Complex postbreeding molt strategies in a songbird migrating along the East Asian Flyway, the Pallas's Grasshopper Warbler Locustella certhiola.

Molt strategies have received relatively little attention in current ornithology, and knowledge concerning the evolution, variability and extent of molt is sparse in many bird species. This is especially true for East Asian Locustella species where assumptions on molt patterns are based on incomplete information. We provide evidence indicating a complex postbreeding molt strategy and variable molt extent among the Pallas's Grasshopper Warbler Locustella certhiola, based on data from six ringing sites situated along its flyway from the breeding grounds to the wintering areas. Detailed study revealed for the first time that in most individuals wing feather molt proceeds from the center both toward the body and the wing-tip, a molt pattern known as divergent molt (which is rare among Palearctic passerines). In the Russian Far East, where both breeding birds and passage migrants occur, a third of the adult birds were molting in late summer. In Central Siberia, at the northwestern limit of its distribution, adult individuals commenced their primary molt partly divergently and partly with unknown sequence. During migration in Mongolia, only descendantly (i.e., from the body toward the wing-tip) molting birds were observed, while further south in Korea, Hong Kong, and Thailand the proportion of potential eccentric and divergent feather renewal was not identifiable since the renewed feathers were already fully grown as expected. We found an increase in the mean number of molted primaries during the progress of the autumn migration. Moderate body mass levels and low-fat and muscle scores were observed in molting adult birds, without any remarkable increase in the later season. According to optimality models, we suggest that an extremely short season of high food abundance in tall grass habitats and a largely overland route allow autumn migration with low fuel loads combined with molt migration in at least a part of the population. This study highlights the importance of further studying molt strategy as well as stopover behavior decisions and the trade-offs among migratory birds that are now facing a panoply of anthropogenic threats along their flyways.

Habitat choice in Phylloscopus warblers: the role of morphology, phylogeny and competition.

We studied ecological and evolutionary aspects of habitat choice in a group of closely related bird species to gain insight into factors influencing bird community structure. Seven species of Phylloscopus warblers breed sympatrically in the middle taiga subzone of Central Siberia. We examine how the distribution of species among habitats is related to morphology, phylogeny and competition, and we compare our results with an earlier study on the ecomorphology of Phylloscopus warblers in Kashmir. We found that in Siberia, large warbler species prefer productive habitats with mostly deciduous vegetation, whereas small species occupy poor coniferous forests. Possible explanations for this finding remain to be tested in the future. Moreover, we found a tendency for species with large feet, small bills and short wings to occupy habitats with an abundance of bush thickets near the ground. In the Kashmir study, competition was considered a major factor in structuring the Phylloscopus community, and patterns of habitat choice were not influenced by phylogenetic relationships. In strong contrast, we found that in the Siberian community, closely related species occupy similar habitats. We discuss whether this conservative evolution of habitat preferences in Siberia may be due to low intensity of interspecific competition or to other ecological factors.