WARMER SPRINGS LEAD TO MISTIMED REPRODUCTION IN GREAT TITS (PARUS-MAJOR)

In seasonal environments, the main selection pressure on the timing of reproduction (the ultimate factor) is synchrony between offspring req uirements and food availability. However, reproduction is initiated mu ch earlier than the time of maximum food requirement of the offspring. Individuals should therefore start reproduction in response to cues ( the proximate factors), available in the environment of reproductive d ecision making, which predict the later environment of selection. With increasing spring temperatures over the past decades, vegetation phen ology has advanced, with a concomitant advancement in the reproduction of some species at higher trophic levels. However, a mismatch between food abundance and offspring needs may occur if changes in the enviro nment of decision making do not match those in the environment of sele ction. Date of egg laying in a great tit (Parus major) population has not advanced over a 23-year period, but selection for early laying has intensified. We believe that this is the first documented case of an adaptive response being hampered because a changing abiotic factor aff ects the environment in which a reproductive decision is made differen tly from the environment in which selection occurs.