Hummingbird chick mimics a toxic caterpillar to evade predators

Jay Falk and Scott Taylor first spotted the white-necked Jacobin hummingbird chick deep in Panama’s rainforest. The tiny bird, smaller than a pinky finger, was covered in brown fuzz.

When they approached, the chick twitched and shook its head, displaying behavior they had never seen in birds before. This movement resembled a poisonous caterpillar found in the same region.

Taylor, an associate professor at CU Boulder, and his team described this mimicry in a paper published in the journal Ecology.

“We know so little about what nesting birds do in the tropics,” said Falk, the paper’s first author and a postdoctoral fellow in Taylor’s lab. “But this could be a very common thing if we had dedicated more efforts to observe and discover new things about the natural world.”

Hidden survival strategies

White-necked Jacobin hummingbirds are common in Central and South America. Males have shimmering blue and green feathers, while females have muted green plumage.

The tropical rainforest poses many dangers for small birds, with predators such as snakes, monkeys, birds, and insects. Studies suggest bird chicks in the tropics face higher predation risks than those in temperate forests.

During a trip to Soberanía National Park in 2024, Falk might have uncovered how these tiny hummingbird chicks survive. Despite regularly observing adult Jacobins at his feeders, he had never seen their chicks or nests.

That changed when co-authors Michael Castaño and Sebastian Gallan-Giraldo found a female Jacobin incubating an egg near a forest trail. The nest, no bigger than Falk’s palm, blended seamlessly with its surroundings.

Hummingbird chick moves like a caterpillar

The researchers observed the nest for a month and saw the chick hatch.

Unlike most hummingbirds, which are born naked, this chick had long brown feathers that made it resemble the nest material. Then, they noticed its strange jerking behavior. Scientists had never recorded such behavior in any hummingbird species.

“I started texting a video to people and asking them, ‘What does this look like?’” said Taylor. “And invariably, they said, ‘That looks like a caterpillar.’ It was very exciting.”

On the chick’s second day, a predatory wasp hovered over the nest while the mother was away. The chick twitched its body and swung its head from side to side, mimicking a caterpillar. A few seconds later, the wasp flew away.

Lessons from other species

Falk and Taylor couldn’t help but think of another study. That one was about a bird, the cinereous mourner chick, living in the Amazon. It didn’t just look different – it had an unusual trick.

Covered in bright orange feathers, it closely resembled a toxic caterpillar. When disturbed, it shook its head just like the real thing.

Curious, the researchers turned their attention to Panama’s caterpillars. What they found was striking. Many were covered in brown hairs that weren’t just for show. Those tiny spikes could cause excruciating pain, even death, to predators.

And just like the hummingbird chick, some of these caterpillars had a habit of shaking their heads when they sensed danger. Was the tiny bird copying them? It certainly seemed possible.

Hummingbird mimics a caterpillar

This mimicry strategy, where a harmless species imitates a dangerous one, is called Batesian mimicry. It is common in butterflies and snakes. For instance, non-venomous milk snakes resemble venomous coral snakes to deter predators.

“A lot of these really classic examples of Batesian mimicry involve butterflies mimicking other butterflies, or snakes mimicking other snakes. But here, we have a bird potentially mimicking an insect, a vertebrate mimicking an invertebrate,” Taylor said.

Though the study was based on a single observation, the team hopes to test their theory by placing artificial chicks with different appearances and behaviors in nests to see which ones avoid predation. They also encourage birdwatchers and citizen scientists to document more hummingbird nests.

“Our perception of the natural world is very biased by our own thoughts about what could be possible,” Taylor said. “It’s incredible what we can discover, but we really have to think broadly.”

The study is published in the journal Ecology.

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