Pyrrolizidine alkaloids are considered the primary defence mechanism in aposematic ithomiine butterflies and arctiid moths. Despite evidence that pyrrolizidine alkaloids are effective against some invertebrate predators, proof for a protective function of pyrrolizidine alkaloids against vertebrate predators is fragmented. The present work shows that the pyrrolizidine alkaloid monocrotaline is unpalatable to the pileated finch, Coryphospingus pileatus, and that the unpalatability is learned through association with a specific colour pattern (blue stripes). In a series of trials, using mealworms as model prey, birds rejected those to which pyrrolizidine alkaloid solution had been applied topically but accepted prey devoid of the alkaloid. Subsequent offerings of prey with pyrrolizidine alkaloid and a painted blue-striped pattern led to consistent rejections by the experimental birds. Birds were then offered blue-striped painted larvae without pyrrolizidine alkaloids ('mimics'), which were rejected at levels similar to the previous trial. The predators learned to recognize the prey as unpalatable items based on their experience in the previous encounters. These results provide evidence for the protective capacity of the pyrrolizidine alkaloid against a vertebrate predator and supports the role of these chemicals in aposematism in the Lepidoptera. (C) 1997 The Association for the Study of Animal Behaviour.544985991