Scrambled Eggs

Warbler Eggs

New Research Reveals How African Songbirds Keep Impostors Out of Their Nests
 July 18, 2015

How’s this for a horror movie pitch?

"In young families around the world... Parents turn their backs for just a minute... And when they do, something is ready and waiting... To take their children and replace them... With something else... It grows up in their home... Eating their food... And waiting!"

Read that in a dramatic voice, add some shots of a creaking door, a little tense string music and you have yourself a trailer! But this story is a fact of life for birds in Europe, North America and Africa.

Brood parasites
, like the cuckoo bird, will steal an egg from a nest and replace it with one of their own, where the unsuspecting parents will raise the impostor chick, often at the cost of their real children.

Cuckoos and their hosts are constantly trying to outsmart each other. The host birds will examine their nests for eggs that don’t look like theirs and throw them away, and in response a cuckoo will try to better disguise her eggs.

Now Duke University graduate student Eleanor Caves has discovered one strategy that the host birds are using to better distinguish their own eggs from those of the cuckoos.

Anyone who has ever seen a dark brown spot on an egg from the store or the bright blue of a robin’s egg knows that eggs can take on different colors and patterns. The birds themselves have some measure of control over the appearance of their eggs. Scientists do not know exactly how this works, but they do know that birds will use their shell glands to add colors and markings to eggs not long before they lay them.

Caves studied about 1,000 eggs from 22 species of Zambian birds, some of which are commonly parasitized by cuckoos or cuckoo finches and some of which are not. She noticed that the commonly parasitized species had much Bird Feeding Cuckoomore variation in egg color, brightness and spots than the species not threatened by cuckoos. Caves hypothesized that this variation, which she calls entropy, makes it harder for a cuckoo to mimic the real eggs.

But Caves broke down this entropy idea even further. She found that rather than parasitized species displaying a wide range of possibilities for a single trait (such as a wide range of spot sizes or shell brightnesses) they displayed little correlation between egg traits (small spots don’t always mean bright eggs, green eggs do not always have few spots, etc.).

What’s more, Caves saw that individual mothers specialize in a single style of egg. That style still contains variation in pattern, color and brightness, but it has a pattern that is easy for the mother to recognize and difficult for a cuckoo bird to mimic — like a signature.

So why go to the trouble of creating an elaborate egg code? What is the harm in raising one extra chick? Cuckoos are often much larger and grow more quickly than the birds they share nests with. Cuckoos often kill other chicks or take so much of the food that the others starve.

So while the egg code may require some extra work on the part of the nesting songbirds, Caves says it can be an effective tool in the evolutionary arms race between parasites and hosts that occurs in nature all the time.

And in case you’re wondering how the cuckoos respond to the egg code, scientists like Caves know that the egg code is not perfect, otherwise cuckoos and other brood parasites would have gone extinct. Scientists have also observed some brood parasites destroying the nests of birds that get rid of their eggs. The operating assumption is the parasites do this not only to encourage the songbirds to build a new nest, giving the cuckoos another chance, but also to send a message of intimidation. The theory is called the “Mafia hypothesis.”

Caves’s work was published in the Proceedings of the Royal Society B.

— Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina