Russell Greenberg, Matthew Etterson and Raymond M. Danner Seasonal dimorphism in the horny bills of sparrows Ecology and Evolution 3
We demonstrate that sexual dimorphism in the bill of two sparrows is restricted to the horny covering (rhamphotheca). Both bill and rhamphotheca are largest in the breeding season and the seasonal increase is found primarily in males. These results highlight the under appreciated role of a dynamic rhamphotheca driving patterns of bill morphology and provide a mechanism for the bill, a structure thought to be shaped by trophic ecology, to be the focus of condition-based sexual selection.
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