Data from: Tropical bird species have less variable body sizes

Ecologists have often predicted that species’ niche breadths should
decline toward the equator. Dan Janzen arrived at this prediction based on
climatic constraints, while Robert MacArthur argued that a latitudinal
gradient in resource specialization drives the pattern. This idea has some
support when it comes to thermal niches, but has rarely been explored for
other niche dimensions. Body size is linked to niche dimensions related to
diet, competition, and environmental tolerance in vertebrates. We
identified 68 pairs of tropical and nontropical sister bird species using
a comprehensive phylogeny and used the VertNet specimen database to ask
whether tropical birds have lower intraspecific body-size variation than
their nontropical sister species. Our results show that tropical species
have less intraspecific variability in body mass ( = 0.0092; p = 0.009).
Variation in body-size variability was poorly explained by both abiotic
and biotic drivers, thus the mechanisms underlying the pattern are still
unclear. The lower variation in body size of tropical bird species may
have evolved in response to more stable climates and resource
environments.