Take one gorgeously indignant (almost apoplectic) mouse, one (almost unfeasibly) laid back bear and that (Almost Entirely Only) One Rhyme and what you get is a picture book that reads with breathless ease: an object lesson not only in rhyming repetition, but visual repetition within which there is to be found a riot of variety of colour, expression, mood and character interplay.
In many animals, parasite load influences body condition and colour expression [e.g. 49], [50], [51] but see [52] [54].
Consequently, pleiotropy in the genes responsible for colour expression would generate covariation between colour polymorphism and a number of individual attributes [10], [13].
Thus, we decided to include also plumage patches where colour expression acts as an indicator of quality or shows condition-dependence.
Structural colours differ from pure pigment-based colours in their mechanisms of development and physical composition [26], which probably affects temporal changes in colour expression, especially the fact that they often present high relative reflectance at short wavelengths.
Clearly different from the carotenoid-based plumage colours studied here, the bill of the blackbird showed the strongest changes in colour expression over the year of all studied patches (compare the scale of colour variation in Figure 3).
Colour expression could change due to microbial activity [3], ectoparasites [4], accumulation of dirt particles [5], feather abrasion [2], [6] [8], application of cosmetics [9], [10] or exposure to ultraviolet (UV) light [11], [12].
Although subtle environmental variation across the hybrid zone could explain the differences in parasite load and body condition, these were uncorrelated with colour expression, suggesting that the differences in coloration reflect heritable divergence.
For example, while pairing in the blue tit may take place early in the year, at the peak of colour expression, variation in male crown colour has been shown to affect female reproductive decisions as late as the chick feeding period [64], [65] when the plumage is considerably faded (Figure 4).
