Variability, heritability and condition-dependence of the multidimensional male colour phenotype in a passerine bird

Elaborate ornamental traits are commonly assumed to be honest signals of individual quality, owing to the presumed costs involved in their production and/or maintenance. Such traits are often highly variable, possibly because of condition-dependence and/or high underlying genetic variation, and it has been suggested that their expression should be more sensitive to condition and/or more heritable than non-ornamental traits. Many bird species display colourful plumage with multiple distinct patches of different developmental origins, forming complex colour phenotypes. Despite this complexity, colourful ornaments are often studied in isolation, without comparison to suitable non-ornamental controls. Based on plumage reflectance data collected over 8 years, we assessed the signalling potential of the multidimensional male colour phenotype in a tropical bird: the purple-crowned fairy-wren Malurus coronatus. Specifically, we tested the predictions that the expression of putative ornamental colours (purple and black - the breeding colours - and blue) is (1) more variable, (2) more heritable and (3) more condition-dependent compared to year-round non-ornamental colours (buff-white and brown). Our results show that ornamental colours exhibit greater levels of variability, and some chromatic components of purple and blue colouration appear slightly heritable (h² = 0.19-0.30). However, contrary to predictions of heightened condition-dependence in ornaments, only brightness of the buff-white and brown colouration increased with male body condition, although brightness of the purple colouration was related to male age as expected. Despite partial support for predictions, the lack of consistent patterns illustrates the complexity of visual signals and highlights the need to study colour phenotypes in their entirety.