Egg hatchability increases with colony size in Cliff Swallows

The percentage of otherwise successful nests containing greater than or equ al to 1 unhatched egg in Cliff Swallows (Petrochelidon pyrrhonota) varied i nversely with colony size in southwestern Nebraska. Colony-site characteris tics other than colony size had no significant effect on egg hatchability: The incidence of unhatched eggs at a colon), site did not var) significantl y with year, mean date of first egg laying, mean incubation period, extent of ectoparasitism, or mean body mass of nestlings or adults. This suggests that increased hatching failure ill smaller colonies was unrelated to ectop arasitism by fleas and cimicid bugs, foraging success, or egg inattendance. Furthermore, because reproductive interference and incidence of brood para sitism do not increase in smaller colonies, these variables seem unlikely t o account for the relationship between egg hatchability and colony size. Mo re frequent matings between genetically similar individuals and reduced opp ortunities for females to seek extrapair fertilizations with outbred males or as fertility insurance might explain reduced egg hatchability in small c olonies. Increased hatching success in larger colonies may he a benefit of coloniality; for Cliff Swallows.