ABSTRACT
By suspending nets within and adjacent to a 2.25 hectare overwintering colony of monarch butterflies in Mexico, we estimated that black-backed orioles and black-headed grosbeaks killed 4,550 to 34,300 and an average of 15,067 butterflies per day. A conservative calculation of mortality through the 135 day overwintering season was 2.034 million butterflies, or about 9% of the colony. The birds preyed selectively upon male butterflies, possibly because of a difference in fat content, or possibly because females contain higher concentrations and larger amounts of cardenolide or other defensive chemicals. The risk to individual monarchs of being killed was much greater on the colony periphery and in thinned areas of the forest. Bird predation thus is sufficient to have played a major role in shaping the evolution of the monarch's overwintering and aggregation behavior. Substantial daily variation in predation intensity occurred, 26% of which was attributable to the birds eating more butterflies on colder days, and 30% of which was attributable to a 7.85 day predation cycle. The hypothesis is put forward that the birds feed cyclically because they build up toxic levels of cardenolides or other defensive chemicals contained in the butterflies. The cyclic predation may reduce total predation on the colony by as much as 50%. Such chemical-based group protection is interpreted as a fortuitous by-product of the evolution of unpalatability through selective processes acting on other phases of the monarch's life history.
