“Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons,” wrote one Charles Darwin in On the Origin of Species. Four years earlier, Darwin had taken to raising pigeons in his own dovecote, hobnobbing with other pigeon fanciers, and carefully measuring the birds. In the diverse breeds, with their fantails, feather-duster feet, and frilly backs, Darwin saw validation for his ideas about evolution. If people could artificially select for such astonishing diversity in just a few generations, nature was surely capable of far more over longer timescales.
Now, 160 years after Darwin published his opus, a team of biologists from the University of Utah have once again turned to pigeons to demonstrate evolution in action. But instead of focusing on the birds themselves, they turned their attention to the pigeons’ parasites.
In a simple experiment, Sarah Bush and Scott Villa placed feather-eating lice on different-colored pigeons and left them there to breed and evolve—for four years. Over that time, the insects adapted to better match the color of their host, which made them harder to spot and pluck off.
For Bush, this was “incredibly exciting”—an experiment that reminded her of the peppered moths she learned about in high school. Those insects normally have speckled white-and-black wings to camouflage against tree bark. But in 19th-century England, when coal factories blanketed trees with soot, the peppered moth quickly evolved into all-black forms. In doing so, it became a textbook example of evolution. Now perhaps Bush’s feather lice can join them.
Feather lice are small, wingless insects that spend their whole lives among the plumage of birds, eating feathers and flakes of skin. The discovery of a 44-million-year-old fossil louse with feathers in its gut suggests that “they’ve been doing the same thing since forever,” says Bush. Today, “there’s pretty much one species of louse per species of bird.” Their presence isn’t welcome, though, and birds will try to preen them from their plumage. The lice, in turn, hide through camouflage: In 2010, Bush and her husband, Dale Clayton, showed that lice tend to match the color of their host’s feathers.