Although changes in the mean arrival and breeding dates of some species are expected in response to climate change, other species may respond to direct or indirect climate effects by modifying the duration of their breeding period. We used a 20-year database for two closely related species that breed in sympatry in Europe, the short-distance migrant Dalmatian Pelican (Pelecanus crispus) and the long-distance migrant Great White Pelican (P. onocrotalus), to test for changes in breeding phenology over the past two decades. Median laying dates of the species covaried over time, indicating that the same or similar environmental factors probably influenced both species at their breeding sites. However, direct climate effects were revealed only for the short-distance migrant species, which showed a rapid advancement in its median laying dates (1.4 days year-1). This shift was associated with (1) an increase in population size and (2) weather variation following warmer and wetter winters. Earlier breeding was associated with enhanced survival of juvenile Dalmatian Pelicans, especially during hot and wet years. The Great White Pelican exhibited no advancement in its median laying date, but it significantly reduced the duration of its laying period (1.7 days year-1) by advancing the latest laying date. Furthermore, the response of the Great White Pelican was related only to factors of population size. Further analyses of breeders' interactions within and between species might help to identify possible indirect effects of environmental changes on both species and should be further considered for their future conservation.
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Vol. 129 • No. 4