For many honest bird species, brood parasites like the cuckoo are no joke. These sneaky free-loaders comprise about one percent of all bird species. Sniffing out false eggs is serious business for many birds. Brood parasites plant eggs in unsuspecting nests and leave the duped foster parents to care for their chicks—usually to the deadly detriment of the foster parents' own babies.

Now, researchers from Harvard University and the University of Cambridge have discovered one way that bird parents likely keep an eye on their own eggs: with special visual signature. The researchers used the same kind of software that companies rely on for facial recognition and image stitching but applied that technology to hundreds of eggs of eight different parasitized bird species. They call the new program NaturePatternMatch.

The host birds, they found, have previously unrecognized egg "signatures"—essentially, secret visual cues that allow them to recognize their own among the imposters. The more intensely the bird species is targeted by cuckoos, the more complex and sophisticated their egg signatures. Some of the host birds, they found, produce exactly the same egg, whereas some show variation within their own clutch or between females within the same species. All of these methods, the team says, would likely be effective strategies for lessening the likelihood of being duped.

"The ability of Common Cuckoos to mimic the appearance of many of their hosts' eggs has been known for centuries," the researchers say in a statement. "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."