Data from: A tale of two seasons: the link between seasonal migration and climatic niches in passerine birds.

The question of whether migratory birds track a specific climatic niche by
seasonal movements has important implications for understanding the
evolution of migration, the factors affecting species’ distributions and
the responses of migrants to climate change. Despite much research,
previous studies of bird migration have produced mixed results. However,
whether migrants track climate is only one half of the question, the other
being why residents remain in the same geographic range year-round. We
provide a literature overview and test the hypothesis of seasonal niche
tracking by evaluating seasonal climatic niche overlap across 437
migratory and resident species from eight clades of passerine birds.
Seasonal climatic niches were based on a new global dataset of breeding
and non-breeding ranges. Overlap between climatic niches was quantified
using ordination methods. We compared niche overlap of migratory species
to two null expectations, 1) a scenario in which they do not migrate and
2) in comparison to the overlap experienced by closely related resident
species, while controlling for breeding location and range size. Partly in
accordance with the hypothesis of niche tracking, we found that the
overlap of breeding vs. non-breeding climatic conditions in migratory
species was greater than the overlap they would experience if they did not
migrate. However, this was only true for migrants breeding outside the
tropics and only relative to the overlap species would experience if they
stayed in the breeding range year-round. In contrast to the hypothesis of
niche tracking, migratory species experienced lower seasonal climatic
niche overlap than resident species, with significant differences between
tropical and non-tropical species. Our study suggests that in seasonal
non-tropical environments migration away from the breeding range may serve
to avoid seasonally harsh climate; however, different factors may drive
seasonal movements in the climatically more stable tropical regions.