Clinal variation in morph frequency in Swainson’s hawk across North America: no support for Gloger’s ecogeographical rule

Abstract

In polymorphic species of birds, clinal variation in plumage coloration is frequently observed. These clines often follow Gloger’s ecogeographical rule, which predicts that greater numbers of dark morphs will breed in wetter areas. More recent research on polymorphic raptors has suggested this pattern might be due to a cryptic advantage of dark morph hawks foraging in conditions with lower solar radiation during their breeding season. We explore this question for Swainson’s hawk (Buteo swainsoni) using the MORPHIC internet application, which sources photographs from the internet and helps to build spatial datasets of visible phenotypes. Using these data, we test for clinal variation in the proportion of dark morph Swainson’s hawks and examine which environmental variables are associated with the morph distribution in the breeding range of this species. We found good evidence for a long- suspected latitudinal cline, with the highest proportion of dark morphs occurring within the west of the breeding range of the species, and a decreasing frequency toward the east of the range. The distribution of these morphs was most closely associated with rainfall and temperature. However, in contrast to Gloger’s rule, we found that dark morphs were more common in drier and cooler areas of their breeding range, conforming to the thermal melanism hypothesis. Similarly, we found no evidence that dark morphs were associated with areas of lower solar radiation. Taking this evidence together, polymorphism in Swainson’s hawks may be affected largely by the energetic needs of individuals on their breeding grounds rather than their non-breeding grounds.