Cases of Conspecific Brood Parasitism in the Willow Warbler (Phylloscopus trochilus, Passeriformes)

A finding of conspecific brood parasitism (CBP) in willow warblers, Phylloscopus trochilus, breeding in western Russia is presented. The data set consists of 41 broods and 214 chicks, and the results are based on an analysis of the willow warbler microsatellites Phtr1, Phtr2, and Phtr3. Two chicks (1%, two chicks of 214) were in broods where they were unrelated to both social parents, and these two chicks were in two different broods (5%, two broods of 41). In each case, the clutch comprised six eggs, one of which had been laid in by a parasitic female. In both nests, chicks hatched out of all eggs and fledged successfully. There is no information about the possible relatedness of the parasitic females to the hosts. In addition, the paper reviews other reports of CBP in the genus Phylloscopus, based on brood size, the morphological characteristics of eggs, and observation data. Such reports are considered to be insufficient for CBP confirmation. Currently, the only reliable evidence for CBP is the data obtained by genetic methods.

Automated summary

The study presents findings of conspecific brood parasitism in willow warblers breeding in western Russia, where 2 chicks were found to be unrelated to their social parents. The study concludes that genetic methods provide the most reliable evidence for confirming conspecific brood parasitism, compared to other factors such as brood size or egg characteristics.