Increase in protandry over time in a long‐distance migratory bird

Abstract

Protandry is a widespread life‐history phenomenon describing how males precede females at the site or state of reproduction. In migratory birds, protandry has an 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 during two time periods, 1979 to 1988 and 2006 to 2016, we aim to investigate protandry dynamics over 38 years in a breeding population of willow warblers (Phylloscopus trochilus). Change in the timing of arrival was analyzed in males and females, and protandry (number of days between male and female arrival) was investigated both at population level and within breeding pairs. Our results show advancement in the arrival time at 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 did not observe any change in sex ratio that could explain the protandry increase, but pronounced temperature change has occurred and been reported in the breeding area and along the migratory route. Typically, natural selection opposes too early arrival in males, but given warmer springs, this counteracting force may be relaxing, enabling an increase in protandry. We discuss whether our results suggest that climate change has induced 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.

Sexes in sync: phenotypic plasticity, sexual selection and phenological synchrony between the sexes in a wild hibernator

“…The selection balance hypothesis was based on Møller's [13] observation of a stronger phenological response to warming spring conditions in male, as compared to female, barn swallows ( Hirundo rustica ), as also more recently reported in willow warblers ( Phylloscopus trochilus [14]). Møller [13] posited that an amelioration of the natural selection costs of early arrival tip the balance in favour of stronger sexual selection on males, leading to ever earlier arrivals of males relative to females (i.e.…”

“…by making resources available earlier in the season), we predict that the balance will tip towards earlier arrival by males, as they are typically subject to stronger sexual selection than females [20–22]. We may then expect the degree of protandry to increase under warmer springs if males plastically advance their phenology to a greater degree than females (figure 1.1 a ; as seen in migrating barn swallows, Hirundo rustica [13], and willow warblers, Phylloscopus trochilus [14]). However, climate warming is projected to increase atmospheric moisture and, over most of North America, increase winter precipitation and heavy precipitation events over time [19,23].…”

Elevational differences in migration phenology of Lazuli Buntings do not support selection-based hypotheses for protandry

“…Understanding how and why individuals vary in the timing of migration has been a central topic in ornithological research and it is well-established that migration timing is controlled by complex interactions between climate, food availability, and photoperiod cues (Gwinner 1977, Dawson 2008, Tøttrup et al 2010, Studds and Marra 2011, Klinner and Schmaljohann 2020, Burnside et al 2021. Additionally, many species show marked differences in migration timing between age and sex classes (Stewart et al 2002, Gill et al 2014, Hedlund et al 2015, Briedis et al 2019, though why these patterns exist is less understood. One common phenomenon observed in migratory passerines is protandry, the arrival of males prior to females within a population (Rubolini et al 2004, Coppack andPulido 2009).…”

“…We therefore predicted we would observe large numbers of buntings at the low-elevation site early during the arrival period and that low-elevation arrivals of both sexes would proceed arrival at high elevations. Given the ubiquity of protandry in migratory songbirds (Hedlund et al 2022), we further predicted protandrous arrivals at both low and high elevations. Lazuli Buntings are a moderate-distance, stopover molt-migrant, meaning they complete their prebasic molt at an intermediate molting ground (the North American monsoonal region of Sonora Mexico and southern Arizona and New Mexico) before arriving on the nonbreeding grounds (Greene et al 2020).…”

“…Data from banding stations, for example, represent passage dates of individuals from multiple distinct populations, each experiencing their own set of evolutionary and en-route factors that can influence migration timing (i.e., weather systems, stop-overs, route distances). As a result, they do not necessarily indicate the degree of protandrous arrivals to the actual breeding location (Coppack and Pulido 2009, Goodenough et al 2014, Briedis et al 2019, Brisson-Curadeau et al 2019, Hedlund et al 2022). In addition, because male birds are generally easier to detect upon arrival to the breeding grounds than are females (due to often more conspicuous behavior or plumage), estimates of protandry that do not account for sex-specific differences in detection probability may be biased (Coppack and Pulido 2009).…”