Inconsistent Trends in the Breeding Phenology of Three Acrocephalus Warblers Nesting in Syntopy

Inter-specific variation in trends of breeding phenology is well documented across cavity nesting passerines. However, general conclusions about the consistency of phenological shifts across closely related open nesting species are yet to be achieved. We used ringing data from 1960 to 2015 during the nesting season of three Acrocephalus species breeding in syntopy at a single fishpond area, “Bohdanečsko” in the Czech Republic, to test whether long-term trends in the timing of the fledging period coincide among Common Reed A. scirpaceus, Marsh A. palustris, and Sedge A. schoenobaenus Warblers. Using logistic regression, we showed that trends in the fledging period were not fully consistent among the three closely related reed warblers. While the middle phases of the fledging period advanced in all species, the early phases were delayed in A. scirpaceus and A. schoenobaenus. The fledging periods became shorter in all species. The variation observed among these species makes it less likely that a single factor, such as climate warming, is the sole culprit of phenological changes. We suggest that interspecific competition and habitat change may have constrained Acrocephalus warblers from adjusting breeding times to increasing spring temperatures.