Intraspecific Variation in Extra-Pair Copulation and Mate Defence in Common Guillemots Uria Aalge

Abstract

Extra-pair copulation (EPC) in common guillemots was frequent during the pre-laying period. Although guillemots are essentially monogamous, males adopted a mixed reproductive strategy (MRS) by attempting to obtain forced extra-pair copulations (FEPC) during the female's fertile period, which was estimated to start 25 days prior to laying. Males spent more time at the colony than females and exhibited highly synchronized cycles of attendance during the pre-laying period in order to defend their partner against EPC attempts. By doing so they probably incurred some energetic cost. Pair copulation started as soon as the adults returned to the colony (4-5 months before the start of egg-laying), when it probably serves a pair-bonding function. From about 25 days before egg-laying (day -25) the PC rate increased, peaking at about day -12. It was estimated that guillemots copulated 22 times in the three weeks prior to egg-laying. The FEPC rate increased towards egg-laying and was more closely correlated with date than with the relative number of males present, although the number of males involved in each FEPC attempt increased towards laying. During the fertile period, 10% of all observed copulation attempts were FEPCs. However, the success rate (the proportion achieving cloacal contact) of FEPCs was much lower (6%) than that of PCs (74%). Females also employed an MRS, occasionally soliciting or co-operating in unforced extra-pair copulations (UEPCs) when their male partner was absent from the colony. UEPCs were much more successful (95%) than FEPCs, but also less frequent. The effect of breeding density on copulation behaviour was examined. FEPCs were more frequent at high breeding densities than at low density. The probability of obtaining FEPC insemination for males with two or more neighbours was about ten times that of males with no neighbours. Similarly, the risk of cuckoldry increased with density. increased with density. Three hypotheses were considered concerning the effect of density on the PC rate: the 'dilution' hypothesis predicted a higher PC rate at high density, and the 'disguise' and 'interference' hypotheses predicted a lower PC rate at high breeding density. The PC rate decreased with increasing density as a result of disruption of PCs by neighbouring males attempting to obtain FEPCs, a result consistent with the 'interference' hypothesis. The reproductive success of males obtaining EPCs, cuckolded males, and non-EPC/non-cuckold males was examined, and a selective advantage of 10% estimated for successful EPC males over cuckolds.