Facultative polygamy and extrapair paternity of willow warblers in different parts of the species’ range

We studied the level of polygyny and extrapair paternity in Willow Warblers Phylloscopus trochilus (L.) with the help of molecular genetic techniques and observations of marked birds during 2006-2008 and 2015-2017 on the territory of European Russia. Seasonal monogamy, typical for this species, prevailed in the study areas. However, there were common cases of extra-pair copulations, which were a confirmation of the facultative polyandry of some females. The frequency of extrapair paternity in different regions was 11,8-37,5 % (on average 19,5 %), and the share of nests with extrapair nestlings was 66,7-83,3 % (on average 72,2 %). We registered polygyny in some males in Karelia and in the north-western part of Russia. The level of polygamy in this region ranged from 7,7 to 16,7 %, averaging 11,5 %, during the research. Apparently, the diversity of reproductive relationships is aimed at maximizing the reproductive potential of individuals and contributes to the genetic diversity of the offspring.