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

Mark C Mainwaring,Sally E Street

doi:10.1002/ece3.8034

Abstract

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. Behavior 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 behavior can substantially modify species’ responses to environmental selection pressures.

Highlights

Ecogeographical rules seek to characterize and explain spatial patterns in biological traits (Gaston et al, 2008)
When including interaction terms for migration and breeding climate, we find support for the predicted effect of migration on conformity to Bergmann's rule: Among sedentary species, body mass increases with breeding latitude (β = 0.006 [0.003, 0.009], pMCMC < 0.001; Table 2a, Figure 1) and decreases with breeding range temperature (β = −0.014 [−0.021, −0.008], pMCMC < 0.001; Table 2b), but this is not the case in short-or long-distance migrants
We find that conformity to Bergmann's rule, which predicts that species inhabiting colder environments should have larger body sizes (Bergmann, 1847), is not uniform across birds of the Western Palearctic, but rather depends on migration and nest design

Summary

| INTRODUCTION

Ecogeographical rules seek to characterize and explain spatial patterns in biological traits (Gaston et al, 2008). We investigate the potential role of nest design and migration as factors influencing conformity to Bergmann's rule in a phylogenetic comparative analysis of Western Palearctic birds We focus on this region due to the dramatic variation in both body size and environmental conditions over large-scale latitudinal gradients in the Northern Hemisphere, in which ambient temperatures are considerably colder in northerly than southerly regions. Since nest structure and location may both influence exposure to climatic conditions (Lamprecht & Schmolz, 2004; Rhodes et al, 2009), we incorporate features of both nest structure and location when classifying species’ nest design, as well as examining their individual effects If both nest design and migration have important effects on conformity to Bergmann's rule, we ought to find support for the predicted effects of nest design within both sedentary and migratory species and for the effects of migration within both open and enclosed nesting species. We fit phylogenetic regression models in which the slope of body mass on latitude or temperature was allowed to differ between species with varying nest designs and/or migratory strategies

| MATERIALS AND METHODS

Findings

| DISCUSSION