Change in migration phenology and biometrics of two conspecific Sylvia species in Hungary

Abstract

The distribution area and the timing of migration of many bird species have recently changed, presumably due to climatic factors. In our study, we estimated the effect of climate change on migration phenology and biometrics of two sister Sylvia species: the medium-distance migrant Blackcap (Sylvia atricapilla) and the long-distance migrant Garden Warbler (Sylvia borin). We used the data of 52,440 Blackcaps and 6,943 Garden Warblers caught and ringed at the Ocsa Bird Ringing Station (47°15′N, 19°15′E) in Hungary between 1984 and 2008. The age and in case of Blackcaps the sex groups were handled separately. The timing of the spring migration of Garden Warblers shifted earlier, whereas it did not change in Blackcaps during the study period. We presume—in accordance with other observations concerning Blackcaps in West-Europe—that it probably has changed earlier than the 1980s. The timing of autumn migration shifted about 2 weeks later in all age and sex groups of Blackcaps. We found significant correlation in the timing of autumn migration of sex groups, the sign of the differences between the timing of males and females changed. Juvenile Garden Warblers shifted their autumn migration 13 days later, but adults did not. Mean body mass decreased while mean wing length increased suggesting a change in the composition of the trans-migrant population. We hypothesise that due to climate change the breeding range of these species expanded northwards, producing an increased ratio of long-winged northern individuals in the trans-migrant populations.