Why This Bizarre Bird in the Amazon Pretends to Be a Caterpillar

As if to prove that we still have much to learn about the animal kingdom, a rarely encountered bird species of the Amazonian region caused a stir in the scientific community around a decade ago when it was caught impersonating a caterpillar! To be more precise, the bird was a Cinereous Mourner (Laniocera hypopyrra) chick. The caterpillar it was impersonating was a toxic moth caterpillar. One example is the flannel moth caterpillar, specifically those in the Megalopygidae family, which are found in the Amazon basin. As you can see in the fascinating clip, the little birds not only resemble a hairy caterpillar but also stretch into an elongated shape and writhe in a very caterpillar-like manner.

About Cinereous Mourners

Cinereous Mourners are primarily grey birds with pale cinnamon spots that measure around 8 inches and weigh around 1.5 ounces. They resemble a combination of a dove and a thrush. This species is distributed throughout the entire Amazon–Orinoco Basin and prefers humid, flooded forests near ravine woodlands. They are typically found in the understory or midstory of the forest. They feed on arthropods and fruit. Their nests are constructed about 6–7 feet above the ground, usually in the branches of an epiphytic fern attached to a tree trunk.

Discovering the Unusual Nestlings

The remarkable discovery was made by researchers from the University of California, Riverside, and two Colombian universities—Universidad Nacional de Colombia and Universidad de Los Andes—during the fall of 2012. Simply finding a Cinereous Mourner nest is exciting enough. At the time, this was only the second nest recorded in that area, although more have been found since. The scientists observed the nest until the eggs hatched, and that is when they noticed something unexpected.

The chicks had very unusual feathers. They had distinctive bright orange barbs with white tips, which were nothing like any other nestlings in the area. What’s more, when the scientists disturbed the chicks, they elongated their bodies and moved their heads slowly from side to side just like a caterpillar. If you glance quickly at the clip, you could easily be fooled into thinking you are watching a caterpillar rather than a baby bird. Their findings were published a few years later in The American Naturalist and were described as an example of a special form of animal mimicry called Batesian mimicry

What Is Batesian Mimicry?

Batesian mimicry is a defense mechanism used by creatures to stop themselves from being eaten by a predator. It involves a non-toxic species mimicking a dangerous or unpleasant species to fool predators. Batesian mimicry requires three elements: a model (a toxic or foul-tasting creature, such as the flannel moth caterpillar observed near the nest), a mimic (the Cinereous Mourner nestlings), and a predator that recognizes and avoids the model. In this case, it would be monkeys and snakes who have encountered toxic moth caterpillars before and know that their bright colors and movements are a sign that they should keep away.

Animals that advertise their unpleasant taste or danger (even to the touch) are called aposematic, and this technique is known as aposematic signaling. It is a way of advertising unpalatability. Batesian mimicry also depends on predators associating aposematic signals with previous unpleasant experiences. In this case, the predator knows that the bright orange color and movements are associated with venomous fur-like spines. They also know that these spines can cause localized pain, as well as more severe symptoms such as swelling, numbness, and even difficulty breathing.

Why Do These Birds Use Batesian Mimicry?

This finding raises the question of why this species uses Batesian mimicry when other bird species in the area do not. The answer probably lies in the particular vulnerability of these chicks. The nests may be built high up in the tree canopy, but that is not sufficient to protect them from hungry snakes and monkeys who also inhabit this zone. To make matters worse, the parents feed the chicks infrequently, visiting the nest about once an hour on average, although this can vary. There is a long time between feedings when the chicks are undefended. Furthermore, due to their limited diet, Cinereous Mourner chicks have a relatively long nesting period. It takes approximately 20 days for them to become sufficiently mature to leave the nest. This combination of slow growth and high nest predation, resulting from limited parental protection, has favored the evolution of an antipredation strategy. Mimicking a toxic caterpillar has proven to be an effective survival strategy!