Reproductive responses of two related coexisting songbird species to environmental changes: global warming, competition, and population sizes

Comparative analyses of interspecific differences in response to climate change can provide important insights into the factors initiating seasonal onset of reproduction in various species and subsequent fitness consequences. We present a comparative analysis based on a 30-year breeding survey of two related migratory songbird species [Reed Warbler Acrocephalus scirpaceus (RW) and Great Reed Warbler A. arundinaceus (GRW)], which coexist in reedbeds by means of various interspecific interactions. The RW advanced breeding by 15 days and shortened its breeding time window, which is a combined effect of higher temperatures, alleviated competition as a consequence of population declines in the dominant GRW, and reduced RW population. Although the breeding period of GRW changed only slightly, its clutch initiation was likewise related to temperature. Most probably, advanced breeding in RW is favoured by changes in food supply and accelerated reed growth, which provides the necessary nest concealment earlier, whereas this does not affect the GRW, a species less vulnerable to nest predation. Clutch size decreased later in the season, so that earlier breeding produced a net increase in both species. An additional increase of clutch size in GRW can be explained by the use of higher-quality territories in today’s smaller population. The main causes of nest losses were predation in RW and adverse weather in GRW, but reproductive success increased over the study period in both species, which was a consequence of larger clutches in RW, but of favourable weather during rearing and fewer total losses in GRW. Our results document that different causal mechanisms are involved in the reproductive changes of the two congeneric species living in the same habitat: RW breeding earlier by making use of competitive release and other ecological improvements, and GRW by benefiting from better rearing conditions. As species respond differentially to climate change depending on ecosystem and biotic interactions, predictions of population dynamics will remain vague until the specific response mechanisms have been elucidated.