ABSTRACT
At present, variation in sperm size in birds seems likely to be a product of sexual selection. However, few studies of sperm size evolution in birds have been undertaken, and it is possible that other factors may explain some of the variation in sperm morphology. For example, sperm size in butterflies increases with body size across species (Gage 1994) and it is possible that some variation in sperm length in birds could be explained by a similar allometric effect. No significant relation between sperm length and body size was found among waders (Johnson and Briskie 1999), but sperm length is negatively related to body size among passerines (Fig. 9.12; Briskie and Montgomerie 1992). Both of these analyses were based on small sample sizes so further work is needed before the influence of allometry on sperm length can be determined.
