Associations between colour parameters
We found a negative association between carotenoid chroma and UV chroma of yellow breast feathers, in line with previous results in other study populations (Johnsen et al. 2003; 2005). This negative association may be due to the fact that higher amounts of carotenoid pigments in the feathers partly conceal feather structures, which results in lower UV reflectance. However, this does not imply that an increase in one of these two colour parameters inevitably leads to a reduction in the other (Jacot & Kempenaers 2007). Indeed, each colour parameter results from a combination of metabolic pathways (pigment-based and structural colouration; Hill 2006): while carotenoids are acquired from food and deposited in the feathers, UV reflectance is produced by an entirely different mechanism related to feather synthesis and it relies on the precise organization of the network of barb and barbules. Moreover, we found that yellow brightness was strongly and positively associated with UV chroma, but not with carotenoid chroma. Hence, the overall reflectance of yellow breast colouration indicates to a large extent the reflectance in the UV region of the spectrum. This is particularly relevant in our model system, since UV colouration is more easily perceived by cavity-nesting birds than carotenoid-based reflectance (Hunt et al. 2003; Avilés et al. 2006; Wiebe and Slagsvold 2009; Wȩgrzyn et al. 2011).