Andreas Nord , Elin Persson, Joshua Kenneth Robertson Tabh , Elisa Thoral

In a recent and impressive analysis of avian morphological data (covering >250 000 male birds from 105 species), Youngflesh et al.1 report that birds breeding in North America have become significantly lighter over the last three decades, coincident with rising breeding season temperatures. Because these observations recapitulate predictions under Bergmann’s Rule (i.e., that the body size of congeners and conspecifics is usually larger near the cooler poles than near the warmer equator), the authors argue a thermoregulatory benefit to their occurrence under the assumption that smaller-bodied animals have lower cooling costs than larger-bodied animals in a warming world. We agree that warmer environments during reproduction may well explain avian body size declines. However, we question whether: (1) changes in heat balance attributed to these declines are sufficiently large to explain observed size reductions, and (2) increased thermoregulatory efficiency during the short windows where reproduction occurs is always relevant, particularly when tenancy in breeding ground temperatures is short (i.e., among migrants). Generalizability of these results may be limited further by sex-specific enquiry, which overlooks the possibility of divergent selection on body size in males and females under climate warming. In this commentary, we expand on these concerns hoping to instigate discussion on knowledge gaps that need closing if we wish to better understand the proximate and ultimate drivers of shapeshifting animals in a changing world.