When a cuckoo finch lays its eggs, it does so in another bird’s nest. And not just one egg, quickly atoned for with an apologetic, “Oops, I didn’t mean to put that there.” No, the cuckoo finch plops multiple eggs into another bird’s nest, bedding them with the put-upon bird’s own eggs.

But why?

“The better to trick their hosts,” says Claire Spottiswoode, a researcher at the University of Cambridge and a co-author on a new paper published this week in Nature Communications.

Cuckoo finches are adroit freeloaders, nesting their eggs with another bird’s eggs in hopes of the besieged bird wasting its time and food on bringing up babies that it mistakes for its own. It also now unfolds that these Frank Abagnales of the bird world are more savvy than previously thought.

It turns out that placing more than one egg in another bird’s nest, as opposed to just one egg, is an adaptive move that improves the cuckoo bird’s odds of foiling the selected foster parent, according to new research from a team of British ecologists working in Choma, Zambia. The insight helps to tell the tale of the cuckoo finch’s long, evolutionary pas de deux with the bird on which it begs off its young, each of them two-stepping through time – sometimes leading, other times following – in hopes of outwitting the other.

The cuckoo finch is a parasite, but it does not look like one – or, at least, not like a parasite as it is often imaged, as a microscopic critter fringed with uncomfortable numbers of legs. Fluffed and puffed in yellow-brown feathers, the female cuckoo finch is a squat but regal bird, like a plump princess. The male bird, not to be outdone, is DayGlo gold.

The cuckoo finch is not at all related to the cuckoo bird (though some cuckoo groups also practice parasitism). In fact, it is out on what Dr. Spottiswoode calls an “evolutionary limb,” some 20 million years removed from its closest relatives, the indigobirds and the whydahs, both of the Viduidae family of African bird parasites.

Brood parasitism – that is, foisting one's young on un-expecting parents – is believed to have evolved seven different times in birds. About 100 different species of birds practice it (about one percent of known bird species). It is also common practice among insects, as well as fair game for some fish, including the so-called Cuckoo Catfish (Synodontis multipunctata). This fish manages to get its eggs into the mouth of another species of fish in Africa’s Lake Tanganyika; the fraudulent eggs then hatch into fish that eat the mother’s own babies – all inside her mouth.

In other words, cuckoo animals (also known as “strange” animals) are ruthless little con artists. And brood parasites, as a group, present to ecologists a list of compelling questions, as tangled as the branches on an evolutionary tree: How did these dupers evolve to pull off their ruses? And, as the host animals adapt to shake off their parasites, how do they manage to keep doing it?

Well, what is known is why animals do it: “brood parasitism is a great strategy if you can get away with it,” says Spottiswoode. “If your offspring require care to develop and grow, and you as a parent can fob off the costs of providing such care onto someone else, you can save the energy that parental care would otherwise consume – and rather spend it on producing more offspring.”

But to get away with it, an animal must be good – Bernie Madoff good. And the cuckoo finch is that good.

Cuckoo finches hoodwink at least three different Cisticola species, as well as the Tawny-flanked Prinia. These are not the most stunning birds, especially relative to the gold-wreathed cuckoo finch. Cristicolas are pale brown (but with proud, primped chests) while the Prinia is as common looking as birds come: small and, in an avian insult, mousy brown.

But the birds do lay some of the prettiest eggs in the proverbial basket: blue with splotches and squiggles; red-brown and pink-brown and pinkish-red; white and cream and olive green.

So, to trick the Prinia and the Crisicola birds into raising cuckoo finches as their own, cuckoo finches must replicate this rainbow suite of eggs. To do so, each female cuckoo finch specializes in imitating the eggs of just one kind of host bird. That’s an already incredible feat, as it’s not quite clear how a female cuckoo finch manages to keep laying one egg design, even after breeding with different males (in fact, little is known in general about how genes control for egg color).

Now, in another impressive demonstration of evolution, scientists have found that cuckoo finches plant more than one interloper egg in the target nest. In tests that involved switching up the ratio of fraudulent to real eggs in a host bird’s nest, the researchers found that the host bird became less likely to toss out intruder eggs as the number of those eggs in the nest increased – as long as the fake eggs were good mimics of the real eggs, of course.

That’s because putting down more than one deceptive egg exploits the host bird’s cognitive limits, the researchers say. Host birds tend to pull out bad eggs by comparing all the eggs to a known good egg template. But it’s must easier for a bird to identify the “odd one out” using that template than it is for it to nose through a confusing crop of fake and genuine eggs: “that egg looks weird, but, hey, so does that one, and that one…so is that the normal egg?”

This is all the more problematic for the host bird if the same parasite keeps frequenting its nest, since the parasitic egg pattern is over time adopted as the host’s template egg.

“We've shown here that the cuckoo finch has an additional strategy against its hosts whereby each female often parasitises the same host nest multiple times,” says Martin Stevens, a researcher at the University of Exeter and a co-author on the paper. “By doing so they confuse hosts about which eggs are theirs and which are the parasite's.”

Separating the good eggs from the bad is no easy task. If the maligned mother bird tosses out a suspicious egg, she could be wrong. If so, she has abandoned one of her own babies. But if the mother keeps all the eggs, including the dubious ones, the eggs might turn to out harbor some little cuckoo finches, each an almost translucent shade of purple and sprouting tuffs of white hair, as if aging backwards.

And that’s a big problem: these homewreckers are born about two days before their foster siblings, and will squack and peep and wrestle and flutter for food. That makes them almost certain to outcompete the mother’s smaller offspring, which will starve within about two days of birth.

“This is thought to be a common issue that hosts assess,” says Dr. Stevens, of the poor bird’s choice: to let the eggs be, or not be?

It's not clear why the host bird doesn't knock the hatched interlopers out of the nest upon realizing its mistake, Spottiswoode said. One possibility, though, is that the host bird doesn't realize its mistake at all. That’s especially possible if it’s been through this harrowing experience before and believes that the big cuckoo birds it raised the first time were its own, Spottiswoode said.

No bird wants to deal with all this drama. So, what’s a Prinia bird to do? Well, there’s always evolution.

Natural selection favors host birds that can tease out the intruder eggs from a batch of beguiling eggs, in order to funnel all their time and resources into bringing up their own babies. So, Prinia birds that produce eggs that look less like the freeloader’s eggs are favored. These host birds also begin to vary their eggs from each other’s eggs, which whittles the chances that the parasite will correctly choose a nest filled with eggs that match its own eggs.

And, if the cuckoo finch gets it wrong – maybe putting smooth blue eggs into a nest full of blue eggs splotched with dark spots – the Prinia bird will spear the unfortunate eggs with its beak and loft them from the nest.

Of course, the parasites don’t want to be lobbed out of their niche. As the host birds become better egg sleuths, the parasites also become better egg forgers. Some 30 years ago, the cuckoo finch laid red eggs. Now it lays blue ones. Since the Prinia is now laying olive ones, ornithologists expect that the cuckoo finch, too, will start plunking down olive-colored eggs.

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It’s a tale as old as time – or, at least, as old as crooks have outwitted the law as fast as it is rewritten to best their ruses, or as long as one state, spotting another state’s mounting pile of arrows or canons or bombs, have lunged into arms-producing overdrive.

That’s right: this is “a co-evolutionary arms race in egg mimicry,” says Spottiswoode. Cuckoo, right?