Male birds are often colourful and ornate. These embellishments demonstrate the wearer is a suitable candidate for fatherhood and is not to be trifled with by other males. Why females are sometimes colourful too is more of a mystery.
One idea is that if the sexes co-operate to raise their young—which birds often do—males as well as females must be choosy about their mates. But Jay Falk of the University of Washington has another explanation. In a paper in Current Biology he suggests it is a way for females to avoid being harassed when they are feeding.
Plumage transvestism, known technically as female-limited polymorphism, is especially common in hummingbirds. Earlier work involving Dr Falk, who was then a PhD student at Cornell University, showed that it has evolved independently in all nine big groups of these birds and is found in nearly a quarter of hummingbird species. This makes them ideal for studying it. But that work was done on museum specimens. Mr Falk, as he then was, wished to take the question into the field.
His reason for doubting sexual selection as the explanation for female-limited polymorphism in hummingbirds was that males of this group do not help raise the young. He therefore tested an alternative—that male-like plumage in females confuses real males. To do so, he decamped to Panama, and spent four years studying a species called the white-necked jacobin.
He started by collecting and recording the details of a lot of birds, eventually fitting some with tiny transponders so that he could follow and identify individuals. Over the course of the study, he captured 436 jacobins, of which he managed to recapture 135 in at least one subsequent year.
Read the full article in The Economist.