Mate guarding in forest-living, territorial Willow Tits

Abstract

A population of Willow Tits was studied during one breeding season in northern Finland, to find out whether copulations outside the pair bond (EPCs) occur in this basically monogamous species and whether mate guarding operates to prevent EPCs. Only two EPCs and two within-pair copulations were seen . The EPCs were not true ones; in both cases, a newly widowed female copulated with a paired male from a neighbouring territory. During the female's fertile period, average and maximum distances between the mates were shorter than during the non-fertile stage. During the fertile period, the pairs also spent more time in the same trees than during the non-fertile period . In the fertile, but not in the non-fertile period, males were the sex that kept near their mates by letting the females initiate most of the site changes. There was no clear difference in intensity of guarding between adult and juvenile males. Thus, male Willow Tits guard their mates during the fertile period and the most probable explanation is the avoidance of cuckoldry. Despite the rarity of witnessed extra-pair copulations, the existence of a mate defence strategy implies that a mixed reproductive strategy could exist in the Willow Tit.