• Crowe, Susan A.
  • Kleven, Oddmund
  • Delmore, Kira E.
  • Laskemoen, Terje
  • Nocera, Joseph J.
  • Lifjeld, Jan T.
  • Robertson, Raleigh J.


Frequent copulation is assumed to be an important male reproductive strategy for paternity assurance in species with female sexual promiscuity. However, the empirical evidence supporting this hypothesis is limited. We examined copulation behaviour in relation to within-pair paternity in the socially monogamous tree swallow, Tachycineta bicolor, a passerine bird species with frequent copulation, a lack of mate guarding and high levels of extrapair paternity. We found that the frequency of within-pair copulation attempts increased and peaked just before the appearance of the first egg. A marked drop in the frequency of copulation attempts was observed once the first egg was laid. Furthermore, the frequency of successful within-pair copulations was highly repeatable for particular pairs within the 3 days prior to appearance of the first egg. Finally, males that obtained a higher number of successful within-pair copulations during this time also sired a larger proportion of the offspring in their own brood. Our findings suggest that frequent copulations just before the start of egg laying is an adaptation to decrease the risk of being cuckolded in the presence of sperm competition, and thus provide empirical support for the paternity assurance hypothesis


Study Site

This study was conducted during April–June 2006 at two nestbox grids in hay fields on the property of the Queen's University Biological Station, Ontario, Canada (44°34′N, 76°19′W). The nestboxes on the grids were situated with an interbox distance of 40 m along a row and 28 m across the diagonal (cf. Stutchbury & Robertson 1987). The two grids used in this study were Bridget's Grid (35 nestboxes, of which 28 were occupied by tree swallows) and Hughson's Grid (25 nestboxes, of which 20 were occupied by tree swallows).

General Field Procedures

Adults were captured using mist nets or traps in their nestboxes (cf. Stutchbury & Robertson 1986) and banded. To reduce the likelihood of nest abandonment as a result of early interference, most adult individuals were not captured upon arrival but after the nestlings hatched. We also collected a small sample (<25 μl) of blood for genetic analyses. The nestboxes were monitored prior to the appearance of the first egg until fledging or breeding failure. Three days after hatching, we obtained a small sample of blood (<25 μl) from the nestlings and collected any unhatched eggs or dead nestlings. Sampling for this study was conducted under Queen's University Animal Care permit 2005-021-R1, Canadian Wildlife Service (CWS) banding permit 10302, and CWS scientific capture permit CA0156.