Sexual size dimorphism in birds

Birds provide excellent model organisms for testing functional explanations of sexual size dimorphism (SSD), since many species are exceptionally wellstudied in nature. This chapter reviews four major functional hypotheses of SSD, and tests these using data on five morphometric traits from over 2,500 bird species. This comprehensive analysis reveals that SSD is male-biased in most avian species and families, and that allometry consistent with Rensch's rule occurs in significantly more avian families than expected by chance. Using cross-species analyses, the chapter shows that the pattern of SSD is most consistent with patterns of sexual selection, specifically with the Mating competition and the Display agility hypotheses. Sexual selection, however, is unlikely to explain all variation in SSD, and further work is essential on ecological use of resources and fecundity selection.