Climate change affects breeding of reed warblers Acrocephalus scirpaceus

Between 1970 and 2006 reed warblers Acrocephalus scirpaceus started breeding progressively earlier; both the initiation of breeding (the earliest first egg dates) and peak of breeding (median first egg dates) advanced. Median first egg dates correlated significantly with increasing May-July mean temperatures. However, in contrast to other studies showing the advancement in laying dates, the end of the season did not shift. As a result, the breeding season is now longer increasing re-nesting opportunities. Individuals are able to re-nest 4-5 times, which might have important implications for the species. It was also found that in warmer seasons the population suffered fewer nest losses. Both factors, higher re-nesting potential and a trend toward fewer losses, should lead to increased fitness of individuals in the studied population.