Using Scent to Deter Predators
The European Roller (aka Eurasian Roller) is the first bird to be identified using scent as a deterrent to predation.
Nestlings will vomit a foul smelling substance onto themselves and in the nest when they are frightened. This supposedly deters any predator with a sense of smell. It also alerts the parents on their return to the nest that it has been disturbed.
Many birds vomit when disturbed, but the target of such behavior is usually a mammal or other non-bird. It is thought that birds rarely use scent cues for themselves.
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