These animal mothers expect other species to raise their young

A mother stealthily approaches a stranger’s home. Darting inside, she drops her bundled baby in the nursery, where another baby is already sleeping in the crib. She runs for her life.

If a human did it, we’d be shocked and horrified. But this is more or less the parenting strategy followed by many of the world’s animals, including about 1 percent of bird species. In some, the abandoned baby in this analogy would next burst free of its swaddle and throw the other infant out the window. 

Although these babies and parents may offend our mammalian sensibilities, their species have evolved their own way of showing they care.

Dumping your baby with other parents or caretakers, in the animal world, is called brood parasitism. It requires finding hosts who are conscientious caretakers themselves. Maybe that’s why it’s so common among birds. The vast majority of bird species have two caring parents: a mom and dad who work together, sometimes with additional helpers, to build a nest, keep the eggs warm, and feed the chicks until they can fly off on their own. If an expectant bird mother manages to sneak her egg into another bird couple’s nest undetected, she knows these strangers will take good care of her offspring.

Probably the most famous brood parasite is the common cuckoo, frequently spotted across Europe and Asia. In North America, the brown-headed cowbird also depends on other species’ parents to raise its young. Still more bird species are able to raise their own young but, in desperate times—maybe they can’t find a nesting site, for example—will dump their eggs in someone else’s nest. 

That means many of the birds we see gracing our neighborhoods or forests may have a dark backstory.

The long con 

A cuckoo mom’s heist starts even before she lays an egg. The common cuckoo has similar plumage to a raptor called a Eurasian sparrowhawk, which may intimidate other birds into giving her space while she scopes out their nests.

When she’s ready to lay, the female cuckoo targets a nest of her preferred host species that already has eggs in it. She has to get the timing just right, so her egg will develop alongside its foster siblings. When the coast is clear, she darts onto the nest. She picks up one or more of the host eggs in her beak, swallowing them whole, then lays her own egg in the space she’s made. Within seconds, she’s flying away, never to return.

(Before naturalists figured out exactly what cuckoos were up to, one hypothesis was that cuckoos deposited their parasitic eggs by vomiting them up—since hunters had sometimes killed cuckoos with these stolen eggs still in their gullets.) 

Now the mother’s job is done, and the baby cuckoo’s work begins. 

The cuckoo newborn hatches a little sooner than its nestmates of a different species. Then the helpless-looking hatchling, its eyes still unopened, fumbles around the nest until it bumps into the unhatched eggs. One by one, it wriggles each egg onto its pink shoulders and arduously heaves it out of the nest. If any of the eggs have hatched, the baby cuckoo will throw the hatchlings themselves overboard. Even if host parents are nearby when all this is going on, they strangely don’t interfere. 

Finally, the cuckoo chick enjoys an empty nest, and the undivided attention of its host parents. Depending on the species, it may grow several times larger than the mom or dad, who nonetheless keep dutifully delivering food to the mouth of their gigantic baby.

There are more than 140 cuckoo species in the world, and about 40 percent are brood parasites. But the strategy appears in other bird species, too, which put their own spin on it.

For example, in sub-Saharan African, the greater honeyguide mother lays her eggs in hollows or burrows where her hosts nest. On her way out, she pierces the other eggs in the nest with her beak. But if any of those eggs survive and hatch, the honeyguide hatchling is ready to handle them: It kills the other babies as they emerge from their eggs, biting them to death with a blade-tipped beak.

Watching an adult honeyguide, you’d never guess it may have this violent past. The species is impressively cooperative: In certain regions, the birds work together with humans to gather honey from wild bees. A honey hunter uses a special call to summon a bird, which then guides the person to a bees’ nest in a tree. After the hunter has subdued the insects and collected the honey, the bird may enjoy a beeswax reward.

Birds aren’t the only kind of wildlife taking advantage of the parental kindness of strangers. There are brood parasitic fish, such as the cuckoo catfish. There are cuckoo bees, cuckoo wasps, and other insects who foist their young onto more attentive parents. But getting into the nests of other species can require elaborate ruses. 

Take a family of insects called blister beetles. The larvae of the blister beetle species Meloe franciscanus are long and dark, and as soon as they hatch they climb together up a nearby plant to form one big clump that looks brown and vaguely fuzzy. Working together, they give off a chemical signal that mimics the sex pheromone of a certain female bee. 

When a male of that species follows the scent trail and tries to mate with the larval blob, the baby beetles immediately let go of their stem and glom onto the bee. And when that male flies off and finds a real female to mate with, the larvae move to her body. Once that female builds a nest, the beetle larvae climb off of her and finally settle down. Eating the food that the mother bee has stored for her own larvae—as well as eating the baby bees themselves—the beetle larvae grow up in comfort and safety. They’ll emerge from the nest as full-grown beetles, ready to lay eggs and start the cycle over.

One battle after another

Dumping your kids with nonconsensual foster parents might seem lazy. But the life of a brood parasite isn’t necessarily easy. For a start, the birds may be known outlaws. Adult honeyguides have been grabbed and killed by their hosts. 

Furthermore, brood parasites and their hosts are engaged in what biologists call an arms race: a nonstop war to evolve new ways of one-upping each other. For example, common cuckoos target a variety of other birds, and common cuckoos in different geographic regions have evolved differently patterned eggshells that match their favorite hosts in those places. Common cuckoos in one region may lay spotted brown eggs, for instance, and elsewhere lay eggs that are solid blue.

An African brood parasite called the cuckoo finch targets a host called the tawny-flanked prinia, which has been evolving eggs that are harder to mimic. Each prinia female lays her own distinctive eggs, which might be white, blue, green or red, with various kinds of spots and speckles. Over the past few decades, the prinia has evolved increasingly diverse eggs—and to keep up, the cuckoo finch has also been evolving more diverse egg patterns. But each female cuckoo finch lays a variety of egg types, which will only survive if she lucks into a match with her host. 

Host birds are quick to throw out eggs that looks like impostors, so brood parasite moms have to lay large numbers of eggs in the hopes of any surviving. In a species called the African cuckoo, parasitic moms lose more than 90 percent of their eggs to sharp-eyed host parents. 

Hosts become much less discriminating once those eggs hatch, though, for reasons scientists don’t understand. When they see a hungry cuckoo mouth, bright red on the inside like a stop sign, other kinds of birds can’t resist putting food into it. After cuckoos have fledged, adult birds of other species who notice a young cuckoo cheeping in the grass may pause to feed it.

But the voracious chicks always want more. In one Japanese cuckoo species, chicks have a bright yellow mouth and another yellow mark under each wing. The baby bird displays these marks when parents approach, making it look like the nest holds extra begging mouths. 

Once those deceitful chicks grow up, they’ll have to learn the ways of their species without any parental model. The honeyguide figures out how to find bees’ nests. The common cuckoo turns away from the parents that raised it and migrates to Africa in the winter. When these birds are ready to lay their own eggs, they’ll develop the furtive habits of their biological mothers.  

They don’t know any other way. Somewhere in their evolutionary past, these birds lost the skill of building a nest and caring for their chicks. So this is what they have to do: Sneak around, make as many eggs as their bodies can stand, and find the best possible homes for their babies. Just like any caring parent, they’re doing everything they can to give their young a chance of growing up.