Typically, we think of this in the context of relatively sedentary cold-adapted species like wolves or bears, where the more northern individuals are bigger and rounder than their more southern relatives.  However, for long distance migrant birds, like nightjars, which breed in the north but winter in warm, equatorial conditions, it is unclear which part of the annual cycle has most influence on Bergmann’s rule.

The main aim of this paper was to  investigate how variation in body size and migration, in Nightjars, is influenced by environmental and geographic conditions on the  breeding grounds. Three migrant northern hemisphere nesting species were studied, including  European Nightjar (Caprimulgus europaeus), Eastern Whip-poor-will (Antrostomus vociferus) and Common Nighthawk (Chordeiles minor).  GPS-tracking data were used to link individuals to breeding and wintering sites, from which geographic and environmental data were extracted, along with migration distance and morphometric data.

The findings showed that environmental and location variables within the  breeding range were the strongest predictors of body size, rather than at  wintering range variables. These showed that body mass (and wing length) within the breeding grounds increased northward for all three species, thus conforming to Bergmann’s rule. However, latitude and longitude correlated strongly with environmental variables and migratory distance; thus, these geographical variables likely encompass many factors that influence body size in nightjars. 

The present study is among the first to use tracking data from individual birds to understand how environmental pressures across the annual cycle are related to body size. The findings highlight the critical role of geographic breeding-ground factors in shaping Bergmannian patterns, offering robust evidence to support nearly two centuries of research.