Birds can overcome the cardenolide defence of monarch butterflies in Mexico

Abstract

Flocks of black-backed orioles (Icterus abeillei Lesson) and black-headed grosbeaks (Pheucticus melanocephalus Swainson) eat several hundred thousand monarch butterflies (Danaus plexippus L.) in the dense overwintering colonies in central Mexico, and in 1979 were responsible for over 60% of the butterfly mortality at several sites¹. Such predation is unusual because, during larval development the aposematically coloured monarch butterfly sequesters cardenolides from its milkweed foodplants (Asclepiadaceae)^2,3. These bitter-tasting heart poisons cause vomiting in 12 species of birds in 9 families (refs 4–9, and L. P. Brower, unpublished observation), although the domestic chicken, Japanese quail, hedgehog, mouse and sheep^8,10,11 have been shown to be insensitive to their emetic effects. Extensive predation of monarch butterflies by birds has never been observed except in Mexico. We report here that the Mexican butterflies are weakly emetic, and that taste discrimination by orioles and cardenolide insensitivity of grosbeaks allow these birds to feed freely on monarch butterflies.