Achromatic plumage variation between and within hybridizing Black-capped and Mountain chickadees

Feather coloration and patterning are major signals influencing mate choice within and between species. However, most studies of the role of plumage in mate choice have focused on colorful species with obvious sexual dichromatism. To better understand how achromatic plumage might influence hybridization, we quantified plumage variation between and within two achromatic songbirds that occasionally hybridize, Black-capped (Poecile atricapillus) and Mountain (P. gambeli) chickadees. We collected feathers from 43 live birds and photographed 155 prepared museum specimens to measure overall plumage color and the size of the throat and cheek patches. Using principal component analyses and generalized linear mixed models, we characterized plumage patterns within and between Black-capped and Mountain chickadees from Colorado to examine plumage color variation and differences in throat and cheek patch size. We found that Black-capped Chickadees (1) were less achromatic and had brighter plumage with more color contrast than Mountain Chickadees, (2) had smaller throat and cheek patches than Mountain Chickadees, and (3) were not sexually dimorphic. We also found that male Mountain Chickadee museum specimens had brighter plumage with more ultraviolet reflectance than female museum specimens (i.e., they are sexually dimorphic). However, we did not observe sexual dimorphism in live Mountain Chickadees, potentially because we did not sample the supercilium. In contrast to previous studies, we found that Black-capped Chickadees are not sexually dimorphic, potentially because plumage is not used in mating decisions for populations at lower latitudes. Between Black-capped and Mountain chickadees, differences in plumage color and patch sizes may influence hybridization if female Mountain Chickadees prefer the brighter plumage and greater color contrast of male Black-capped Chickadees. Our results will guide future work exploring the role plumage might play in hybridization between Black-capped and Mountain chickadees by informing plumage manipulation experiments investigating the influence of brighter plumage and color contrast in hybridization.