Reproductive responses of two related coexisting songbird species to environmental changes: global warming, competition, and population sizes |
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Authors: | Thomas Schaefer G?sta Ledebur Josef Beier and Bernd Leisler |
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Institution: | (1) Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee, 78315 Radolfzell, Germany |
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Abstract: | 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. |
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Keywords: | Acrocephalus Great reed warbler Reed bed Breeding success Phenology |
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