Conformity to Bergmann’s rule in birds depends on nest design and migration

Ecogeographical rules attempt to explain large-scale spatial patterns in
biological traits. One of the most enduring examples is Bergmann’s rule,
which states that species should be larger in colder climates due to the
thermoregulatory advantages of larger body size. Support for Bergmann’s
rule, however, is not consistent across taxonomic groups, raising
questions about what factors may moderate its effect. Behaviour may play a
crucial, yet so far underexplored, role in mediating the extent to which
species are subject to environmental selection pressures in colder
climates. Here, we tested the hypothesis that nest design and migration
influence conformity to Bergmann’s rule in a phylogenetic comparative
analysis of the birds of the Western Palearctic, a group encompassing
dramatic variation in both climate and body mass. We predicted that
migratory species and those with more protected nest designs would conform
less to the rule than sedentary species and those with more exposed nests.
We find that sedentary, but not short- or long-distance migrating species,
are larger in colder climates. Among sedentary species, conformity to
Bergmann’s rule depends, further, on nest design: species with open nests,
in which parents and offspring are most exposed to adverse climatic
conditions during breeding, conform most strongly to the rule. Our
findings suggest that enclosed nests and migration enable small birds to
breed in colder environments than their body size would otherwise allow.
Therefore, we conclude that behaviour can substantially modify species’
responses to environmental selection pressures.