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

Authors: Johanna Hedlund, Thord Fransson, Cecilia Kullberg, Jan-Olov Persson, and Sven Jakobsson

Year: 2022

Publication: Ecology and Evolution

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Keywords: bird migration, climate change, phenology, Phylloscopus, protandry, willow warbler

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.

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