Statistical analysis
The random sampling procedure in the null model was repeated 1000 times, thus generating 1000 simulated wintering distributions for each population. Each time, climate was extracted and seasonal niche overlap was calculated in the same way as for the observed wintering distributions. We then calculated the rank of the observed seasonal niche overlap among the 1000 simulated ones. Ranks were rescaled between 0–1 and their distribution was tested for skewness towards low values (indicating seasonal niche tracking) by one-sample Kolmogorov-Smirnov tests against a uniform distribution.