Chaffinches are common birds across Europe. The bright colours of the male Chaffinch include a beautiful deep salmon-pink face and breast, with an iridescent blue head and shoulders, with white wing bars and yellow stripes. Males do everything they can to make themselves gaudy and attract females in the breeding season. However, all those colours in their feathers come at a considerable cost. They use a great deal of their available energy to create the complex chemicals which make such intense and beautiful colours in their feathers.
Many of the most spectacular colours of birds aren’t actually pigments at all. Those blue feathers probably have no blue pigment in them. The colour comes from the microscopic structure of the feathers themselves. It’s iridescence caused by structural colouration from wave interference. It’s not blue, it just looks blue.
Birds on a plant-based diet find it especially difficult for to get all the energy they need to make themselves attractive, compared with those feeding on nutritious insects. It’s also difficult for male birds which are ill with viruses or blood parasites to make the bright, colourful plumage they need for breeding success. That’s because unwell birds spend their energy fighting infections rather than decorating themselves. You can’t be bright and breezy when you’re under the weather.
Female Chaffinches prefer brightly coloured males because they show clear, visible signs of having a good diet and good health. That means the chicks that come from the eggs they lay will have a good chance of survival; they will grow up to find good quantities of healthy food and resist common illnesses. That bright beauty, however, is a thin disguise. It’s only the visible tips of the feathers which are brightly coloured and all the rest, buried underneath, are much duller. All it takes is a gust of wind to show the dull feathers hidden underneath.
Beauty really is just feather deep.