By John Upton
When Eurasian rollers forage for insect prey for their young, they’re not just on a quest for nourishing fat and protein. They’re fossicking through an ecological armory for chemical weapons.
Some plants produce toxins to deter herbivores. Some insects that eat those plants use those plant toxins for their own defense. Eurasian roller chicks use the plant toxins from those insects to produce a pungent orange liquid — an unsavory concoction that scientists have concluded is used as a defense against predators.
A team of Spanish researchers found that Eurasian roller nestlings vomited when they picked them up, but not when they approached the young birds, talked to them or gently prodded them. “This fact suggests that the vomit might be produced in response to some kind of predators that actively grasp and move prey during the predation event such as snakes, rats and mustelids, which are common predators of hole-nesting species as rollers,” the scientists wrote in a paper published in the journal PLOS ONE.
The researchers collected the puke and smeared some of it on pieces of chicken, which they offered (with the smeared side hidden) to 25 dogs alongside a similar chunk of poultry smeared only with water. Some of the mutts strangely showed no appetite for chicken whatsoever. But 18 of the 20 dogs with a hankering for hen opted first for the untreated meat, indicating that the smell is off-putting for a predator. Most of those 18 dogs subsequently wolfed down the vomit-smeared chicken, but six of them left it entirely alone.
“One could wonder about the nestling advantage of this defence,” the scientists wrote. “Kin selection is a possible answer to that question because a predator that finds the first nestling of a brood of five to be distasteful may leave alive the others.”
From where do the chicks get the hydroxybenzoic and hydroxycinnamic acids, phenolic acids and psoralen needed to produce their unpalatable puke?
The scientists matched these compounds to toxins produced by plants to deter animals from feeding on them. Many insects have developed an immunity to such toxins, and some use the plants’ toxins to defend themselves. That’s the case for many of the grasshoppers upon which the rollers prey, and the scientists believe that the chicks are, in turn, purloining the poisons from the grasshoppers to defend themselves.
But that’s not all — the scientists think that the parent birds might also be hunting for more-poisonous insects, such as centipedes, that most other birds would never touch.
“Grasshoppers are the main prey that rollers hunt to feed their nestlings,” they wrote. “Furthermore, rollers feed their offspring with a large share of poisonous arthropods that are avoided by most of the other sympatric insectivorous birds. This suggests that rollers are resistant to these toxic substances and could have the ability to sequester chemicals from their protected prey to defend themselves.”