Influence of temperature and rainfall on the timing of the spring migration of Eurasian Wren <i>Troglodytes troglodytes</i> through the southern Baltic coast

Climate factors shape the spring phenology of migrant birds in Europe. We examined carry-over effects of temperature and rainfall in four regions of Europe on the spring migration timing of a medium-distance migrant, Eurasian Wren, at the southern Baltic coast over 60 years. We modelled the effects of these climate factors on the timing of the start (10%), median (50%) and end (90%) of wrens’ spring passage at the bird ringing station Hel (N Poland) during 1964–2023. The dates of start and median of spring passage have not changed, but the end shifted earlier by 5.7 days over these 60 years. The start, median and end dates of wren passage on the Baltic coast were early after warm autumn, winter or spring, but delayed after high autumn and winter rainfall in central and northern Europe. Analogous climate conditions in southern and south-western Europe had the opposite effect on wrens’ timing of passage. High spring rainfall in central and southern Europe delayed spring passage at Hel. High rainfall in early summer in northern Europe was related to early wren passage the following spring, but such conditions in late summer resulted in a delay of spring passage. We assume that temperature and rainfall in these regions influence the timing of wren spring passage through their effect on food availability, individual fitness, migration distance and proportion of immatures among migrants. We suggest that varied humidity, which caused different insect abundance for wrens, explains the varied effects of rainfall in various regions of Europe on the timing of wren spring passage at Hel. We show that wrens’ spring migration phenology in the Baltic region is shaped by a combination of carry-over effects of many environmental factors. They affect wrens in regions of Europe where they stay previously, even months before their spring migration.