Testing the simple and complex versions of Gloger’s rule in the Variable Antshrike (Thamnophilus caerulescens, Thamnophilidae)

Gloger’s rule is a classic ecogeographical principle that, in its simplest version, predicts animals should be darker in warmer and wetter climates. In a rarely tested more complex version, it also predicts animals should be more rufous in warmer and drier climates. The Variable Antshrike (Thamnophilus caerulescens) is a widely distributed South American passerine that presents an impressive amount of plumage color variation and occupies a wide variety of climatic conditions. Moreover, genetic and vocal evidence indicate ongoing hybridization in south-central Bolivia among 3 populations with very distinct plumages. We collected color data from 232 specimens from throughout this species’ distribution to test the predictions of Gloger’s rule. We found a negative correlation between brightness and precipitation, consistent with the simple version of Gloger’s rule. In contrast, we found that birds were darker in cooler climates, contrary to the simple version of Gloger’s rule, but consistent with recent findings in other taxa. We found support for both predictions of the complex Gloger’s rule and suggest it might be driven by background matching. We conclude by concurring with a recent suggestion that the simple version of Gloger’s rule should be reformulated exclusively in terms of humidity.