Babies of a bird species called the Eurasian roller vomit a foul-smelling orange liquid as a defence mechanism against predators, biologists have discovered. Offspring of the bright-blue jackdaw-sized bird -- Latin name Coracias garrulus -- throw up the repugnant fluid when they are frightened in their nests, according to a paper appearing on Wednesday in the journal Biology Letters. Covered in vomit, the nestlings not surprisingly become less attractive as a snack, the team says. But the smell also alerts parents, returning to the nest, that a threatening incident has happened in their absence, they believe. The scientists tested the \"olfactory cue\" theory by visiting nests with 10-day-old nestlings inside. They used a small paintbrush to daub a tiny amount of either lemon juice or vomit on the inside of the nest. Parents returning to a vomit-treated nest reacted with great caution, delaying the time when they would settle in the home. Previous research has found that birds have a surprisingly wide range of defensive reactions. For instance, the northern fulmar (Fulmaris glacialis) yawks up stomach oils against intruders that makes them lose their waterproof coating. And the common eider (Somateria mollissima) and northern shoveler (Anas acuta) have the ability to spray faeces on their eggs to deter mammal egg-thieves. However, the Eurasian roller is the first bird that has been found to use a scent, derived in response to a threat, as a means of communication, says the paper. In that regard, it joins many other animals, from insects to humans, that use the \"smell of fear\" to warn fellow members of their species of an attack. The study is led by Deseada Parejo of Spain\'s Estacio Experimental de Zonas Aridas.