Copulation behaviour of the tree swallow, Tachycineta bicolor: paternity assurance in the presence of sperm competition

Abstract

Tree swallows engage in extra-pair copulations, creating an opportunity for sperm competition. The aim of this study was to determine whether the pattern of within-pair copulation was consistent with the use of frequent copulation by the male to assure paternity. Tree swallows attempted copulation frequently (65 times per clutch), 78% of which resulted in cloacal contact. Copulations were most frequent from 8 days prior to egg laying until early in egg laying. Over the whole season, pairs averaged (± se) 0·53±0·09 copulations/h in the morning and 0·22±0·05 copulations/h in the afternoon ( N=17 pairs). Early in the breeding season (prior to the egg-laying period), copulations occurred most often in the early morning. During the laying period, eggs were laid in the early morning and copulations occurred more often in the afternoon/evening. Females perched less often (and were therefore less available for copulation) during egg-laying days than directly before. Pairs spent less time together during egg laying than directly before egg laying. There was a trend towards higher copulation attempt rates under denser nesting conditions. Evidence supports the idea that males are attempting to protect their paternity by frequent copulation. The results also suggest that females dictate to some extent when copulations occur such that males do not necessarily copulate at times that are optimal for assuring paternity.