Warning coloration: a conspicuous appearance carried by an animal that is costly to attack because it is toxic, distasteful or armed.
An aposematic signal pairs a defence with a display that predators learn, or are born knowing, to avoid. It predicts that a conspicuous defended form is attacked less often than an inconspicuous one in the same place. Müllerian mimics are defended species that share a warning pattern; Batesian mimics are harmless species that copy one.
Bright does not mean warning. A vivid colour can equally be a mating signal or a by-product, so the label needs evidence both that the animal is defended and that the colour is associated with fewer attacks.
In a field experiment in Costa Rica, red clay models resembling the local poison frog were attacked by birds less often than brown models.
It is one of the few proposed purposes of animal colour that has been tested by field experiment, which makes it a benchmark for how much evidence the other proposed purposes have.
Aposematism is tested by counting attacks. Researchers set out artificial prey that differ only in appearance and score the marks predators leave. In Costa Rica, red clay models of a poison frog were attacked by birds less often than brown ones. Plasticine replicas of harmless ringed snakes were attacked far more often where venomous coral snakes are absent than where they live. Butterflies whose colour form matched the local distasteful species survived better than those that did not. Each result holds for one system in one place, and motionless models are a proxy for predation on the real animal.
A brightly coloured animal is described as 'warning predators' because it is bright.
The claim skips both halves of the evidence: that the animal is defended, and that the colour is associated with fewer attacks. Without them the colour may be a mating signal, a by-product or, as experiments on iridescent beetles found, a form of concealment.
A field experiment paints models to match the animal's colour as a person sees it.
Birds, the usual predators, are sensitive to ultraviolet, so a paint that matches to a human eye may not match to a bird's. Careful studies check the paint against the animal's reflectance spectrum.