Evol Ecol Res 13: 35-44 (2011)     Full PDF if your library subscribes.

The evolution of sexual bill-size dimorphism in shorebirds: a morphometric test of the resource partitioning hypothesis

Silke Nebel^1,2 and Graham J. Thompson¹

¹Department of Biology and  ²Advanced Facility for Avian Research, University of Western Ontario, London, Ontario, Canada

Correspondence: S. Nebel, Department of Biology, University of Western Ontario, London, Ontario N6A 5B7, Canada.
e-mail: snebel2@uwo.ca

ABSTRACT

Question: Is sexual dimorphism in shorebirds an adaptation to reduce resource competition between males and females?

Hypothesis: If selection for resource partitioning between the sexes has contributed to dimorphism, then the degree of sexual size dimorphism (SSD) in resource-exploiting characters – such as those related to feeding – should exceed that expected from sexual selection alone.

Data: Morphometric data from 151 species of shorebirds (Charadriiformes).

Methods: We compared the degree and direction of SSD between a resource-exploiting trait (bill length) and a non-resource-exploiting trait (body size). If bill-SSD exceeds body size-SSD, then resource partitioning may contribute to morphological differentiation of the sexes in shorebirds. We also tested whether SSD is more pronounced in migrant than non-migrant species.

Conclusions: From species level data it appears that bill dimorphism is more pronounced than body size dimorphism, and is more common in migrant than non-migrant shorebirds. This observation from extant taxa suggests a role for resource partitioning in promoting SSD. The phylogenetic effect on both of these correlations is significant, however, to the extent that shared ancestry, rather than contemporary selection, is sufficient to explain why resource-exploiting traits and migrants are more sexually dimorphic.

Keywords: bird migration, resource partitioning hypothesis, sexual bill-size dimorphism, sexual selection, shorebirds.

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