Copulation and mate guarding in the willow warbler

Abstract

Copulation behaviour and mate guarding were studied in a willow warbler, Phylloscopus trochilus, population in south-west Sweden. Males tried vigorously to copulate with their mates from the time of pair formation but successful copulations were observed only from 4 days before egg laying. The copulation rate was highest in the morning. After egg laying commenced females were reluctant to copulate and only 29% of all copulations occurred after egg laying had begun. The observed temporal copulatory pattern is inconsistent with the fertilization window hypothesis, which predicts copulations early in the morning until no more ova are fertilizable, i.e. the laying of the second from last egg. Most pair copulations were initiated by female solicitation, indicating that females are able to control copulation activity. Intrusions into the territory by other males coincided with the fertile period of the female, and peaked 3 days before egg laying during the period of female solicitation. Females also solicited from experimentally induced intruders for 3 days prior to egg laying, indicating that females do not discriminate between males at this stage in the breeding cycle. Only one of 61 observed intrusions resulted in a successful extra-pair copulation. Males intensively guarded their mates before egg laying started and they reacted very strongly to intruders at this stage whereas the reaction was much weaker once egg laying started.