Why female house finches prefer redheads over blonds

Walk into any clothing store and you’ll see that the women’s section is more colourful (and bigger!) than the men’s section. Not so in the bird world. In most bird species, males are more colourfully and elaborately dressed than females. This type of sexual dimorphism in which males and females look dramatically different is frequently driven by sexual selection. That is, female preferences for bright colours and ornate decorations have pushed each successive generation of males to evolve more and more flamboyant plumage. But why do female birds prefer brightly coloured males in the first place?

One idea is that ornamental traits like coat colour are indicators that can provide useful information about important survival traits. For example, the theory of parasite-mediated sexual selection proposes that the quality of a male’s ornamental display signals how well he can resist infection by parasites. To test this theory, many researchers are turning to the house finch, a common bird found across North America. Males have variable red-to-yellow colouration on their head, breast and rump whereas female finches are a rather boring greyish brown in colour. The bright red males enjoy more popularity among the females than their more drab yellow rivals. Many factors contribute to male colouration, including nutrition and parasite exposure during feather growth.

Male house finches vary in their colouration from red to yellow. (Image: Diane Pierce, National Geographic)
Male house finches vary in their colouration from red to yellow. (Image: Diane Pierce, National Geographic)

Speaking of parasites, let’s talk about the complex relationship between house finch feather colour and parasite infection. Several studies, including this one in 2004, showed that male house finches infected with the bacteria Mycoplasma gallisepticum develop more yellow and less bright feathers than uninfected males fed the same diet. Yellow males also seem to be at a disadvantage when it comes to surviving an infection. In the mid-to late-1990s, an M. gallisepticum epidemic hit the house finch population in the eastern United States. Surveys conducted before and after the epidemic found that red males survived better than yellow males, which drove the eastern house finch populations to become more homogenously red than populations in the rest of the country. In laboratory experiments, red males from unexposed populations resolved symptoms of M. gallisepticum infection faster than yellow males. So what exactly is going on? How does feather colour affect parasite infection and vice versa? Continue reading