Testis size increases with colony size in cliff swallows

By using a sample of over 800 male cliff swallows (Petrochelidon pyrrhonota) that died during a rare climatic event in our Nebraska study area in 1996, we investigated how testis size was related to body size, age, parasite load, a bird's past colony-size history, and spleen size. Testis volume increased with body size. After correcting for body size, testis volume was lowest for birds age 1 and 2 years but did not vary with age for males 3 years old or more. Birds occupying parasite-free (fumigated) colonies had significantly larger testes than did birds at nonfumigated sites. Testis volume increased significantly with the size of the breeding colonies a bird had used in the past. Testis volume had no relationship with spleen volume after correcting for body size. The results show within a species that larger testes are favored in more social environments, probably reflecting a response to increased rates of extrapair copulation (and thus sperm competition) among cliff swallows in large colonies. The presence of ectoparasites, by inflating levels of plasma corticosterone, may in turn reduce testis mass. These data provide no support for the hypothesis that large testes, perhaps by producing more testosterone, are immunosuppressive and thus costly for that reason.