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.