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 relationship between breeding latitude and body mass during all post-embryonic life stages, from early nestlings through the oldest adults. We observed this same pattern with historic eggs, but contemporary eggs exhibited a negative relationship between latitude and egg size.

Main conclusions: A positive latitude-body size relationship was present just after hatching, suggesting a genetic component to this pattern. As latitude increases, selection for larger altricial nestlings may help explain the existence of this pattern in adults of migratory species, as even rare extreme cold weather events may cause mortality during this sensitive stage. Although this pattern was present in all contemporary post-embryonic stages and historic eggs, the reversal observed in contemporary eggs may be related to a complicated (and dynamic) selective landscapes that may be associated with ongoing global climate change. Although speculative, global changes may be in the process of altering this long-standing ecological principle, but more research is necessary.