Migratory shorebird adheres to Bergmann's Rule by responding to environmental conditions through the annual lifecycle

Abstract

The inverse relationship between body size and environmental temperature is a widespread ecogeographic pattern. However, the underlying forces that produce this pattern are unclear in many taxa. Expectations are particularly unclear for migratory species, as individuals may escape environmental extremes and reorient themselves along the environmental gradient. In addition, some aspects of body size are largely fixed while others are environmentally flexible and may vary seasonally. Here, we used a long‐term dataset that tracked multiple populations of the migratory piping plover Charadrius melodus across their breeding and non‐breeding ranges to investigate ecogeographic patterns of phenotypically flexible (body mass) and fixed (wing length) size traits in relation to latitude (Bergmann's Rule), environmental temperature (heat conservation hypothesis), and migratory distance. We found that body mass was correlated with both latitude and temperature across the breeding and non‐breeding ranges, which is consistent with predictions of Bergmann's Rule and heat conservation. However, wing length was correlated with latitude and temperature only on the breeding range. This discrepancy resulted from low migratory connectivity across seasons and the tendency for individuals with longer wings to migrate farther than those with shorter wings. Ultimately, these results suggest that wing length may be driven more by conditions experienced during the breeding season or tradeoffs related to migration, whereas body mass is modified by environmental conditions experienced throughout the annual lifecycle.

Generalized evidence for Bergmann’s rule: body size variation in a cosmopolitan owl genus

“…Being conceived for endotherms, where patterns coherent with such a prediction have been observed in a plethora of taxa in many regions of the globe (e.g. Ashton, 2002Ashton, , 2004Brown & Lee, 1969;Gibson et al, 2019;Meiri & Dayan, 2003;Olson et al, 2009;Rodríguez et al, 2006;Torres-Romero et al, 2016), recent studies have confirmed the validity of this biogeographical rule also in some ectotherms (e.g. Arnett & Gotelli, 1999;Ashton & Feldman, 2003;Chown & Gaston, 2010;Olalla-Tárraga & Rodríguez, 2007;Olalla-Tárraga et al, 2006;Pallarés et al, 2019).…”

“…Ashton et al, 2002;Ashton, 2004;Blackburn et al, 2019;Blackburn & Ruggiero, 2001;Meiri, 2011;Meiri & Dayan, 2003;Olson et al, 2009) and at the within-species level on a continental geographical scale (e.g. Fan et al, 2019;Gibson et al, 2019;Graves, 1991;James, 1970;Jones et al, 2005). However, comprehensive intraspecific analyses on cosmopolitan taxa are almost null (but see Murphy, 1985).…”

Bergmann's rule is followed at multiple stages of postembryonic development in a long‐distance migratory songbird

“…Previous tests of Bergmann's rule have come to inconsistent conclusions when focusing on migratory organisms, with some taxa displaying weak or no adherence (Ashton, 2002; Collins et al., 2017) and others exhibiting adherence across breeding (e.g., Jones et al., 2005) and nonbreeding distributions (e.g., Gibson et al., 2019). The occurrence of the latitude‐body mass cline nearly immediately after hatching that we observed in prothonotary warblers provides a unique perspective that may help explain some of these inconsistencies observed in migratory species.…”

“…Additionally, many migrants (e.g., Nearctic‐Neotropical avian species) typically only encounter temperate environments during a relatively short breeding season, which may not represent a strong enough selective pressure for this pattern to become established. Thus, because of this putative lack of a strong evolutionary mechanism, and the possibility that multiple mechanisms contribute to latitudinal variation in size (Blackburn et al., 1999), adherence to Bergmann's rule may be considered paradoxical in migratory species and has been understudied (but see Collins, Relyea, Blustein, & Badami, 2017; Gibson et al., 2019; Jones et al., 2005; Van Buskirk, Mulvihill, & Leberman, 2010).…”

“…It is possible that the perception of a lack of selective pressures that could result in the expression of Bergmann's rule in migratory species may be related to an overwhelming focus on adult organisms (e.g., Gibson et al., 2019; Husby, Hille, & Visser, 2011; Murphy, 1985). Selective forces related to thermoregulation (and hence body size) may in fact be strongest during juvenile stages, as these endotherms typically develop during specific thermal conditions (Salewski, Hochachka, & Fiedler, 2010).…”