Different components of the Great Bowerbird (Chlamydera nuchalis) multimodal signal associate with distinct male traits

Females may use male sexual signal components to assess male quality. The signals of male Great Bowerbirds (Chlamydera nuchalis) include the bower and its decorations, plumage characteristics and a ritualised vocal and physical display. We investigated morphological, constructed, physiological and behavioural traits to estimate their effect on mating success. Over 2 years, we measured the number of bower decorations, the length and spectral properties of male nuchal crests, circulating testosterone (T-levels) and quantified male displays when solitary (Display Rate, year 1) or with a visitor present (Courtship Rate, year 2). We found that crest-length and luminance, and several decoration types, predicted T-levels in both years. Some decoration types predicted both T-levels and crest-length, but other decoration types predicted crest-length but not T-levels. Decorations that predicted Display Rate or Courtship Rate were independent of those predicting T-levels and crest characteristics. Thus, females could use different classes of bower decorations to assess male behaviour, morphology and endocrine status, with some signals being redundant and others serving as independent signals of male qualities. Although many species have complex multimodal signals, few construct signal elements detached from morphology. Bowers provide a unique constructed signal allowing females to evaluate male signals either in the absence or presence of the male. This temporal and spatial separation of signal elements may add to the complexity of signal relationships in bowerbirds but potentially provides females with more information than would be possible with constitutive male traits alone.