Effects of climate change on the degree of protandry in migratory songbirds

Current climate change is affecting the timing of life-history events of birds, such as the timing of spring arrival at the breeding grounds. Interspecific differences in the advancement of spring migration have hitherto been attributed to differences in exposure to climatic variables in winter or on migration, without a more detailed consideration of sex-specific differences in the timing and extent of migration. Since males and females migrate during different times of the season or may segregate into different wintering grounds, we expect climate change to differentially affect the arrival timing of males and females. Furthermore, sexual selection theory predicts that the degree of protandry (i.e. the time difference between male and female arrival) should increase when the forces of natural selection opposing early arrival relax and the earliest males benefit from improved mating opportunities. Here, we explore whether the degree of protandry has changed during a period of climatic warming using long-term phenological data collected at 5 North European bird observatories. We show for 4 sexually dichromatic songbird species that spring migration has generally advanced, but that the degree of protandry has not changed. Moreover, we provide evidence that variation in large-scale weather conditions, as reflected in the North Atlantic Oscillation (NAO) index, have not affected the degree of protandry. We hypothesize that a parallel shift in the timing of male and female migration may be due to similar phenotypic plastic responses of males and females to climatic changes or reflect a correlated selection response in males and females. In addition, we discuss the possibility that the observed variation in protandry could result from seasonal shifts in local environmental conditions under which birds of both sexes ‘fall out’ and are sampled.