Plumage coloration is shaped by a suite of factors including sexual selection, predation risk and diet. Bright colours, such as yellows, that are seen in many species, are often associated with males and attributed to sexual selection, indicating quality as a mate. Interestingly, most evolutionary transitions in plumage coloration have tended towards increasing similarity across the two sexes, driven by natural selection.
Research carried out in North America suggests that certain ecological conditions, often coupled with particular types of mating system, favour more brightly coloured males because of increased competition for mates. These conditions may not apply as readily here in the UK, so we have fewer brightly coloured species. However, it is difficult to untangle the influence of other factors, such as predation risk or the availability of carotenoid-rich foods, which allow the development of bright yellow pigments.