A small bird about the size of a sparrow, the collared flycatcher is something of a celebrity among ecologists and evolutionary biologists. Every autumn, the bird leaves the Baltic island of Gotland, east of the Swedish mainland, to travel to Africa. It returns to its nordic home in May to mate and breed, making it possible for researchers to track the bird's life history from generation to generation.

“The special thing about these birds is that they migrate such a long way and then come back to almost the same spot,” says Anna Qvarnström of Uppsala University, who has studied the birds during their mating and nesting season every year for the past ten. “They are very nice to study too,” she says. “Some birds get nervous if you try to touch them, but you can handle flycatchers quite easily.”

Qvarnström's lab has been using the flycatcher (Ficedula albicollis) as a model for studies of sexual selection. Sexual selection drives the evolution of certain traits that will help an animal (usually a male) to attract a mate. Often the traits are very elaborate and conspicuous — take the peacock's tail. But how do mate preferences evolve? In other words, why would an animal (usually a female) spend time and effort on deciding who to mate? One theory suggests that females are looking to get ‘good genes’ out of the mating game.

Qvarnström set out to test the underlying assumptions of this theory using flycatchers in the wild. Male flycatchers have a white patch on their foreheads, the size of which renders them either more or less desirable to the opposite sex. If females like to mate with males with a larger patch and this mating provides good genes to offspring, then patch size and an affinity for large-patched males should evolve hand in hand.

But the study, published on page 84 of this issue, finds that this is not the case.

Taking advantage of data collected during the past 24 years by her lab and others at the university, Qvarnström and her colleagues examined the inheritance of patch size, female mate preference and fitness in 8,500 flycatchers. “If you look at all these aspects together, there is no correlation between the inheritance of mate preference and ornament,” she says.

The challenging part of this study was getting enough data for the pedigrees and good estimates of fitness, adds Qvarnström. “Many birds die during the migration so we needed data from many years.”

So what is driving the females' love of a large white patch? If the flycatcher female is not choosing her mate for his good genes, she may have other motives. “Females may be discriminating among potential mates in order to obtain suitable resources, such as a nice nest-site, rather than suitable genes,” points out Qvarnström.

She plans to continue her yearly visits to the birds as the lab explores different aspects of sexual selection. “It is nice to mark them as chicks,” she says, “then see them again the next year as adults and know that they have been all the way to Africa.”