Elaborate patterns on eggs help thwart cuckoos

A comparison of Cuckoo Finch eggs and hosts. Host (left) and parasitic (right) eggs show the range of polymorphism among females, with each egg coming from a different clutch. Photo: Dr Claire Spottiswoode (commons.wikimedia.org).
A comparison of Cuckoo Finch eggs and hosts. Host (left) and parasitic (right) eggs show the range of polymorphism among females, with each egg coming from a different clutch. Photo: Dr Claire Spottiswoode (commons.wikimedia.org).
A new study has found that African songbirds victimised by cuckoos create elaborate patterns on their eggshells to help them recognise forgeries.

However, because evolution isn't perfect, the arms race continues. This high-stakes cold war is waged with colours and patterns in the scrublands of southern Zambia. It’s a battle that’s probably being fought everywhere there are birds 'brood parasites' laying eggs in the nests of another bird species.

During Master’s research at the University of Cambridge, lead author Eleanor Caves methodically examined how two songbird species pattern their eggs to try to stay one step ahead of two species of parasitic bird.

“We don’t know at what stage of this evolutionary arms race we’re seeing these species,” Caves said, but her study has found that birds that are heavily parasitized are able to 'sign' their eggs with colour and pattern traits in unpredictable combinations.

The hundreds of eggs used in her study had been meticulously collected and catalogued by a retired British military officer, Major John F R  Colebrook-Robjent, on his ranch in the Choma district of Southern Zambia over a 35-year period. He painstakingly drilled and blew out each of the tiny eggs and marked them with a set number to indicate which ones belonged in the same clutch.

Colebrook-Robjent’s egg collection grew to at least 14,000 clutches, from raptors to waterbirds to the songbirds Caves studied. Upon his death in 2008, the collection was bequeathed to the British Museum, but it’s still housed on a game farm in Zambia, which proved lucky for Caves.

“It’s a hugely valuable scientific resource,” said Caves, who is the first author on the study with colleagues from Britain and South Africa. She spent a week just sorting and organising the eggs she found in the collection, then set up an outdoor lab to replicate natural lighting in the area where they were collected. Caves characterised each egg according to five measurements of background colour and brightness.

Taking such measurements outdoors on a Zambian game farm came with unique challenges. “Sometimes I had to grab the eggs and run inside because baboons were coming,” Caves said.

Caves and colleagues also analysed digital photographs of the eggs to quantify their markings and the contrast between patterns and background colour. In all, the researchers measured nine pattern variables or traits for each egg. The study includes at least 10 clutches each from 11 warbler species and 11 weaver species.

The parasitic Cuckoo Finches and Diederik Cuckoos produce a wide array of egg patterns in an attempt to mimic the variety of their hosts. However, an individual parasitic bird isn’t always careful to lay in a nest that closely matches the pattern it's producing, so rejections are common.

As for the songbirds, a female will lay one pattern-type with just a little variability during her whole breeding 'career'. The colours and patterns are created in the shell gland of the mother bird hours before the egg is laid, but the specifics of the system are still a mystery.

Variation by itself is not new. What’s new is being able to study how host birds arrange combinations of individual traits that add up the overall pattern of the eggs, Caves said. She enlisted the help of Duke statistician Edwin Iversen to help her assess the 'differential entropy' of the patterns.

Multi-component visual signatures that are individually distinctive like this are important to other species in the animal world such as paper wasps, which have individual patterns on their faces and abdomens to help identify nest-mates.

Understanding how to make individually distinctive signals from just a few components might also have security uses, like devising a bank note that’s more difficult to forge, Caves said.

Caves, E, Stevens, M, Iversen, E, and Spottiswoode, C. 2015. Hosts of brood parasites have evolved egg phenotypic signatures with elevated information content. Proceedings of the Royal Society B DOI: 10.1098/rspb.2015.0598.
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