Data and code from: Bergmann’s rule: Why does body size increase with latitude?

Bergmann’s rule describes the tendency for endothermic body size to
increase with latitude, a pattern often attributed to climatic factors.
However, the underlying developmental and evolutionary mechanisms remain
debated. Latitudinal gradients in temperature and precipitation are often
thought to be responsible for Bergmann’s rule, but climate may generate
such geographical patterns via several alternative pathways. We examined
geographic patterns in body size of 5,596 burrowing owls (Athene
cunicularia) throughout western North America to inform the underlying
processes responsible for range-wide phenotypic variation. Burrowing owls
followed Begmann’s rule, with larger individuals in northern latitudes. We
also detected a longitudinal gradient, with the largest owls in the
northwest of the breeding range. We considered several mechanisms to
explain geographic gradients in burrowing owl body size: developmental
plasticity, reversible plasticity, and local adaptation. Developmental
plasticity links body size to early-life environmental conditions, causing
permanent changes; reversible plasticity involves temporary physiological
shifts; local adaptation reflects evolutionary changes optimizing body
size to local conditions. We tested predictions generated by each of these
three mechanisms to assess how resource availability and climate shape
continental body size patterns in burrowing owls. Spring precipitation
explained variation in adult body mass, with contrasting effects in warm
versus cold climates, supporting both local adaptation to thermal extremes
and reversible plasticity in response to resource availability. Drought
explained variation in juvenile body mass, supporting the developmental
plasticity mechanism via resource availability. Juvenile mass was more
influenced by maximum temperature during the prior breeding season than by
natal year conditions, suggesting that extreme heat may affect parental
condition or suppress resource availability into the following year,
consistent with developmental or reversible plasticity. Extreme
temperatures and drought during the prior breeding season explained
variation in adult tarsus length, more so than 21-year mean climate
conditions or geographic gradients, supporting the developmental
plasticity mechanism via thermal or drought stress. Body size of burrowing
owls largely conforms to Bergmann’s rule and appears to reflect both acute
and evolutionary responses to resource availability and thermoregulatory
mechanisms depending on whether a trait was more plastic (body mass) or
more fixed (tarsus) during adulthood.