An imposter mimicking an imposter: What sounds like the premise for a cheesy spy film is in fact legitimate scientific research. A group of biologists and animal behaviourists from across the United States and Argentina have designed 3-D-printed eggs to replace parasitic eggs in bird nests so that ornithologists can study how birds identify and react to unwelcome houseguests even more closely.

The original imposter eggs are produced by brood parasites—animals that rely on the parenting of another species to raise their young. Cowbirds are some of the best-known beneficiaries of this technique: They’re renowned for placing their eggs in other species’ nests and letting the oblivious owners act as surrogate parents.

To test how birds react to such egg-regious intruders, researchers fill the nests with phony shells. In the past, egg props have been fashioned out of plaster, wood, and plastic. But those creations failed as the birds found the handiwork unconvincing and chucked the eggs before the experiment really began. Researches realized that they needed to come up with a more realistic-looking egg.

To create more convincing duplicates, the researchers drew up computerized digital models of American Robin and Brown-headed Cowbird eggs, then built prototypes on a 3-D printer, leaving the fake shells hollow so they could be filled with liquid or gel. The filling makes the replicas responsive to temperature and gives them some weight, so that they’re more convincing when placed in the nest. Some eggs were painted with a bluish-green hue to represent robin eggs; others were painted beige, to make them look like a cowbird’s. Both sets were then tested out in a robin’s nest.

The eggs were so convincing that the robins accepted every one of the bluish-green models, and tossed out almost 80 percent of the cowbird mimics.

In the future, such observations will enable researchers to study how exactly birds use shape, color, pattern, and size as cues to help them recognize their own eggs. The 3-D shells could even be useful for revealing how birds count their eggs—and use their beaks to fling the bad ones out.

In the end, it's all too bad for the ingenious cowbird, which might soon find itself in stiff competition with researchers for a good host nest.