Complementarity in Allen's and Bergmann's rules among birds

Area of Science:

• Zoology
• Ecology
• Evolutionary Biology

Background:

• Endotherms often exhibit larger bodies (Bergmann's rule) and shorter appendages (Allen's rule) in colder climates.
• Many species deviate from these established ecological rules, with limited explanations for these exceptions.

Purpose of the Study:

• To investigate the complementary changes in body and appendage size in response to thermal gradients.
• To explain frequent exceptions to Bergmann's and Allen's rules in non-migratory terrestrial birds.

Main Methods:

• Analysis of geographic, ecological, phylogenetic, and morphological data for 6,974 bird species.
• Correlating changes in bill size with changes in body size across thermal gradients.
• Examining species richness, distribution, and ecological constraints in relation to trait variation.

Main Results:

• A negative correlation was observed between family-wide changes in bill size and body size across thermal gradients.
• Most bird families showed subtle, coordinated adjustments in both body and bill size, supporting rule complementarity.
• Families with pronounced gradients in either body or bill size were often more speciose, widely distributed, or ecologically restricted.

Conclusions:

• Bergmann's and Allen's rules are validated through the concept of complementary trait adjustments.
• Body and bill size serve as proxies for the underlying surface-to-volume ratio, a key factor in thermoregulation.
• Understanding these complementary adaptations provides insights into evolutionary responses to climate change.