Climate changes and post-nuptial migration strategy by two reedbed passerines

Phenology of biological systems is the expression of selective pressures forcing organisms to match their energy requirements to seasonal variations in resource availability. For longdistance migrant organisms, migration strategy is shaped by the availability and quality of stopovers during a precise time period. Ongoing climate changes alter the usual spatial and temporal distribution of resources, and, as a result, migrant species return earlier to their breeding grounds. Less is known on the evolution and determinants of timing of departure to wintering grounds, i.e. postbreeding migration. We analysed timing of post-nuptial migration and stopover strategy with capture–mark–recapture (CMR) data in 2 reedbed insectivorous passerines: the sedge warbler Acrocephalus schoenobaenus, specialised on a thermo-dependant prey, and the generalist reed warbler A. scirpaceus. Spring temperatures increased through years (0.16 ± 0.047°C yr–1), and studied species migrated earlier in recent years (–0.73 ± 0.086 d yr–1). Autumnal migration phenology was negatively related to spring (March) temperature (–2.54 ± 0.19 d °C–1), whereas it was not related to autumnal temperatures. We suggest that this results from a major time shift (i.e. advance) in biomass production caused by global warming. Stopover duration (estimated by CMR analysis) was positively related to body mass gain, with a trend for a stronger effect in the specialised species. However, neither stopover duration nor body mass gain were related to spring temperature, suggesting that the advance in migration timing efficiently adapted bird phenology to temporal variations in resources.