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Seasonal dimorphism in the horny bills of sparrows
Citation:
Greenberg, R., M. Etterson, AND R. Danner. Seasonal dimorphism in the horny bills of sparrows. Ecology and Evolution. Wiley-Blackwell Publishing, Hoboken, NJ, 3(2):389-398, (2013).
Impact/Purpose:
This manuscript describes seasonal patterns of bill growth and wear in two species of saltmarsh sparrows. The primary objective of the manuscript was to develop a periodic model of bill growth and wear and to show how such patterns differ among sexes. The resulting model is used to evaluate potential hypotheses for the evolution of large bill size in saltmarsh sparrows. The primary audience for this manuscript is the academic community and in particular evolutionary ecologists and ornithologists.
Description:
Bill shape and size are often viewed as species-specific adaptations for feeding, but they sometimes vary between sexes, suggesting that sexual selection or inter-sexual competition may also be important. Hypotheses to explain sexual bill size dimorphism in birds avian bill size include intraspecific divergence in feeding niche or thermoregulatory demands, intrasexual selection based on increased competition among males, or female preference. Birds also show seasonal changes in bill size due to shifts in the balance between growth rate and wear, which may be due to patterns in diet, nutrition, or endogenous rhythms in growth. Insight into the function of sexual dimorphism can be gained by using the novel approach of digital x-ray imaging to examine the degree to which the skeletal core or the rhamphotheca (keratinized sheath) contribute to overall dimorphism. The rhamphotheca is ever-growing and ever-wearing, varying in size throughout life; whereas the skeletal core shows determinant growth. Tidal marsh sparrows are more dimorphic in bill size than related taxa. We investigated dimorphism and seasonality in the size of the overall bill, the skeletal core, and the rhamphotheca in two tidal marsh taxa using both external measurement and radiographs of museum study skins. Overall bill size varied by sex and season, with males having larger bills than females, and bill size increasing from non-breeding to breeding season disproportionately more in males. Skeletal bill size varied seasonally increasing in size from the non-breeding to the breeding periods. The rhamphotheca varied primarily with sex; males had a larger rhamphotheca (corrected for skeletal bill size) which showed a greater seasonal increase than females. The size of the rhamphotheca may be a condition-based character that is shaped by sexual selection, vis-à-vis male –male interactions or female preference. These results add to the growing body of evidence that bill size is influenced
