Bergmann’s rule

Some birds are bigger in colder places. Is that mainly inherited adaptation, the conditions in which they grow, or changes in body condition?[1] [2]

Birds of the same species can be larger in colder parts of their range. What produces this pattern, and why do some measurements or species depart from it? Leg-bone length and body mass need separate treatment: skeletal growth can leave a lasting difference, whereas mass can also change with current reserves.

Inherited differences suited to local conditions are called local adaptation. Lasting effects of the environment during growth are developmental plasticity. Burrowing-owl measurements examine these possibilities alongside reversible condition changes. Comparisons of Australian museum birds also test whether geographic size trends match changes over time.[1] [2]

The pattern is not universal, and these causes are not completely separated.[1] [2]

Current observational studies constrain, but do not uniquely identify, their causal contributions across metrics and species.[1] [2]

A heavier bird may have a larger frame or simply more stored reserves. Those differences tell different stories about inheritance, early growth and present conditions. Understanding them helps explain what a climate-related size pattern can tell us about how animals respond to their environment.[1] [2]

Geographic differences can reflect evolutionary history, conditions experienced while growing or the resources available now. Each gives a different interpretation of an animal’s form and its response to environmental change. The mismatch between spatial and temporal trends warns that a present-day climate gradient alone cannot tell us how morphology will change during warming.[1] [2]