Birds in the News

Proceed With Caution

Baby Cinereous Mourners disguise themselves to look like caterpillars, and researchers have the photos to prove it.

The picture above shows a...
a) Bird
b) Horned melon
c) Deadly caterpillar
d) Cheeto past its expiration date

If you guessed a then you're correct (and also maybe psychic).

Meet the Cinereous Mourner, the first feathered caterpillar mimic known to man. The tropical lowlands, in which this bird lives, are crammed with nightmarish predators, such as monkeys and snakes. Rather than trying to outmuscle its foes, the bird outwits them. Newborn mourners cover themselves in neon-orange plumage to channel the noxious larvae of the flannel moth, which live in the same neighborhood. They don't skimp on the details either: A close-up of the nestling's feathers shows that they're tipped in white barbs that resemble the pale ends of a caterpillar's stinging hairs.

The guileful guise was initially noticed in museum specimens from England in 2012. But the miming behavior had not been seen in the wild: that is until Gustavo Londoño, a scientist from the University of California, Riverside, decided to pick it up as a labor-intensive side project. Londoño was already in Peru's Manu National Park, leading a long-term survey on birds living at different altitudes of the rainforest. It took him and his team eight years to find a Cinereous Mourner nest—the first to ever be recorded. To do so, the researchers had to spot mature mourners and follow them back to their nests. This was no easy task. Despite being classified as songbirds, the species is quiet and austere, with sooty wings, haphazard orange flecks, and a simple yellow eye ring. And though it has a robust population, the bird is spread thin across the upper half of South America, making it even more difficult to detect.

In 2013, Londonõ's crew stumbled upon a second nest near the Madre de Dios River. This one held a viable egg. Over the next couple of weeks the researchers took photographs of the newborn bird, tracking slight changes in its plumage. When Londoño approached the nestling for measurements, the bird bobbed its head from side-to-side, just like a roving caterpillar. It was a way for the juvenile to ramp up its defense while under "attack." It then repeated the behavior during parental visits. Each time the adults came home they would belt out a few notes: a code to tell the offspring that it was safe to beg for food. When left alone, the nestling would assume the caterpillar position and lay silently on its belly.

This is the critter that inspired the charade:

Morphological mimicry is popular among caterpillars, but it's a rare feat in the bird world. The Cinereous Mourner lives in a cutthroat environment; according to Londoño, up to 90 percent of juveniles fall victim to carnivores. To ensure that the remaining few make it out of the nest alive, the species has to get creative and slow down its development. It takes about a year for Cinereous Mourners to shed their orange cloaks and fledge. In most other species, it only takes two weeks to a month. The delay probably results from a strategic feeding schedule: Adults space out their deliveries to draw less attention to the nest. On the other hand, belated growth spurts could cause young birds to be more susceptible to attackers. It's a risky tradeoff that the mourners must make to reduce the pressure of predation, says Londoño.

So why does the bird fake it? Why doesn't it stuff its feathers full of poison? It all depends on the the diet, Londoño says. Cinereous Mourners eat caterpillars (not the lethal kind) rather than beetles and ants, which are what the Hooded Pitohui uses to stock up on toxins. Both species exhibit bright colors as a sign of caution—pitohuis as adults and mourners as nestlings. It's a behavior known as aposematism, which again, is not often found in birds. 

Londoño's next pet project involves the mourner's closest of kin. He wants to trace the origin of the imitation game and see if it's been picked up by other birds in the locale. One step for the Cinereous Mourner, one giant leap for birdkind.

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