Null model
To test whether migratory populations are tracking climate conditions at finer scale (i.e., regionally), we devised a null model against which seasonal climate overlaps (two-dimensional, thermal and precipitation) were compared. This null model consists in randomized sampling around energetically optimal wintering destinations. For each genetically distinct population, we kept the empirical breeding destinations (i.e. occupied ecoregions during the breeding season) and we sampled wintering destinations as follows. We randomly sampled \(N\) distinct ecoregions among the set of ecoregions satisfying the condition:\(d_{O}<\frac{1}{2}d_{\max}\), where \(d_{O}\)is the geographic distance to the centroid of the set of occupied ecoregions during the wintering season simulated by ORSIM, and \(d_{\max}\) is the maximum distance separating any pair of ecoregions occupied by the population during the wintering season simulated by ORSIM; and \(N\) is the observed number of ecoregions occupied by the population during the wintering season. The distance separating pairs of ecoregions was calculated as the great circle distance between the ecoregions’ centroids.