Occurrence of extra-pair copulation in the tree swallow (Tachycineta bicolor)

Summary A new method to mark sperm transfer events between birds, using microspheres inserted into males' cloacae, was employed to assess the frequency of extra-pair copulation (EPC) in a population of tree swallows (Tachycineta bicolor) in Ontario, Canada, during the summer of 1988. We inserted 25 males with microspheres; spheres from 8 (32%) of these males were found in extra-pair females, from which we infer that males pursue a mixed reproductive strategy of monogamy coupled with seeking EPCs. The cloacae of 10 of 41 (24.4%) females were found to contain microspheres from non-mates. EPCs occur more frequently between neighboring birds than between non-neighbors. Only two within-pair copulations were detected with this method, suggesting that it underestimates the occurrence of all copulations, extra-pair or within-pair. Copulation watches revealed that within-pair copulations occur very frequently. We propose that frequent pair copulations are used by males to ensure their paternity in their mate's offspring. Offprint requests to: R.J. Robertson