THE HIGH-FREQUENCY OF EXTRA-PAIR PATERNITY IN TREE SWALLOWS IS NOT ANARTIFACT OF NESTBOXES

A common criticism of nestbox studies is one of creating artificial ne sting conditions and breeding behavior different from what would be se en under natural conditions, We assessed the frequency of extrapair pa ternity (percentage of broods with at least one extra-pair young) in 2 5 families of tree swallows (Tachycineta bicolor) nesting in natural c avities and compared it to that in a nestbox population. We found that 84% of females nesting in natural cavities obtained fertilizations fr om extra-pair males. These extra-pair males fathered 69% of all nestli ngs. Studies of tree swallows breeding in nestboxes have shown that 50 -87% of broads contained extra-pair young, with extra-pair males fathe ring 38-53% of all the young. In broods with extra-pair paternity, nat ural cavities contained a significantly greater proportion of extra-pa ir young than did nestboxes. Despite differences in nesting habitat an d female age structure, the frequency of extrapair paternity did not d iffer significantly between the natural-cavity and nestbox populations , Therefore, the presence of extra-pair paternity ill tree swallows is not an artifact of nestboxes or of artificial nesting conditions.