Extent of a white plumage patch covaries with testosterone levels in female Pied Flycatchers Ficedula hypoleuca

It is not clear at present if variation in testosterone (T) levels is associated with variation in plumage signal expression in female birds. In Iberian populations of Pied Flycatchers Ficedula hypoleuca, some females exhibit a distinctive white forehead patch similar to that found in males. Both sexes also exhibit conspicuous white patches on wings that vary greatly in size. Males show markedly larger wing patches. These patches are exhibited in social interactions. We have aimed at detecting if naturally occurring variation in T circulation is linked to the expression of forehead patch presence and folded wing white patch area in female Pied Flycatchers. There was marked variation in female plumage patches and in T levels. The area of the wing white patch but not the presence of a forehead patch in females was associated with higher circulating T levels during the incubation phase when controlling for female age and hatching success. Older females, and females suffering a reduced hatching success, also presented lower levels of T. Our study indicates that a female plumage trait that is equivalent to a sexually selected trait in males may signal T levels. Females, like males, may use plumage traits to signal their T-mediated aggressive disposition.