Increasing body condition of autumn migrating Eurasian blackcaps Sylvia atricapilla over four decades

Abstract

The effects of recent climate change on the phenology of avian breeding and migration, geographical range shifts, and decreasing migration distances are well documented. Only a few studies have focused on the long-term influence of these changes on the body condition of migrants, which is important as it is likely to affect the fitness of the whole population. Eurasian blackcaps have adapted their migratory behaviour to changing ecological factors. These changes seem to favour the species and its migratory populations have increased over the last decades in Europe. In the present study, we analysed the body condition index of juvenile blackcaps at their stop-over site in the southern Baltic region during autumn migration from 1967 to 2006. This index was distinctly higher since the 1990s than in the middle of the 1960s, when the number of migrating blackcaps was at least twofold lower. Moreover, there was a reduction in recapture rates of blackcaps from 14% to 6% during these periods which may imply that warblers were staying for a shorter time at the stop-over site as their body condition was higher. We hypothesized that higher berry crop availability resulted in the higher condition of migrating blackcaps. Body condition index also reflecting fuel load in migrants was higher in long-distance blackcaps compared to the short-distance ones. As the former are time-minimizing migrants, their higher body condition/fuel load may allow them to migrate faster to wintering grounds in sub-Saharan Africa.