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.