Parasites beware. Victimized birds are fighting back against forgeries with sophisticated signatures to authenticate their eggs. Scientists have discovered evidence of an evolutionary arms race between cuckoo cheats and parasitized birds.

The common cuckoo (Cuculus canorus) is the classic brood parasite. They sneak their eggs into other birds’ nests, and not only do the host parents end up taking care of the cuckoo babies, but those freeloading chicks also push out all the rightful nestlings. Whenever host species develop the ability to reject imposter eggs, the cuckoo improves its ability to lay better mimics over evolutionary time.

Here’s just one snapshot. Using a new visual recognition algorithm, a team led by Harvard’s Mary Caswell Stoddard investigated the eggshell patterns of birds targeted by cuckoos. Their tool, called NaturePatternMatch, extracts and compares recognizable features. It detects the same wavelengths seen by birds and recreates the avian cognitive processes for recognition. 

“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,” Stoddard explains in a news release. 

Working among the egg collections of the Natural History Museum in London, the team studied the pigmentation patterns on 689 eggs laid by eight different targeted species. They found that, in order to distinguish their own from those of cuckoos, host birds have evolved “highly recognizable patterns” on their eggs that help them reject counterfeit eggs before they hatch. “Now we know that host birds can employ their own defenses, just as a bank adds special watermarks to its dollar bills,” Stoddard tells Nature. 

Hosts with the most sophisticated egg pattern signatures are the ones subjected to the most intense (and the best) cuckoo egg mimicry. Their findings were reported in Nature Communications last week. Furthermore, the birds go about thwarting cuckoo trickery in different ways. Effective eggs pattern signatures can be replicable, distinctive, and complex -- but they shouldn’t have all three features at the same time. 

Some species, such as the red-backed shrike (Lanius collurio), evolved egg patterns that are highly repeatable within a single clutch; that means all the siblings have the same eggshell markings. Others like the great reed warbler (Acrocephalus arundinaceus, pictured with cuckoo baby) use patterns that differ dramatically from female to female within a population. Still, others produce egg patterns are visually complex. Heavily targeted bramblings (Fringilla montifringilla) evolved highly recognizable markings: sparse blotches that are unevenly spaced. 

“Some species use two of these strategies, but none uses all three,” says study coauthor Rebecca Kilner from the University of Cambridge. “A signature like this would be too complex to be easily recognized.”

Another paper investigating Eurasian magpies (Pica pica) and great spotted cuckoos (Clamator glandarius) found that older magpies were able to reject fraudulent eggs better than new moms. 

Images: Mary Caswell Stoddard/Natural History Museum (top) & David Kjaer (middle)