The cloacal protuberance (CP) of passerines functions primarily to store spermatozoa. When gently squeezed, the CP 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 dimensions and papilla length of mated male Tree Swallows in western Michigan. Male CP volumes were greatest when their mates were laying eggs and declined to the nestling period. Papilla lengths did not vary over the course of the breeding season. Cloacal protuberance dimensions and papilla length were 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 course of the breeding season is consistent with the steady decrease in reproductive opportunities for male swallows. Although it seems likely, whether or not the papilla of Tree Swallows functions as an intromittent organ awaits further study of the anatomical mechanics of copulation in these birds.