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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