Variation in which chromosome region is strongly associated with breeding altitude and latitude?

Chromosome 3.

While Chromosomes 1 and 5 are clearly tied to the physiological requirements of the migration distance itself, Chromosome 3 shows differentiation across the Scandinavian divide but exhibits a different adaptive correlation. Variation found within the highly differentiated region on Chromosome 3 is strongly associated not with the migratory phenotype directly, but rather with environmental variables related to the breeding location, specifically breeding altitude and latitude. The functional significance of this regional variation is further hinted at by the presence of genes involved in calcium channel function, such as $RYR2$, a protein known in other high-altitude vertebrates to be involved in adaptation to colder, higher elevation environments. This suggests this segment is adapting the warbler to local climatic or topographical breeding conditions, independent of the fueling needs of the subsequent long-distance flight.