Zebra finch males are very colourful with orange legs and cheeks and bright red beaks, but the females are much plainer. So why are the males so colourful? The males’ red legs and beaks are full of colour pigments called carotenoids that are found in fruits, seeds and carrots. So Steve Leonard feeds half the males a diet high in carotenoids to make them redder. He then puts those ones in a special chamber next to a duller male who has not been fed the pigments. The female - separated from them by a pane of glass - is then allowed to choose a male by perching by his particular window. The result is that the female focuses her attentions on the male with the redder beak. In fact, in these tests, nine out of ten females preferred him. But why would they fancy the most colourful male? The answer is that the same red pigments not only make the male look good, but also help his body fight diseases. A dull male is probably weaker and not a good bet as a mate. So a pretty male does turn out to be a good choice of mate after all. A brightly-coloured male will also pass on his parasite-fighting genes to his babies. So it pays for a girl to demand the best.