Left-sided directional bias of cloacal contacts during tree swallow copulations

Most female birds have only a left ovary and associated oviduct with entry to the oviduct on the left side of the urodeum of the cloaca. We hypothesiz ed that male cloacal contacts during copulation would occur from the left s ide of females because this would put sperm closer to the entrance of the o viduct. We observed that cloacal contacts from the left outnumbered those f rom the right by a margin of 3:1 during tree swallow, Tachycineta bicolor, copulations at nestboxes in western Michigan in 1999. The directional bias of cloacal contacts may have an adaptive function. (C) 2001 The Association for the Study of Animal Behaviour.