In a series of masterful experiments, Lincoln Brower and his colleagues proved that monarchs obtain poisons (cardiac glycosides) from their food plants, that these poisons make birds ill, and that birds that have been sickened by eating a monarch thereafter reject monarchs on sight.
He raised some monarchs on a species of milkweed that does not contain cardiac glycosides and others on a species that does. Blue jays that had been kept in captivity long enough to forget previous experiences with monarchs eagerly consumed monarchs that had been raised on the milkweed that does not contain cardiac glycosides.
They continued to eat nontoxic monarchs until they were given monarchs that had been raised on the species of milkweed that does contain cardiac glycosides. Shortly after eating a toxic monarch, they showed obvious signs of distress and then vomited and retched.
Thereafter these bluejays rejected both toxic and nontoxic monarchs, and some retched at the sight of a monarch.
