Climatic variation in Africa and Europe has combined effects on timing of spring migration in a long-distance migrant Willow Warbler <i>Phylloscopus trochilus</i>.

In each case we obtained a well-behaved scatter plot showing a decreasing trend, with no outliers.

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This is analogous to studies analysing trends in the dates of percentiles of passage, where a negative trend indicates an advance of passage over the years and a positive trend means a delay ( Tøttrup, Thorup & Rahbek, 2006a ; Miles et al., 2017 ; Lehikoinen et al., 2019 ).

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