It might not look it, but this bird’s nest is a tomb.

Deep within its recesses are layers of eggs, all doomed, all certain to die. And when they do, they will lie in a nest that could accurately be described as a war grave.

That's because these eggs, and the developing chicks within, are victims of one of the most extraordinary conflicts in the animal world, an evolutionary struggle for life and death that has left behind countless unhatched baby birds buried within generations of nests.

They have fallen foul of a perpetual conflict between the small, pretty yellow warbler, and its larger, drabber archenemy, the brown-headed cowbird.

There is a certain pattern to the conflict. Generally speaking, the cowbird is the aggressor. Female cowbirds seek out the nests of other species, such as the yellow warbler, and lay an egg within. Though unrelated, female cowbirds use the same devious trick as the common cuckoo, hoping their victims will not notice their invasive act, and will adopt, incubate and raise the cowbird chick on their behalf.

But some yellow warblers have learnt to fight back, taking extreme measures.

Rather than being forced into adopting another bird’s young, they escalate the conflict to a frightening new level. They cover the cowbird’s egg with fresh nest material, burying it, sealing it within a grassy tomb. In doing so, they will also bury their own clutch of eggs, condemning their own young to death.

The warblers’ egg-burying is a puzzle. Not all females do it

The cycle can repeat many times if the cowbird returns to lay again. As a result, sometimes yellow warblers entomb multiple laters, each with at least one cowbird egg alongside their own. Sometimes the victor is the yellow warbler, often the persistent cowbird.

Though unusual, these egg tombs are a reminder of something more common; around the world, birds are engaged in a series of egg wars.

Tactics include surveillance, deception, aggression, and as in any conflict, arms races, where each side improves its armoury or techniques to counter those of the other. Decades of research by scientists are unveiling the true nature of these egg wars, but mysteries still remain.

The battle between the yellow warbler (Setophaga petechia), and the brown-headed cowbird (Molothrus ater), a so-called brood parasite, is a good example.

The warblers’ egg-burying is a puzzle. Not all females do it. Sometimes, when confronted by a cowbird egg in her nest, a female will desert it entirely. Yellow warblers have short breeding seasons; if she leaves early enough, she can spend five to ten days building a new nest, then raising a new clutch elsewhere.

Egg burying may have an unusual origin

Yellow warblers can also distinguish foreign eggs from their own, decades of experiments since the 1980s by the University of Manitoba’s Dr. Spencer Sealy and his students have revealed.

So why do warblers bury eggs, including their own, rather than just ejecting the invader? The process of adding a new layer can take two days, and doesn’t prevent the cowbird from returning, repeating the insult. In some places, such as in Manitoba, Canada, this ding-dong continues until a warbler is sitting upon two to five layers of entombed eggs.

Egg burying may have an unusual origin, research suggests. Warblers bury almost any foreign object that enters their nest, including oddly shaped bits of wood, placed by scientists, that are painted the same colour and pattern as eggs. So burying may be a form of nest sanitation, suggest studies by Mélanie Guigueno at the University of Manitoba, one of Sealy’s former graduate students.

More likely, the warblers are forced to bury eggs, because they can’t eject them. Cowbirds lay eggs that look similar to yellow warblers’, which aids the deception. But cowbirds are much bigger birds, and their eggs are twice the size.

Warblers don't seem capable of grasping these big eggs with their tiny beaks to turf them out. Puncturing the cowbird egg is also risky. Cowbird’s eggs are more rounded, with a tough shell, so are difficult to break into. “They could also accidentally puncture and damage their own eggs in the process,” explains Guigueno, who’s explored many aspects of this unusual egg war.

Wars waged by cowbirds tend to be less costly than those waged by cuckoos

Even the thickness of cowbird eggshells is no accident. The large, thick eggs are themselves weapons in the egg war. Other brood parasites have independently evolved thick eggs, and their own chicks have evolved special adaptations to break out of them. Cuckoo hatchlings are large, strong, tactical shell-peckers, with a strategically located egg tooth. They also have remarkably dense neck muscles.

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The wars waged by cowbirds tend to be less costly than those waged by cuckoos, to their hosts at least.

Cowbirds tend to pluck a single egg from the host nest, replacing it with their own. As a result, some host chicks may survive despite growing up alongside their larger, hungrier, more aggressive cowbird foster sibling. So while there is a cost to this enforced fostering, for the host parents it often means a reduction rather than a total loss of breeding success.

These devious deadbeat parents parasitise nests of more than 200 bird species

As a result, “cowbird parasitism is much less costly than cuckoo parasitism,” says Dr. Mark Hauber of Hunter College, New York, in the US, who has spent much of his career studying brood parasites including cowbirds and cuckoos in North America and Europe, exploring their repertoire of dirty tricks.

Common cuckoos are less tolerant. Once hatched, “cuckoos toss all the host eggs and nest mates out of the nest and grow up alone,” says Hauber.

Cuckoos have been waging eggs wars on their hosts for more than ten million years, far longer than cowbirds, which evolved the tactic less than five million years ago. That may explain why cuckoos have evolved a more dominating strategy. It also explains something about the appearance of cuckoo and cowbird eggs.

Cuckoos are surprisingly stealthy

“Cowbirds are generalists, which means that a single individual female can parasitise multiple different host species,” explains Hauber. In fact, as a species, they parasitise nests of more than 200 bird species. Because they put their eggs in so many nest types, that makes it hard for cowbirds to lay eggs that look like their hosts’. And if a host bird species starts rejecting cowbird eggs, the cowbird can easily switch to exploit another.

