Predation on Aposematic Ithomiine Butterflies by Tanagers (Pipraeidea melanonota)

Tanagers (Pipraeidea melanonota)have been found preying heavily and selectively on winter concentrations of 22 species of ithomiine butterflies in the region of Sumare, Sao Paulo. Flying birds catch resting butterflies in the cold of the early morning, take them to favored perches, and squeeze out and eat the abdominal contents, leaving the remainder of the butterfly as testimony to the predation. No other insectivorous birds in the region have been observed to attack these butterflies, nor are any literature reports known for such predation on ithomiines, which are well-known aposematic models for mimicry rings throughout the Neotropics. The commonest ithomiine species in the colonies, Mechanitis polymnia, is attacked in far greater proportion than its true abundance, probably because of its conspicuous coloration and roosting habits, and its more sluggish behavior than the other two common larger species, which are taken in proportions lower than expected from analysis of net captures. This behavior pattern is probably a function of abundance in the butterflies, thereby leading to the density-dependent predation observed in the system.