Bergmann's rule is followed at multiple stages of post-embryonic development in a long-distance migratory songbird

Aim: We assessed the presence of Bergmann’s rule in a model migratory species across all life stages on their breeding grounds. We then compared historic and contemporary egg sizes to understand if the biogeographic patterns during this developmental life stage have changed over time.

Location: Eastern North America.

Time period: 1865 to 2019.

Major taxa: Prothonotary warbler, Protonotaria citrea

Methods: We measured the size of eggs, young and old nestlings, and adults (of two age classes and both sexes) from across the full latitudinal breeding range of the species to assess the relationship between latitude and body size across several life stages in this altricial migratory species. We also assessed potential changes in this pattern over time by comparing historic eggs (museum specimens collected across a similarly broad latitudinal range) with the contemporary latitudinal-egg size relationship. 

Results: In accordance with Bergmann’s rule, we found a positive relatio...