In A Nutshell
Cuckoos (and some other birds species, like the cowbird) are known for laying their eggs in the nests of other birds in hopes of conning someone else into raising their young. Host species have been found to be fighting back, though. Some have developed a complicated system of patterns that seem to use a system similar to facial recognition software to allow birds to recognize their own eggs. Other birds have evolved to lay different colored eggs in one clutch, making it harder for cuckoo eggs to blend in. And if a yellow warbler discovers an intruder, they simply entomb the egg—along with their own—in the bottom of the nest and try again on top of the remains.
The Whole Bushel
The European cuckoo is one of the biggest jerks in the bird world. They’re not the only species that does it, but they’re certainly the most well-studied for their strange nesting adaptation that allows them to take advantage of the competition.
Female cuckoos lay their eggs in the nests of other birds. These other birds will, in theory, take care of the hatchling as if it was their own. The cuckoo hatchling usually doesn’t just end up as the weird one out in a happy family, though, as one of their first instincts is to throw everything else out of the nest. This way, they make sure they’re the only one the parent birds are caring for, which is often the only way the reluctant, adoptive parents can cope with the bigger, more demanding offspring.
It’s a phenomenon called brood parasitism, and about 40 percent of all cuckoo species do it, along with other species like the cowbird. Some birds will recognize the impostor eggs and get rid of them, while others, called acceptor species, will raise the chick as though it were its own.
Species that practice brood parasitism usually have eggs that look like the eggs of the other species. But now, researchers have found that species that are frequently the targets of brood parasitism have evolved a mechanism to help them find impostor eggs, and it’s a natural sort of equivalent to facial recognition software.
Researchers from Harvard University and the University of Cambridge developed some computer software to help our brains do what bird brains have evolved to do naturally: interpret the spots and speckles on bird eggs. They looked at the eggs of eight different species usually used as host parents by the common cuckoo, and when the computer mapped out the markings of the host eggs, they found a series of patterns. The resultant mapping was similar to how computers use facial recognition software to match names with faces. Host parents were laying eggs with specific, recognizable markings that indicated to the parents that they were real.
The researchers also compared the eggs’ distinctive patterns to watermarks on paper currency. The watermarks confirm legitimacy when checked by authorities, but counterfeiters will struggle to recreate them or even miss them entirely.
They’ve also found that in some cases, the same pattern is repeated in all the eggs laid by a particular species. In other cases, though, each female has her own distinctive pattern that identifies the egg as hers and hers alone.
Other birds—like the tawny-flanked prinia, a common target of the African cuckoo finch—have gone an entirely different route. While the cuckoo counts on their eggs looking pretty similar to the eggs of its chosen host, the prinia has evolved to lay all different kinds of eggs, with different colors and patterns all in the same nest. That makes the cuckoo’s job a lot harder, and when the prinia parents find an egg that doesn’t belong in their nest, they’ve been seen tossing out the intruders.
Yellow warblers, a frequent target of cowbirds, have been found doing something that is a little brutally terrifying. If yellow warblers find a cowbird egg in their nests, they don’t toss it out: They bury it with layers and layers of nesting material, sometimes alongside their own eggs. The birds sacrifice their own young to take out one of the enemy, and some birds do it over and over again, burying intruder eggs and their own. Some nests have been found with layers of up to five clutches of eggs, making the first home of countless warbler chicks the entombed remains of brothers, sisters, and enemies.
Show Me The Proof
Featured photo credit: Alan Manson
Phys.org: Birds evolve ‘signature’ patterns to distinguish cuckoo eggs from their own
Stanford: Brood Parasitism
BBC Earth: Some birds wage perpetual war against each other, leaving countless numbers of victims
Phys.org: Biological arms races in birds result in sophisticated defenses against cuckoos