How Cuckoos Lay Their Deceptive Blue Eggs

Female cuckoos are brilliant masters of disguise - at least when it comes to laying their eggs. A new study finds that mothers invade other species' nests and lay eggs that look remarkably similar to those that actually belong there, in an effort to hide the foreign eggs in plain sight.

For about a century, researchers have been investigating how different female cuckoos manage to lay eggs in such a wide variety of colors and patterns so that unsuspecting birds can't tell the difference between their own eggs and imposters.

Now, scientists at the Norwegian University of Science and Technology (NTNU) have solved one piece of the egg puzzle: The gene that causes cuckoos to lay blue eggs is determined by the mother alone. [The 7 Weirdest Moms in the Animal Kingdom]

"The enigma for scientists is the distinct colors and patterns of eggs mimicking different host species," said lead study author Frode Fossøy, a research scientist in the Department of Biology at NTNU. "We do know that males and females from different host races mate with each other. If not, each host race would quickly become a separate species."

If the genes that affect egg color were carried by both males and females, any mating could result in eggs that are a mixture of the two colors and patterns, which would thus not mimic the host bird's eggs at all. In other words, the male cuckoo's genes could mess up the disguise.

Birds have Z and W chromosomes, which work similarly to X and Y chromosomes in mammals. Male birds have ZZ and females have ZW, and so the gene for blue eggs could be carried on the Z chromosome, the researchers said. Another explanation could be that it is passed on in mitochondrial DNA, which scientists think is only passed on by mothers, they added.

The researchers homed in only on the genes for blue eggs, not any other colors, but they studied a wide variety of samples, including some eggs that are more than 100 years old.

Blue eggs are thought to have originated from Asia, around 2.6 million years ago. In Europe, blue eggs are most commonly found in common redstart nests, but have also been found in pied flycatcher, winchat and wheatear nests, the researchers said. These are the species that cuckoos are likely to try to dupe by depositing their blue eggs in the other birds' nests.

However, the cuckoo's plan isn't foolproof, and sometimes the wily birds are found out and their eggs are kicked out and destroyed by suspecting host birds.

"There is a continuously ongoing arms race between cuckoos and their hosts," Fossøy told Live Science. "As cuckoo eggs evolve to be more like the hosts' eggs, the hosts themselves become better and better at recognizing and ejecting cuckoo eggs from their nests."

It is not known whether the cuckoos or the host birds are winning the arms race, but Fossøy said many host species are declining in population throughout Europe, and cuckoos are also declining as a result.

Fossøy said he and his colleagues are interested in studying other egg colors in addition to blue ones. The blue eggs had a long documented history that made them ideal for this research, but more genetic data is needed to ask the same questions of other colored eggs, the researchers said.

"We have recently sequenced the complete genomes of several cuckoos and this will be the focus of our work for the next couple of years," Fossøy said. "Hopefully, we will be able to say something about the other egg colors during this work."

The new study was published in the journal Nature Communications.

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This marsh warbler has a cuckoo chick in its nest. It was unable to differentiate between the cuckoo egg and the rest of the eggs in its nest, researchers say.

Eggs come in a multitude of colors and patterns, from subtle pastels to vivid bright hues. Now a new study, published in Current Biology, supports a centuries-old theory that shell variation, at least for some birds, helps to camouflage eggs.

The masters of egg disguise turn out to be Japanese quail. This photo amazingly features eggs just from this one species. Mother quails somehow learn the patterning of their own eggs and choose laying spots to hide them best.

“We currently do not know the mechanisms by which the (mother) bird learns its own egg patterns,” lead author P. George Lovell of Abertay University and the University of St. Andrews, told Discovery News.

Nevertheless, the ground-nesting birds often perfectly match the eggs to substrate, helping to prevent the precious contents from being some hungry predator’s dinner.

Close-ups of the individual Japanese quail eggs really show how well the bird moms can match their eggs to the environment.

One of the first scientists to theorize that coloration and speckles evolved for camouflage against predators was Alfred Russel Wallace, who gained fame as Charles Darwin’s co-discoverer of the principle of natural selection.

Yet another Japanese quail egg matches its environment, but the egg is not invisible to us. “Some have asked why they can still see the eggs if they are camouflaged,” Lovell said. “It’s important to remember that the eggs are less visible, not invisible…The photographs are taken quite close up to the egg, and a predator wouldn’t necessarily be that close. It would be scanning an area, rather than staring straight at the egg.”

Japanese quail themselves are brown and speckled, like their eggs. The bodies of many birds appear to match their eggs. A parent bird could then likely better shield the egg when resting upon it.

Speckles appear to be key to camouflage, at least for eggs.

Avian expert Innes Cuthill of the University of Bristol’s School of Biological Sciences, told Discovery News, “Alfred Russel Wallace concluded that the brown and speckled colors of many eggs had evolved as camouflage against nest predators and that the ancestral color of white only persisted in species whose eggs were laid in well-protected cavities, were covered by nest material or, in birds such as ostriches, could be defended by their parents.”

Brown with speckles is just one egg color/pattern combo, as these reed warbler eggs reveal. Their greenish hue, set off by the green leaves surrounding the nest, would make them less visible to predators scanning the area from a distance.

This nest also includes a, slightly larger, common cuckoo egg. Demonstrating another bit of bird trickery, common cuckoo mothers frequently match the appearance of their eggs to those of reed warblers, which wind up caring for the hatchlings.

Sometimes avian parents enjoy the best of both worlds: camouflage for their eggs as well as fooling other birds. Cuckoo finches can be deadbeat parents, matching their eggs (seen in the inner circle) to those of the tawny-flanked prinia (outer circle).  The prinia parents are sometimes fooled into caring for the cuckoo finch eggs and later hatchlings.

A mother cowbird must have worked hard to find this perfect spot for her eggs. “Interestingly, all birds seem more concerned in minimizing the mismatch between nest and the darker speckles on their eggs than the mismatch between nest and the underlying, predominant egg color, but particularly so for birds with more dark speckling,” Cuthill said. 

Lovell and his team speculate that the dark and light markings serve as a disruptive camouflage, breaking up the outline of the otherwise revealing oval shape. That shape, for humans and countless other predators, serves as a visual signal for good eats.

This cockatiel, with its proud expression, has a right to boast. Its egg not only matches the mother bird’s coloration, but it also matches the environment. In the wild, a hunter would have a hard time finding such an egg.

By itself, the egg of a swan is just as visible as a bright white egg on a breakfast plate. Under the mother swan, however, the egg disappears.

This Adelie penguin laid a speckled egg that matches its environment. The matching phenomenon could lead to a twist to the old riddle: Which came first, the chicken or the egg? In this case, the teaser could be: Which came first, the egg color and pattern, or the choice of nesting site?

An even more compelling mystery concerns a common sight in springtime—bright blue eggs in a robin’s nest. To human eyes, they are beautifully unmistakable. They stand out from both the nest and the parent birds, so why are they bright blue?

No one yet knows for sure. Biologists do know that pigment glands in the mother bird’s body deposit the blue coloration onto the eggs, so it must have a critical function. Perhaps the dark color camouflages the eggs when the eggs are at the bottom of a dark nest, or the blue might blend in with the sky for some viewers. Some researchers have even speculated that the striking color helps mother birds to find their own eggs.

For now, however, the case of the mysteriously blue robin’s egg has yet to be cracked.