Joint evolution of the biogeography and phenology of seasonal migration

In migratory species, the temporal phases of the annual cycle are linked
to seasonally shifting geographic ranges. Here, we investigate the
spatiotemporal structure of the annual cycle in a phylogenetic comparative
framework by developing a method to demarcate the pacing of annual cycle
stages using eBird, a massive avian occurrence dataset, and applying it to
migratory passerine birds breeding in North America. The analyses focus on
150 species of migratory passerine birds breeding in North America,
filtered from a starting list of 206 species based on data quality. The
analyses use eBird data averaged across the years 2002-2021 to calculate
daily distributional centroids of the geographic range of each species.
Next, the centroids are used to estimate demarcations of the annual cycle
stages for downstream analyses. Our analyses reveal a striking negative
correlation between the durations of the breeding versus nonbreeding
stationary periods, indicating that a tradeoff between the lengths of the
two stationary periods is the primary axis of variation in annual cycle
pacing. Our results further show that the duration of annual occupancy in
the breeding versus stationary nonbreeding ranges predicts the geographic
separation of these seasonal ranges, demonstrating that the ratio of time
spent on stationary breeding versus nonbreeding locations evolves in
tandem with a species’ migration distance. By contrast, the amount of time
during which species undergo seasonal migration—that is, the duration of
the seasonal periods when species’ geographic ranges shift
latitudinally—varies relatively little across species compared to the
length of the stationary periods. Our study helps untangle the complexity
of seasonal distributions and schedules to reveal integrated evolution of
the biogeography of the migratory cycle, its pacing, and life history
tradeoffs among species.