Blue-footed boobies dirty their eggs to hide them from predators

If you lay something on the ground, it’s bound to get dirty. And that’s a good thing — at least for blue-footed boobies, which lay their eggs in shallow nests dug into the dirt. The dirtier the egg, the less likely it is to get eaten, a new study finds.

The boobies lay beautiful blue eggs that quickly turn white and then slowly acquire the color of the ground they’re laid on as mom and dad rotate and jostle the eggs with their feet. Within a couple of weeks, the eggs match the soil. Is there a point to the coloration? Or are the eggs just dirty?

Fernando Mayani-Parás of the National Autonomous University of Mexico in Mexico City and colleagues examined data from more than 3,600 eggs laid from 1981 to 2011 on the Mexican island of Isla Isabel. And they closely monitored eggs laid in dozens of nests in 2012.

The eggs proved to be very vulnerable when they were still relatively white. Of the 3,668 eggs laid in that 20-year period, 47 percent were lost to predation in the first five days, the researchers report in the October American Naturalist . Attacks, mostly by Heermann’s gulls , declined in the days afterward and then, at about 15 days of incubation — just about the time the eggs were dirtiest—the rate of predation leveled off.

But parent birds might be acting differently in those first days after egg laying, and that might account for the differing rates of predation. So the researchers removed the parents, laying chicken eggs, which are similar in size to booby eggs, in artificial nests. “Soiled eggs were less likely to be taken [by gulls] than clean ones,” the researchers write. And the dirty eggs were also more likely to be destroyed by boobies wandering by than were clean ones.

With a species of bird that lays its eggs on the ground, we might expect that those eggs would have evolved some sort of camouflage that helps them blend in and provides protection against predators. But that built-in camouflage may not be helpful if the birds don’t always nest on the same kind of ground. “By darkening egg color behaviorally, boobies can potentially exploit a greater range of nest substrates than might otherwise be possible because the extent of egg camouflage can be flexibly manipulated to suit the local nest environment,” the researchers suggest.