Migratory distance and Latitude
To estimate migratory distance, we divided species into three categories: Migrants, which have no spatial overlap between their breeding and nonbreeding distributions; non-migrants, which have complete overlap between breeding and nonbreeding distributions, and partial migrants, which have some overlap between breeding and nonbreeding distributions. Non-migrants were always set to zero migratory distance. Using shapefiles of breeding and non-breeding distribution (Birdlife 2016) we calculated six separate estimates of migratory distance: 1: distance between the mid latitudes of each distribution, 2 & 3: distance between the maximum and minimum latitudes of each distribution respectively, 4: distance between maximum latitude of breeding distribution and minimum latitude of nonbreeding distributions, 5: distance between minimum latitude of the breeding distribution and maximum latitude of the nonbreeding distribution, and 6: the great circle distance between the centroids of the points. We used linear models to examine the autocorrelation between these variables and chose the first measure of migratory (distance between mid-latitudes) distance to use in further analysis, because it best predicted the other measurements of migration. We calculated the latitude of the breeding and winter ranges of each species as the mean latitude value of each shapefile.