To see how quickly the lice can adapt, Bush, Clayton, and their colleagues captured regular urban pigeons from around Salt Lake City and fumigated their feathers with carbon dioxide. Lice fell off them in droves, and the team transferred 2,400 of these insects onto 96 captive pigeons—some white, some black, and some gray.

Read: The lonely lives of dolphin lice

For four years, the pigeons did whatever pigeons do. Meanwhile, for the lice, oceans rose, empires fell, and 60 generations came and went. Over that time, their colors changed. The lice on black pigeons became slightly darker, the ones on white birds became much brighter, and the ones on gray birds stayed the same.

But these changes occurred only if the pigeons could preen themselves. Bush stopped half the birds from doing so by fitting them with poultry bits—plastic clip-ons that prevented them from closing the very tips of their beaks. On those birds, the lice suffered no risk of removal, and their colors stayed the same. (The birds that couldn’t preen also ended up with 20 times as many lice—a clear sign of the strong evolutionary pressure that a beak can exert.) This clearly shows that the lice don’t automatically blend in when they arrive in a new environment. They do so specifically to avoid the attention of their hosts.

Bush et al. 2019

All the lice that Bush used belonged to the same species—Columbicola columbae. But by the end of the experiment, these individuals began to resemble other species that have been parasitizing different pigeons for millions of years. For example, the lightest individuals were just as light as Columbicola wolffhuegeli, a species that lives on one of the whitest pigeons—the pied imperial of Australia.

C. columbae and C. wolffhuegeli have been evolving independently for at least 20 million years, but in just four years, the former had changed enough to resemble the latter in color (although many other differences separate the two species).

This study reminds me of another ambitious evolutionary experiment that I wrote about earlier this year. In the hills of rural Nebraska, Rowan Barrett and his colleagues placed mice in large outdoor enclosures, built on light sand or dark soil. Over time, individuals that better matched their backgrounds were less likely to be eaten by owls—just as lice that blended in among their host plumage were less likely to be preened off.

Read: The wild experiment that showed evolution in real time

Barrett’s team went one step further, though. It identified a gene that’s responsible for the rodents’ fur color, and it showed how variations in that gene became more or less common over the course of the experiment. It would be great if Bush and her colleagues could do the same for their lice, says Jessica Light from Texas A&M University. Still, as it stands, their experiment is already groundbreaking: Biologists “rarely, if ever” do evolutionary studies of this kind with parasites, says Light.