Eurasian Roller Nestlings Use Vomit as a Defensive Weapon

Nestlings confined to a nest, unable to fly or make a rapid escape, have very few options when a predator enters. Eurasian roller (Coracias garrulus) nestlings, however, have developed an unusual response. When strongly disturbed, they regurgitate an odorous orange liquid from their mouths. Ornithologists had observed this behavior before, but it remained unclear whether the vomit was simply a physiological response to distress or served a specific function.

Eurasian roller (Coracias garrulus)（Image source：Artur Mikołajewski， CC BY 3.0 ）

Chemical defenses are widespread throughout the animal kingdom. Many arthropods and amphibians use toxic or unpalatable chemicals to reduce the risk of predation, and some animals can obtain defensive compounds from their diet and retain them for their own protection. Such examples are relatively uncommon among birds, leading researchers to suspect that the fluid expelled by roller nestlings might also function as a form of chemical defense.

The diet of roller nestlings consists largely of insects in the order Orthoptera, including many grasshoppers. When researchers analyzed prey brought to the nest by the parents, 103 of the 112 prey items identified from video recordings belonged to Orthoptera, accounting for 92% of the total. Grasshoppers themselves are well known for regurgitating fluid when threatened, and these secretions may contain plant-derived secondary compounds that make them distasteful and discourage predators from attacking.

Nest of the Eurasian roller（Image source：Dûrzan cîrano， CC BY-SA 3.0 ）

Eurasian roller nestlings（Courtesy of Nadia Silva ）

To determine what actually triggers vomiting in roller nestlings, the researchers exposed them sequentially to several different stimuli. They first spoke loudly to the birds, then showed them a human face, gently touched them, and finally picked them up and moved them. None of the 43 nestlings vomited in response to sound, visual stimulation, or simple touch. Vomiting occurred only after the birds were picked up and moved. Many nestlings began showing this behavior before their eyes had even opened, further indicating that visual cues were not required. The behavior first appeared at an average age of about 6.7 days and disappeared at around 19.6 days, close to the time of fledging.

This timing is consistent with a defensive function. While confined to cavity nests, young rollers have little ability to escape. Predators such as snakes, rats, and mustelids may seize or move a nestling after entering the nest. Such handling could trigger vomiting, exposing the predator to the strongly scented and unpleasant-tasting fluid before the chick is swallowed.

But where does this fluid come from? To investigate its origin, the researchers fitted some nestlings with special neck collars that prevented food delivered by the parents from passing into the digestive tract. After one hour, vomit production had decreased in 12 of the 14 nestlings wearing collars, compared with only 4 of the 14 nestlings without collars. This indicated that the production of vomit was closely associated with recent food intake.

The researchers then analyzed 16 vomit samples and detected hydroxybenzoic acid and hydroxycinnamic acid in every sample, although only trace amounts were present in some cases. The mean concentration of hydroxybenzoic acid was approximately 481 ppb, while that of hydroxycinnamic acid was about 150 ppb. Traces of psoralen were also detected in several samples. Hydroxybenzoic and hydroxycinnamic acids are phenolic acids commonly found in plant tissues and can deter herbivorous insects from feeding. Psoralen belongs to a group of compounds known as furanocoumarins, which are also produced by plants and can contribute to defenses against pathogens and herbivores.

These findings suggest a chemical pathway extending across three trophic levels. Plants produce defensive compounds, phytophagous insects consume those plants and may retain some of their chemical defenses, and rollers then prey upon these insects. Compounds originating from plants may consequently reach the nestlings through their prey and ultimately appear in the fluid they regurgitate.

Orange oral fluid produced by a Eurasian roller nestling（Courtesy of Parejo D et al ）

The next question was whether this chemical mixture could actually discourage a predator. To test this, the researchers used domestic dogs as a model for generalist mammalian predators. Each dog was offered two pieces of raw chicken: one smeared with distilled water and the other with fresh vomit from a roller nestling.

Of the 25 dogs tested, five did not respond. Among the remaining 20, 18 chose the chicken treated with distilled water first, while only two initially ate the meat coated with nestling vomit. Some dogs eventually consumed the second piece as well, but only after a delay. Of the 18 dogs that first selected the control meat, 12 later ate the vomit-treated meat after waiting an average of about 118 seconds, while the other six never ate it during the experiment. The results showed that roller nestling vomit could substantially reduce the attractiveness of otherwise palatable meat to dogs.

Because the odor of the vomit appears to be volatile, its deterrent effect may weaken with time. This could explain why some dogs initially avoided the treated meat but eventually ate it several minutes later. Even so, about 30% of the responsive dogs refused to eat the vomit-treated meat altogether, even when it remained as their second option.

The results therefore suggest that vomiting may provide a short-lived but potentially important defense during the critical moments immediately after a predator seizes a nestling. The unpleasant taste and odor may cause a predator to release its prey or abandon the attack. Even if the first nestling has already been bitten, the experience could discourage the predator from continuing to consume the remaining chicks in the brood, potentially improving the survival chances of their siblings.

Author: Shui-Ye You

Reference:

Parejo D et al. (2013). Armed Rollers: Does Nestling's Vomit Function as a Defence against Predators? PLOS One.