Birds stamp signature on eggs to thwart Cuckoo cheats

Many birds have evolved signature patterns on their eggs to distinguish them from those laid by a cuckoo cheat, just like banks insert watermarks on currency to deter counterfeiters, a new Harvard and Cambridge study has found.

The study shows that these signature patterns provide a powerful defence 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 recognise 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 recognisable 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 recognised," said Stoddard.

Using the tool, the researchers studied the pigmentation patterns on hundreds of eggs laid by eight different bird species (hosts) targeted by the Common Cuckoo.

They discovered that some hosts, like the Brambling, have evolved highly recognisable egg patterns characterised by distinctive blotches and markings.

By contrast, other hosts have failed to evolve recognisable egg patterns, instead laying eggs with few identifiable markings.

Those hosts with the best egg pattern signatures, the researchers found, are those that have been subjected to the most intense cuckoo mimicry.

The Common Cuckoo and its hosts are locked in different stages of 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 colour and patterning of those laid by its host.

"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 recognisable patterns on their own eggs, just like a bank might insert watermarks on its currency to deter counterfeiters," said Stoddard.

"The surprising discovery of this study is that hosts achieve egg recognition in different ways," said Kilner.

Some host species have evolved egg patterns that are highly repeatable within a single clutch, while other species have evolved eggs with patterns that differ dramatically from female to female in a population.

Still other host species produce egg patterns with high visual complexity. Each strategy is effective, increasing the likelihood that a given host will identify and reject a foreign egg.

The finding was published in the journal Nature Communications.