Because some female superb fairywrens sing when they are either inside the nest or close to it—instead of farther away as do the males—they inadvertently reveal its location to predators, the scientists say. “That’s a high cost for singing,” says Kleindorfer, who adds it is “clearly” creating evolutionary pressures on the females. Indeed, females never initiated the chatter song while inside their nests, instead only singing in response to their mate. And half of the females studied (six out of 12) never uttered one twitter while inside or near their nests—a difference that may be due to learning or some other factor, because singing is a variable behavior, not a fixed trait. That means that the female fairywrens (and songbirds in general) are not programmed to sing, but can change when, where, and how often they croon in response to what’s happening around them