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Bad eggs

Reed warbler feeding cuckoo chick

A reed warbler feeding a “brood parasite” common cuckoo chick. Photo by Harald Olsen, via Wikimedia

For decades, researchers have been trying to figure out how birds identify and reject the eggs that other birds, known as “brood parasites,” sometimes sneak into their nests. These rogue birds don’t build their own nests, they “dump and drive”—they lay their eggs in the nests of other birds, which then may incubate and raise the imposter chicks, often at the expense of their own.

The brood parasite species include cuckoos, cowbirds, black-headed ducks, indigobirds, whydahs, and honeyguides.

This evolutionary battle between host species and brood parasites goes beyond just dropping eggs off in a random nest to hatch and mature. The imposter eggs that most closely mimic the host eggs are more likely to endure, so brood parasites learn which species’ nests to impose on. And, over time, as it turns out, host species learn to identify imposter eggs, which they then reject. So the brood parasites start producing eggs that look even more like the host species’ eggs. And so on. 

Eggs, Part 1: Watching what birds do

To study this egg rejection behavior and figure out how bird parents identify impostor eggs, ornithologists have long been doing experiments that involve adding artificial eggs to nests and observing what the parents do. The problem is that making convincing, uniform, artificial eggs out of traditional materials like plastic, wood, and plaster-of-Paris is surprisingly challenging. The fake eggs are time-consuming to produce, prone to human error, and hard for other researchers to replicate exactly, which is important in confirming scientific findings.

3D cowbird eggs

Manufacturing 3D cowbird eggs. Photo by Mark Hauber.

According to a new study published in the open-access journal PeerJ, researchers have devised a way to get around these limitations by creating digital models of the eggs of brood parasites and producing them using 3D printers. This process produces hollow eggs that can be filled with water or gel, closely mimicking not just the weight but even the thermodynamic properties of real eggs.

“3D printing technology is not just in our future, but has already revolutionized medical and basic sciences,” says Prof. Mark Hauber, an animal behaviorist at Hunter College of the City University of New York, the study’s senior author. “Now it steps out into the world of wild birds, allowing standardized egg rejection experiments to be conducted throughout the world.”

Eggs, Part 2: Understanding how they do it

Cuckoo egg in reed warbler nest

Four clutches of reed warbler eggs, each containing one (larger) cuckoo egg
Photo by Chiswick Chap, via Wikimedia

In 2014 scientists at Harvard’s Natural History Museum and the University of Cambridge demonstrated that many birds affected by the parasitic Common Cuckoo—which lays its lethal offspring in other birds’ nests—have evolved distinctive patterns on their eggs in order to distinguish them from those laid by a cuckoo cheat.

The study revealed that these signature patterns provide a powerful defense against cuckoo trickery, helping host birds to reject cuckoo eggs before they hatch and destroy the host’s own brood.

To determine how a bird brain might perceive and recognize complex pattern information, Dr Mary Caswell Stoddard at Harvard University and Professor Rebecca Kilner and Dr Christopher Town at the University of Cambridge developed a new computer vision tool, NaturePatternMatch. The tool extracts and compares recognizable features in visual scenes, recreating processes known to be important for recognition tasks in vertebrates.

“We harnessed the same computer technology used for diverse pattern recognition tasks, like face recognition and image stitching, to determine what visual features on a bird’s eggs might be easily recognized,” explains Stoddard.

Using the tool, the researchers studied the pigmentation patterns on hundreds of eggs laid by eight different host bird species targeted by the Common Cuckoo. They discovered that some hosts, like the Brambling, have evolved highly recognizable egg patterns characterized by distinctive blotches and markings, while other hosts have failed to evolve recognizable egg patterns, instead laying eggs with few identifiable markings. In an interesting twist, hosts with the best egg pattern signatures, the researchers found, are those that have been subjected to the most intense cuckoo mimicry.

Comparing patterns on eggs

Using algorithms to analyze and compare patterns on eggs. Photo by Mary Caswell Stoddard/Natural History Museum

The Common Cuckoo and its hosts are locked in different stages of what many scientists call a co-evolutionary arms race. If a particular host species – over evolutionary time – develops the ability to reject foreign cuckoo eggs, the cuckoo improves its ability to lay eggs that closely match the color and patterning of those laid by its host.

Stoddard says, “The ability of Common Cuckoos to mimic the appearance of many of their hosts’ eggs has been known for centuries. The astonishing finding here is that hosts can fight back against cuckoo mimicry by evolving highly recognizable patterns on their own eggs, just like a bank might insert watermarks on its currency to deter counterfeiters.”