Protandry is a widespread life-history phenomenon describing how males precede females at the site or state of reproduction. In migratory birds, protandry has important influence on individual fitness, the migratory syndrome and phenological response to climate change. Despite its significance, accurate analyses on the dynamics of protandry using data sets collected at the breeding site, are lacking. Basing our study on records collected daily, spanning a period of 38 years, we aim to investigate protandry dynamics over time in a breeding population of willow warblers (Phylloscopus trochilus). Temporal change in the timing of arrival was analysed in males and females, and protandry (number of days between male and female arrival) was investigate both at population level and within breeding pairs. Our results show advancement in the arrival time to the breeding site in both sexes, but male arrival has advanced to a greater extent, leading to an increase in protandry both at the population level and within breeding pairs. We do not observe any change in sex ratio that could explain the protandry increase, but pronounced temperature change has occurred at the breeding area and along the migratory route. Typically, natural selection opposes earlier arrival in males, but given warmer springs, this counteracting force may be relaxing, enabling an increase in protandry. We discuss whether our results suggests that climate change has caused sex-specific effects, if these could be evolutionary and whether the timing of important life-history stages such as arrival at the breeding site may change at different rates in males and females following environmental shifts.  

This data was collected by observation of wild migratory birds, in south Sweden, between 1979-2016. The data has been processed in Excel and in Stata.