NIEBOER E, KRAAIJEVELD K & VAN LOON AJ (2017) Changes in the timing of the autumn migration of Willow Warblers Phylloscopus trochilus on the island of Schiermonnikoog in 1969-2010. LIMOSA 90 (2): 72-76.

Scandinavian Willow Warblers migrate through the Netherlands during late summer/early autumn, mainly between 15 Augustus and 30 September. Here we use a time series spanning 41 years of bird ringing on the island of Schiermonnikoog to show that nowadays Willow Warblers migrate a week later compared to the 1970s. The number of Willow Warblers captured per year did not change over time. In 1969-1981 the median passage date was 26 August, while it fell around 2 September during 1993-2010. This change appears to have occurred relatively abruptly during the 1980s. This delay could be caused by the lengthening of the breeding season in northern Europe due to climate change, a change in the composition of the breeding origins of the birds (e.g. nowadays birds with a more northern or eastern origin are captured), or a change in the proportion of juvenile birds. Intriguingly, a similar delay in autumn migration was reported for Christiansø (Denmark) but the timing of autumn migration of Willow Warblers on Helgoland (Germany) did not change over time, despite that the latter ringing station is likely to sample birds with a very similar breeding origin.

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