Individual and seasonal variation in external genitalia of male Tree Swallows

The cloacal protuberance (CP) of passerines functions primarily to store sp ermatozoa. When gently squeezed, the cr of male Tree Swallows (Tachycineta bicolor) everts to expose a small papilla from which semen can be expressed . From 1994 to 1999, I examined individual and seasonal variation in CP dim ensions and papilla length of mated male Tree Swallows in western Michigan. Male CP volumes were greatest when their mates were laying eggs and declin ed to the nestling period. Papilla lengths did not vary over the course of the breeding season. Cloacal protuberance dimensions and papilla length wer e not associated with the age class of a male's mate, the date she started laying eggs, or number of eggs she laid. The decrease in CP size over the c ourse of the breeding season is consistent With the steady decrease in repr oductive opportunities for male swallows. Although it seems likely, whether or not the papilla of Tree Swallows functions as an intromittent organ awa its further study of the anatomical mechanics of copulation in these birds.