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.
Sexing hummingbirds is hard. As with most avians, the males have no penis. To work out a bird’s sex, he took a blood sample. This showed that nearly 30% of females had male-like colouration. A jacobin’s age, though, is easy to estimate from its beak. Younger birds’ beaks have more serrations. On this basis, he discovered that all young jacobins, regardless of sex, sport male-like plumage. Only later in life do some females develop distinctive muted colouration—a fact confirmed by some of the recaptures, which had changed plumage in the intervening period.
This is the reverse of normal for sexual selection, in which there is no point in immature individuals pretending they are mature, nor mature females pretending to be males. So he carried out experiments at hummingbird feeding stations filled with sugarwater to try to find out what benefits adult-male-like plumage brings.
First, he put pairs of stuffed hummingbirds by these stations—mixing males, females with male plumage, and females with female plumage—and recorded how they were treated. A clear pattern emerged. As might be expected, stuffed females in female plumage received a lot of sexual attention from males, whilst those in male plumage were spared. More surprisingly, they were also on the receiving end of more aggression, perhaps because males see females as easier to drive away from contested food sources. Both courtship and hostility, though, distract from the serious business of feeding, and are therefore harmful.
He then started varying the amount of sugar in the sugarwater, while measuring the frequency and length of visits to feeders. Female birds with male-like plumage, he discovered, fed more frequently and for longer than those with muted colours. At feeders with higher sugar concentrations, the difference was yet more pronounced—probably because, the stakes being higher, birds with muted colours were experiencing more harassment.
And that, perhaps, explains why hummingbirds as a group have evolved female-limited polymorphism so often—for, of all known vertebrates, they have the highest metabolic demand per gram of body weight. Anything detracting from feeding will get short shrift from natural selection. If looking like an adult male while actually being juvenile, female or both gets the job done, such plumage will evolve.
The remaining puzzle is why some adult females, having adopted the disguise as juveniles, then go on to abandon it. Perhaps the muted female colours make them less conspicuous to predators, especially at their nests. But polymorphisms of this sort are normally maintained because each option is advantageous when rare in a population, and disadvantageous when common. The details of how such a trade-off—between access to food and protection from predators—would work in this case remain unclear. Dr Falk has therefore only half-answered the question he set out to investigate. Answering the other half might require another trip to Panama. ■