When Eurasian rollers feed their babies grasshoppers, centipedes, and other insects, the chicks aren’t just getting the nutrition they need to grow—they’re getting an arsenal.
When animals can’t make their own defenses, they often borrow them from elsewhere. Poison dart frogs hang on to the toxic alkaloids in the beetles and mites that they eat, and then secrete the toxins through their skin. The caterpillars of tobacco hornworms eat tobacco leaves and then exhale the nicotine in a cloud of “defensive halitosis.” African crested rats gnaw on the roots and bark of certain trees and then slobber the poison onto their fur.
Rollers—stocky, blue-and-cinnamon-colored birds related to kingfishers—also take out a chemical loan to defend themselves, but they go through a middleman. Their diets consist mostly of poisonous insects, and they’re not only resistant to the bugs’ toxins, but are able to absorb them and use them for themselves. Many of these insects, in turn, sequester those same toxins after eating poisonous plants that they’d become resistant to. At two different points in the food chain, animals have worked out ways both to defeat and co-opt their meals’ defenses—and chemicals originally produced by plants to protect them get passed around to do the same for other organisms.
Poison dart frogs advertise their toxicity with bright colors and bold patterns. Young rollers also warn potential predators that they don’t taste good, but do it in a more active, and unsavory, manner—they puke up a pungent orange liquid at them.
Scientists had puzzled for years over roller chicks’ propensity for vomiting. Because it costs the young birds precious bodily fluids, they thought it had to have an important purpose. Deseada Parejo, a biologist at Spain’s Arid Zones Experimental Research Station, first encountered the brightly-colored vomit while she was studying the species’ family dynamics a few years ago. One day, when she plucked a chick from a nest to measure its size and weight, it let about a teaspoon’s worth of vomit loose at her. The next chick she grabbed did the same thing. And the next. And the next.
She describes the smell of the puke as orange juice and insects, and she’s not the only one who notices it. Roller parents who return to a nest that smells of vomit approach their home more cautiously than usual and spend less time feeding the kids they’d left there, as if they’re scared of something. Maybe the vomit is a defensive reaction, Parejo thought, and the lingering smell also warns the parents that a predator still might be lurking near the nest.
To test the idea, Parejo and her team went to south-eastern Spain, where they worked with wild rollers to see what they ate, what prompted them to puke, what their vomit contained, and how other animals reacted to the sticky orange liquid. The birds’ diet was almost 90 percent grasshoppers, with some butterflies and centipedes mixed in. What was left of those bugs came back up at the researchers whenever they picked the birds up or moved them, but not when they touched the birds gently or simply got close to them without any contact. The chicks also puked more after they’d just eaten. When the scientists deprived them of food for an hour or more, fewer of the birds puked.
When Parejo’s team analyzed the vomit, they discovered hydrobenzoic and hydroxycinnamic acids and psoralen, which are all produced by plants to deter pathogens and insects. The same chemicals have been found in the secretions of different grasshoppers and other insects, including ones hunted by rollers.
The pieces were starting to fall into place: The rollers were eating toxic insects, and poisons originally produced by plants had found their way into the birds’ puke. Their vomiting also depended directly on recently consumed food (that is, they didn’t seem to have a way to produce the toxic stew on their own) and they only employed the trick when they were seriously disturbed or harassed. Specifically, Parejo had to grasp and move the chicks, the same way that a predator like a snake or weasel might try to grab one and flee, to get a reaction.
It definitely looked like the puke was a defensive weapon, but did it work? To see if the third-hand poisons would deter the chicks’ predators, the team wetted pieces of chicken meat with either roller vomit or water than then offered both treats to dogs. The pooches overwhelmingly went after the water-covered chicken first, but almost two-thirds of them also ate the vomit meat a few minutes after their first snack.
This suggests that the smell of the vomit is part of the defensive effect, but that it only works for a short time. Biting a puke-covered, toxin-filled bird would also give the predator a bad taste in their mouth and a serious stomach ache—or worse—but by that time the bird is already in danger of injury or death. What’s the point of the defense, then, if a hatchling might wind up in an animal’s jaws anyway?
Parejo thinks the underlying strategy has to do with the survival of the nest, and not necessarily every bird in it. If a predator bites or eats one roller chick and doesn’t like the taste, it will probably leave the others alive and seek tastier prey. If the smell of the vomit alone isn’t enough to drive an animal away, the trick still works because it only costs one casualty instead of the whole brood.