Why do afternoon copulations mainly occur after the egg-laying peak date in a colony of Great Skuas on Skúvoy, Faroe Islands?

In Great Skuas Stercorarius skua, copulations are often preceded by courtship feeding and occur in the morning and afternoon. We surveyed copulations in a colony of Great Skuas on Skúvoy, Faroe Islands during afternoons throughout the breeding season of 2013. The afternoon copulation frequency peaked 2.5 weeks after the peak in laying dates. This is unexpected because literature suggests that a pair copulates most frequently around a week before egg laying. As we were not able to link each copulation to a specific pair, several explanations are possible. First, if these afternoon copulations were pre-laying copulations, they were presumably mostly performed by pairs laying late in the season. A possible mechanism could be that young pairs and fish-eating pairs, which breed later in the season, make longer foraging trips and therefore feed their mate, and thus copulate, later in the day. These copulations may also reflect an increased copulation rate of young pairs, to strengthen the pair bond or compensate for low copulation success. Alternatively, if these copulations were post-laying copulations, they may be a response to mate feeding that continues during the incubation phase, and may strengthen the pair bond. We argue that potential individual and population dif- ferences should be taken into account when describing copulation behaviour at the species level.