Birds Evolve Signature, Colorful Patterns to Combat the Parasitic Cuckoo

Bird eggs are speckled and spotted, displaying intricate patterns on their delicate shells. Now, scientists have found out exactly why these eggs display such colorful markings. It turns out that birds affected by the parasitic Common Cuckoo may have evolved distinctive patterns on their eggs in order to distinguish them from those laid by a cuckoo.

Cuckoos don't make their own nests. Instead, they lay their eggs in other birds' nests and fly away. Once their offspring hatches, it will push the other natural eggs out of the nest and the surrogate parents will care for it until it's old enough to fly away. That's why it's crucial for birds to recognize the imposter egg before it hatches.

In order to see how a bird brain might perceive and recognize complex pattern information, the researchers developed a new computer vision tool called NATUREPATTERNMATCH. The tool extracts and compares recognizable features in visual scenes, recreating processes known to be important for recognition tasks in vertebrates.

The researchers used the tool to study the pigmentation patterns on hundreds of eggs laid by eight different species that act as hosts for the Common Cuckoo. In the end, they found that the eggs had evolved highly recognizable patterns that were characterized by distinctive blotches and markings. More specifically, the eggs with the best pattern signatures were ones that have been subjected to the most intense cuckoo mimicry.

"The ability of Common Cuckoos to mimic the appearance of many of their hosts' eggs has been known for centuries," said Mary Caswell Stoddard, one of the researchers, in a news release. "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."

The findings reveal that host birds and cuckoos are evolving in an ever-competing arms race. As cuckoos become more adept at mimicry, host bird eggs evolve more intricate patterns. The findings reveal a bit more about bird evolution and show how birds manage to cope with the parasitic cuckoo.

The findings are published in the journal Nature Communications.