Factors influencing the amount of extra-pair young in a willow warbler population Phylloscopus trochilus

We studied a willow warbler (Phylloscopus trochilus) population breeding in Øvre Heimdalen, in Valdres, Norway. Several previous studies have found large differences in the amount of EPY (extra- pair young) in the broods of different males in the same species, many of these species have previously been regarded as strictly monogamous. An earlier study of the willow warbler population in Øvre Heimdalen by Gro Bjørnstad et. al found that about 40% of the chicks had a biological father different from the one who was feeding them. We wanted to find out if this could be explained by differences in male abilities and/ or differences between the different females. The use of genetic markers to study paternity in bird species have become very common, and has provided the researchers with an excellent tool. By comparing the number of offspring sired by a male to his body condition and behaviour it has become possible to understand which male characters are important to female choice of partner. In several bird species the number of EPY has been linked to male condition and indirectly to his genetic viability. We used microsatellite markers and gel-electrophoresis to determine how many of the offspring in a male s social nest that was sired by him, and to link this number to different variables, such as male song rate, body condition, and if the male was courtship feeding or not. We found relationships between courtship feeding and the number of offspring sired by the male in his social nest, and a difference between the unpaired and the paired males in song output. Weather also seems to play a part in determining the number of EP offspring in the population. The amount of EPY in the clutches was largest in the years with the most favourable weather. The results also indicate that female body condition might play a part in determining how many of the offspring that are sired by the pair male. It seems that older and/ or larger parents are less likely to engage in EPC s, the reason for this might be that both are more confident that their partner are of high quality, and so invest more in the clutch