Of all the birds in the world, none may be as clever and cruel as brood parasites. These opportunistic animals dump their eggs into another species’ nest to avoid wasting time and energy on childcare (some female parasites literally have a larger memory complex in their brain to help them remember and hone in on targets). Their life strategy is built on tricking other birds, so perhaps they’ve earned the nasty reputation that precedes them—some parasites’ very first act upon hatching is to kill their adopted “siblings.” This is survival of the fittest at its most ruthless, and with about 100 parasitic bird species out there, it seems to be working.

Many hosts don’t take this assault lying down, of course. Over time, they’ve evolved brilliant defenses against these crazy tactics. And as both sides continue to fight to get the upper hand, bizarre stories and discoveries keep coming out of the woodwork. Take it from Claire Spottiswoode, who’s been studying brood parasites for a decade, with both the University of Cambridge in England and the University of Cape Town in South Africa, and has been loving it. “I can’t think of a topic that shows evolution more vividly,” she says. “You can’t not be fascinated.” She does her field research in Zambia, home to 31 species of brood parasites.

Spottiswoode says there are three major ways the freeloaders try to outsmart their hosts. “At each of those stages the parasite is trying to deceive the host, and at each stage the host is evolving,” she says.

Step One: Invade the Nest

The parasite waits patiently to try to sneak its spawn into another bird’s nest. As soon as the owners leave, it darts into their home and lays a single egg. And sometimes, rather than biding its time, it adopts a disguise to scare its victims right out of their nests. A pair of papers from 2008 and 2011 proposed that Old World cuckoos evolved to resemble raptors—with barred underparts and decurved bills—to keep hosts at bay. “Hosts are less likely to approach them closely because they’re worried they’ll be eaten,” Spottiswoode says.

Other parasites use the “wolf in sheep’s clothing” method to breach the nest, Spottiswoode says. Her team published a paper last year that showed that female Cuckoo Finches go undercover as harmless Southern Red Bishops to fool their host, the Tawny-flanked Prinia. (They often foil birders, too.) But the prinia is catching on: It now attacks both Cuckoo Finches and Southern Red Bishops to avoid taking any chances.

The prinia isn’t the only host species that errs on the side of caution: Many go berserk when they spot a parasitic bird near their nest, Spottiswoode says. “They attack [the invaders]. They land on them. They peck them.” Some species even rely on several adults to keep a lookout, making them particularly adept at repelling invaders. Black-collared Barbets in Africa are one example: They’ve been observed to team up and kill parasitic Lesser Honeyguides by crushing them with their big beaks.

Meanwhile, some hosts defend their nests by rigging them. Weaverbirds, for instance, construct narrow doorways to keep the freeloaders out—or trap them inside. “There are striking examples of entrance tubes catching cuckoos as they try to enter the nest,” says David C. Lahti, an assistant professor of biology at Queens College in New York. “And then the weaverbirds tear them apart."

Step Two: Disguise the Eggs

The parasites’ eggs have a number of tricks up their shells, too. They often mimic host eggs in color and shape—so the nest owners don’t notice the eggs aren’t theirs—and also have thicker shells and shorter incubation times, which allow the invading offspring to hatch first. It’s a real problem for species that aren’t used to brood parasitism, especially in North America, where songbirds have been exposed to Brown-headed Cowbirds due to habitat fragmentation. “Many birds wouldn’t even make the distinction between a purple egg and a white egg,” Spottiswoode says. The endangered Kirtland’s Warbler, for instance, can’t tell the difference between warbler and cowbird eggs; they’ll raise pretty much anything that’s in their nests. Thanks to the parasites' overwhelming success, at one point there were fewer than 200 pairs of Kirtland’s Warblers. The species now depends on humans to cull cowbirds to survive.

But not all hosts are so naive. Birds like Pied Wagtails and Red-faced Cisticolas can pick up on the slightest differences by using visual cues. Another recent study by Spottiswoode’s team found that in some species, females go so far as to lay eggs with unique spots, squiggles, and colors that serve as a sort of signature. If they spot a forgery, they’ll push the egg overboard, not incubate it, or abandon the nest altogether. Scientists have also discovered that Superb Fairy-wrens in Australia essentially teach their embryos a password—one that only fairy-wrens can learn—while they’re still in the egg. If the nestlings don’t tweet the password as part of their begging calls later, then the fairy-wrens know that they’ve got the wrong chicks.

Step Three: Murder the ‘Siblings’

If the parasite makes it out of its egg (cue the Jaws theme), then it’s often time for a real massacre. In the dramatic clip above, a newborn Common Cuckoo wiggles its way around the Reed Warbler’s nest, using its shoulders and back to push out all three of the host’s eggs. Brood parasites, it seems, are basically born evil—the honeyguide, for example, while still blind and featherless, will stab the host’s chicks to death with its hooked beak.

Once the real offspring are out of the picture, the parasites are left alone to be raised by their adoptive parents. This can result in an almost comical mismatch during mealtime: a small and exhausted warbler feeding a mammoth Brown-headed Cowbird chick, trying to satisfy the black hole of its beak. Common Cuckoo chicks can even echo the begging calls of a whole brood of Reed Warblers to get triple the amount of food.

Left: Honeyguide chick attacking and stabbing bee-eater host chicks; Right: African Cuckoo hatchling ejects the host egg of a Fork-tailed Drongo. Photos: Claire Spottiswoode

These serial-killer chicks do meet their match in some hosts, however. In 2010, scientists discovered that two gerygone species in Australia have learned to recognize Little Bronze-Cuckoo chicks and fling them out of the nest as soon as they hatch. (As Spottiswoode points out, “we’re still left with the mystery of why [this defense is] not more widespread.”) Reed Warblers, on the other hand, have been found to use a community-wide alert system where birds use alarm calls to point out cuckoo eggs to their neighbors.

But once again, the parasites have come up with a back-up plan. Bronze-cuckoo species in Australia have young that closely resemble their host nestlings. Even more bizarre are the parasitic indigobirds of Africa: They have freaky markings on the roofs of their mouths that mimic those of their nest mates. These patterns are found in a number of finches, perhaps to serve as guides for when the parents have to feed their chicks in the dark. But in a parasite-infested nest, they doubly serve as signs of ruin.