The Influence of Winter and Spring Large-Scale Climate Patterns on the Timing of Male and Female Blackcap Spring Passage at the Baltic Coast.

Sex-specific adjustments of bird migration phenology to climate change are understudied. We analysed blackcap migration timing using spring (26 March-16 May) ringing data from Bukowo-Kopań (N Poland) in 1982-2025. We assessed the influence of large-scale climate indices in winter (December-February) and spring (March-May) at the beginning (p10), median (p50), and end (p90) of passage of each sex, using multiple regression. Protandry (earlier arrival of males) at the beginning of passage averaged 2.2 days and remained stable over 44 years, because p10 dates advanced by 9 days in both sexes. This aligns with 22-year studies from northern Europe, and with the mating opportunity hypothesis. The median (p50) of passage advanced by 7-9 days, but p90 showed no trends during 1982-2025. Positive winter and spring North Atlantic Oscillation (NAO) and spring Scandinavian Pattern (SCI) were associated with early p10 and p50, and positive winter Indian Ocean Dipole (IOD) with delayed p90, in both sexes alike. These similar responses likely occur because these climate patterns affect broad non-breeding areas of both sexes. We suggest that female blackcaps are as adaptive as males, and advanced migration maximises reproductive success and the chance of second broods in both sexes.