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.