The bird that mimics a toxic caterpillar
13:29 11 December 2014 by Sandhya Sekar
Species: Laniocera hypopyrra
Habitat: Lowland Amazonian rainforests in South America
In the warm, humid tropical forests of the Peruvian Amazon, a rather drab grey bird makes its way to a cup-shaped nest made of dry leaves, its beak stuffed with a juicy insect.
Inside the nest, however, there are no gaping mouths and crying chicks begging for food. Instead, a creature starts sliding around inside the nest. On closer inspection it seems that a huge, bright orange caterpillar has ravaged the nest.
But something magical happens next. The bird gives out a call and the caterpillar transforms into a hungry chick eager to get its meal.
The chick is as different from its parents as can be. It mimics the look and movements of a poisonous species of the caterpillar family Magalopygidae, to avoid being eaten by snakes and monkeys. This trick is known as Batesian mimicry, and is more common in invertebrates. Think of the harmless butterflies mimicking poisonous species from the same area. Among birds, the only other recorded example is of burrowing owls producing hissing sounds like an agitated rattlesnake when disturbed.
But in Laniocera hypopyrra‘s (the cinereous mourner) mimicry is more complex, involving both looks and behaviour, something not spotted in birds before.
The medium-sized bird, found in lowland forests in the Amazon basin in South America, has one brood per year and mainly feeds on insects. It builds unlined, cup-shaped nests from dry leaves in areas with relatively open undergrowth.
Cinereous mourner parents spend a lot of time foraging away from the nest, and return to the nest only once each hour. This is a very low feeding rate for such a small bird, and the chicks grow slowly, taking around 20 days to start flying. This gives ample time for predators to eat up chicks.
The intense predation pressure, which could be as high as 80 per cent among birds in habitats where the mourner lives seems to have driven the evolution of complex anti-predatory strategies in the species.
Gustavo Londono, now at the University of California, came across a nest of the species in 2012, and later set up cameras to observe what happens in it.
The chick has bright orange feathers, some lined with black ridges and tipped with white barbs, so it looks and moves like a caterpillar.
The most risky period in their life is the first 18 days, after that the orange down feathers and barbs thin out, and body and wing feathers emerge fully. During those early days the chick is also similar in size to the 12–centimetre-long moth caterpillar.
By bobbing its head from side to side it gives the appearance of a sliding caterpillar. Movement and appearance combined would probably be enough to deter predators, Londono says.
Adding to the visual illusion is the unusual behaviour of the chick – it does not beg for food as soon as the parent appears, it assumes the new arrival is a potential predator and so it behaves accordingly until the parent gives a particular call.
As well as mimicry, the just-hatched chicks’ bright orange colour could double up as camouflage, as they blend in nicely with the dried leaves lining the nest.
“Mimicry plays a major role in deterring predators, but camouflage is also likely to occur when the nestling is on the nest,” says Londono. “To know more about the relative importance of these strategies in this species, we need studies with larger sample sizes.”
Journal reference: American Naturalist, DOI: 10.1086/679106