This strategy has served the cowbird so well that it can easily parasitise unfamiliar host species.

Cowbirds prefer open, cleared habitat. The recent conversion of North American forest to farmland has allowed them to flourish, and exploit unprepared migrant species that haven’t yet learned to discriminate against them, even if cowbird eggs are of a size, colour, shape, and pattern quite different from the new hosts'.

Cowbirds have been linked to the decline of the endangered Kirtland's warbler (Setophaga kirtlandii), migratory songbirds and forest-dwelling tropical migrants.

Eurasian common cuckoos, in contrast, are specialists, creating elaborate eggs with matching colours and spots that mimic those of specific hosts. This tactic is so refined that genetically distinct “gentes” or races of common cuckoo produce eggs containing the same palette of egg tints as each host species.

There is continued selection for improvement

Cuckoos are also surprisingly stealthy, given their size.

They spy on their victims, watching hosts such as reed warblers (Acrocephalus scirpaceus) from concealed perches. Their cryptic feathers act as camouflage.

When the time is right, cuckoos lay their eggs swiftly. Females lay every other day, one egg per nest, laying up to 25 per breeding season. So “she’s got to be a fantastic birdwatcher, knowing exactly the stage of nesting of all the reed warblers in her territory,” says Dr Nick Davies at the University of Cambridge in the UK, author of the recently released book Cuckoo: Cheating by Nature.

Precisely timing her egg drop, the female cuckoo glides down to the nest, takes out one reed warbler egg, lays her own in its place, and then she’s off, all in a mere ten seconds. Often eating the egg she snatches, “she gets a meal on the job,” quips Davies. His experiments show that secrecy and speed are essential to success. Hosts alerted to a cuckoo's presence are more likely to reject the egg.

But if cuckoos are so clever, it infers their hosts are a little stupid. But studies have revealed that things are not so simple.

Different prinia females can lay one of four different colours of eggs – red, white, blue, and olive green

In response to these secretive, egg-popping machines, hosts have evolved their own discreet behaviour and unpredictable laying. Mobbing of marauders is another strategy reed warblers use to keep nests free of cuckoo eggs. The noisy kafuffle of cuckoos being mobbed acts as a neighborhood watch system, warning other would-be cuckoo hosts to step up nest security.

Assuming cuckoos do successfully sneak their eggs into a host nest, how do hosts distinguish their own from the usurpers? Cuckoo hosts appear to learn what their own eggs look like upon laying their first clutch. “Then, they will reject eggs that differ from that learned set,” explains Davies. In the reed warblers studied by Davies, despite well-matched eggs, about 20% of cuckoo eggs are chucked out of the nest, “so there is continued selection for improvement,” he says.

We don’t yet know how they spot the alien egg in their nests. “It’s probably a combination of cues,” says Davies, including colour, size, shape, and the pattern of spots.

But it is not just cowbirds and cuckoos waging war on unsuspecting birds.

In Africa, the Cuckoo finch (Anomalospiza imberbis) is embroiled in an egg war with the Tawny-flanked prinia (Prinia subflava). Both birds bring a surprising level of sophistication to the conflict.

Cuckoo finches, to date, seem unable to lay olive green eggs

Prinia females lay unique clutches of eggs, with each clutch varying enormously in colour, spotting and patterns, compared to those laid by other females. “Different prinia females can lay one of four different colours of eggs – red, white, blue, and olive green,” explains Dr Claire Spottiswoode of the University of Cambridge, who studies this arms race in Zambia, supported by 40 local nest-finding assistants.

On top of that, each female prinia also lays eggs that all share a unique signature, a fine scrawl on each egg that is so unique even the researchers can use them to tell prinia and Cuckoo finch eggs apart.

Over evolutionary time, the Cuckoo finches respond by trying to match the colours of the prinia eggs, in an escalating arms race.

But both bird species struggle to win the egg war.

Cuckoo finches, to date, seem unable to lay olive green eggs. Prinias have seized on this weakness; since the 1980s, studies show they are laying more olive green eggs in response, making it harder to be duped.

Oddly though, prinias have a weapon they themselves don’t use. The little scribbles they leave on their eggs are a perfect way to identify their own eggs from those of the Cuckoo finch. But they often accept Cuckoo finch eggs that lack them. “We’re baffled that they’ve got a foolproof signature but don’t seem to use it,” says Spottiswoode.

We can’t be certain because we don’t have a time machine

That leads to perhaps the most fundamental question of all: Are any of these egg wars winnable?

It’s difficult to know, explains Spottiswoode, “because in all the arms races that [may] have been won by hosts, we no longer see the parasite.”

One possible winner is an African songbird, the Rattling cisticola (Cisticola chiniana). It isn’t currently parasitised, yet has high levels of defense. “If you put a foreign egg in their nest, they are extremely good at getting shot of it immediately,” she says.

Chaffinches (Fringilla coelebs) may be another egg war champion against the Eurasian common cuckoo. “It’s very good at detecting and rejecting foreign eggs,” says Spottiswoode. “This is probably because it was exploited by cuckoos in the past – but of course we can’t be certain because we don’t have a time machine.”

But the chances are, as long as there are birds such as cowbirds, cuckoos, and cuckoo finches, plus hosts available to be duped, the egg wars will continue as some tragic, never-ending story.

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