Arrival dates of male and female willow warblers (Phylloscopus trochilus) to their breeding site in Sweden 1979-2016

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.

先雄后雌（protandry）是一类广泛分布的生活史现象，指雄性在繁殖位点或繁殖状态上先于雌性出现。在迁徙鸟类中，先雄后雌现象对个体适合度、迁徙综合征以及应对气候变化的物候响应均具有重要影响。尽管该现象意义重大，但目前仍缺乏依托繁殖地收集的数据集对先雄后雌动态开展的精准分析。本研究基于历时38年的每日观测记录，旨在探究柳莺（Phylloscopus trochilus）繁殖种群内先雄后雌现象随时间的动态变化。我们对雄性与雌性的抵达时间时序变化展开分析，并分别在种群水平及繁殖对内部探究了先雄后雌程度（即雌雄抵达间隔天数）。研究结果显示，两性抵达繁殖地的时间均有所提前，但雄性抵达时间的提前幅度更大，进而使得种群水平与繁殖对内部的先雄后雌程度均有所上升。我们未观测到可解释先雄后雌程度上升的性比变化，但繁殖地及迁徙途经区域均发生了显著的气温变化。通常而言，自然选择会抑制雄性过早抵达，但随着春季气温升高，这类反选择压力可能有所缓解，从而使得先雄后雌程度得以提升。本文还讨论了：研究结果是否表明气候变化已产生性别特异性效应，这些效应是否可被归为进化性变化，以及在环境变迁背景下，抵达繁殖地等关键生活史阶段的时间是否会在雌雄两性间出现不同速率的改变。