Sex is also a matter of timing : an overview and an update of protandry studies in birds

Earlier emergence or arrival of either sex at the breeding grounds is common in animals. Since the widespread occurrence of differences in the timing of appearance of males and females at the breeding grounds has been appreciated decades ago, diverse hypotheses have flourished to account for the selection pressures leading to the evolution of earlier male (protandry) or female (protogyny) arrival. In birds, males typically arrive from migration or, in resident species, start socio-sexual activities earlier than females, although protogyny occurs in a restricted set of, mostly polyandrous species. Here, I will first review the numerous hypotheses and briefly sketch modellistic approaches relevant to the interpretation of the evolution of protandry in birds. I will then review the current knowledge on the patterns of variation in the relative timing of arrival of males and females across bird species. The experimental study of the function of earlier male or delayed female arrival is hampered by obvious practical limitations. However, the hypotheses for the evolution of protandry in birds have been empirically scrutinized in a few comparative analyses of across species-variation in protandry in relation to variation in traits that can differently constrain timing of arrival of either sex or influence the costs of early arrival, as well as in relation to the intensity of sexual selection via eg sperm competition. I will discuss the findings of these comparative studies while emphasizing the lack of data on sexually monomorphic species, which owes to the only recent development of molecular techniques for sexing birds, and the methodological issues in the study of protandry. I will therefore present novel findings on inter-specific variation in spring migration protandry in a large set of both monochromatic and dichromatic species obtained by extensive sampling at Ventotene island and molecular sexing. Climate change has affected migration phenology of birds. However, climate change is likely to differentially affect the selection regime acting on migration and arrival dates of males and females, suggesting that microevolutionary change in protandry may be under way. In the end of the talk, I will outline current and future scenarios for the change in protandry under the effect of climate change and how selection for protandry may be predicted to hasten or, conversely, to slow down adaptation to rapidly changing environments.