Seasonal but not sex-biased gene expression of the carotenoid ketolase, , in the sexually dichromatic southern red bishop ().

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic gene expression in the seasonally and sexually dichromatic southern red bishop () in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids.