-cup nests are visited by a larger pool of predators. This shows that using snake skin could be a strategy that has evolved to deter smaller predators likely to access cavity nests. In contrast, predators that visit open-cup nests are less likely to respond fearfully to cues such as snake skin.
Shed snake skin is used as nest construction material by 78 bird species. Of these, 77 are passerines, also known as perching birds. The authors attribute this striking pattern to their relatively higher intelligence, which has likely driven the diversification of nest types, along with an array of interesting nesting behaviors. This seems particularly true for small-bodied birds that may not be able to physically fend off predators and, therefore, have come to rely on ingenuous strategies such as incorporating snake skin to deter predators.
This study shows that behavioral choices, such as selecting unusual nest-building materials, are advantageous for specific nest morphologies and associated predator communities. More broadly, it highlights the importance of ecological context in assessing the fitness boost conferred by a behavior.
Megha Srigyan is a 4th-year PhD student in the Paleogenomics Lab at UC Santa Cruz. She studies how ancient and historical DNA can be used to inform species taxonomy and conservation. Currently, she is studying tricolored blackbirds in California and burrowing owls throughout North and South America. Outside of work, she enjoys traveling, reading, and taking walks in nature.
