Survival of caterpillars in the face of predation by birds: Predator-free space, caterpillar mimicry and protective coloration

Caterpillars have been evolutionarily shaped by visually-orienting predators, in particular, birds. Predation events on caterpillars are difficult to witness. Studies of bird-caterpillar interactions have focused mainly on the numerical consequences of predation, and antipredator defenses have not generally been tested with free-living predators. Artificial clay models provide a novel tool to answer questions about caterpillar antipredator defenses in the face of predation by birds. Clay models allow manipulation of both caterpillar morphology and caterpillar placement within the different microhabitats in which it resides. Cryptic and aposematic colored caterpillar models were exposed to free-ranging predaceous birds. Survival times of different colored and different sized models in various habitat, weather, fellow herbivore, and vegetative microhabitats were measured. I found that: (1) aposematic models [which mimicked monarch butterfly caterpillars (Danaus plexippus )] had the greatest survival time, (2) models in fields had shorter survival times than those placed in forests, (3) smaller models survived longer than larger models and (4) seasonal patterns of predation coincided with the bird breeding season. No difference in survival time was detected for models on the surface versus undersides of leaves or for models on intact versus damaged leaves. Additional experiments suggest predator-free space exists on the common milkweed (Asclepias syriaca). While milkweed is best known as host to many aposematic insects, cryptic insects are also encountered on its foliage. Predator-free space for both cryptic and aposematic models was observed. Finally, the protective value of the aposematic coloration of the monarch caterpillar was shown to be effective in the field, both on and off of milkweed hosts. Thus, the potential of the monarch caterpillar as a model for mimicry with other similarly colored caterpillars [e.g. the black swallowtail caterpillar (Papilio polyxenes) and the clouded crimson caterpillar (Schinia guarae)] was established. By estimating caterpillar survival through the use of clay models the effectiveness of antipredator defenses can be measured. This technique enables testing of defenses in the face of free-ranging birds, something that had been lacking in studies of caterpillars due to the difficulty in witnessing predation events in the field.