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

Species geographic ranges and range limits are thought to be determined by climate, and across climatic gradients the morphology of populations varies non-randomly. Ecogeographic rules seek to characterise such variation, with Bergmann9s rule positing that organisms inhabiting colder environments are typically larger-bodied than those inhabiting warmer environments. While Bergmann9s rule has been supported across a range of taxonomic groups, how organisms behaviour may moderate its effect remains unclear. Here we investigate whether conformity to Bergmann9s rule among birds of the Western Palearctic varies in relation to nest design and migratory behaviour, using phylogenetic comparative analyses. We test predictions using data on nest structure and location, migration, body mass, latitudinal distribution, annual mean temperature and phylogenetic relatedness for a sample of >500 species. We find that conformity to Bergmann9s rule depends strongly on migratory behaviour: non-migratory species breeding at colder, more northerly latitudes are larger-bodied, while body mass is unaffected by climate in short- and long-distance migrants. Among non-migratory species, conformity to Bergmann9s rule depends, further, on nest design: species with more open nests, who are thus most exposed to adverse climatic conditions while breeding, conform most strongly to Bergmann9s rule. Our findings suggest that enclosed nesting and migration allow smaller bodied species to breed in colder environments than their body size would otherwise allow. Therefore, we conclude that organisms behaviour can strongly affect exposure to environmental selection pressures.