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| 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. | |
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