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On the basis of field studies conducted in 1973–2000 it was found for four species of Phylloscopus warblers breeding in the taiga zone of NW Russia that their northern marginal populations, more remote from wintering grounds, have considerably shorter period of staying in the breeding region as compared with southern populations (because of later arrival and earlier departure), shorter interval between male and female arrival and lower population densities. It determines the regional features of their behavior during pre-nesting period: reduction of time of its first phase (before female’s arrival) and more “peaceful” delimiting of nesting territories. The “companionship” stage (according to D. May) is absent as the warblers from northern populations arrive in the nesting sites being physiologically readier for breeding. Although socially monogamous pairs dominate, the cases of facultative polygamy occur, especially in Willow Warbler. The molecular-genetic analysis has shown that in this species the frequency of polygyny (5–17 %) is much lower than the frequency of polyandry (68,4 %), i.e. in majority of Willow Warbler’s nests the bastard nestlings (15,3 % of total nestling number) are found. This is obviously conditioned by the excess of adult males, which is quite typical for all Phylloscopus species. Refs 31.

The adaptation mechanisms in Phylloscopus warblers to habitat conditions associated with their reproduction, namely, the sex ratio, family structure, duration of sexual activity, etc., have been studied. The brood sex ratio is established to be about 1 : 1 among juveniles, but males begin to predominate with time. In the reproductive period, some males remain unmated for a long time or even over the whole season. On the other hand, nesting males create pairs with second females. Visual observations and the results of a family structure analysis by means of molecular genetic methods have shown that female "adultery" and/or forcible copulations with alien males occur from time to time, which leads to the appearance of extra pair paternity. A PCR analysis revealed a high level of genetic diversity within the local populations of all the Phylloscopus warbler species, but the diversity for wood and greenish warblers (new species for the region) was lower than that for chiff chaffs and willow warblers (species native to the region).

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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.

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