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 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 three 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 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.