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By Root 765 0

or debris; some blend into the background of bark that they choose to rest on when not feeding; others cover themselves with debris taken from their background. In a slide demonstration of caterpillars that I have given several times, I take the audience on a virtual caterpillar hunt featuring various showy caterpillars perched on their food plants. After a number of practice slides, to get the audience used to finding caterpillars in the projected images, I give a test: I show a picture with real hidden caterpillars, or things that look like them, or both. Invariably even professional entomologists have been fooled, either not seeing the real caterpillars that were in plain sight and magnified 1,000 times on the screen, or pointing out something they wrongly assumed to be a caterpillar. My own skill at finding caterpillars despite their various tricks involved looking for fresh feeding damage on leaves, in order to focus the search on a smaller area, since most (though not all) caterpillars don’t move far.

After I had found the first clipped-off partially eaten leaves in Minnesota, I searched and eventually found the inconspicuous stubs of leaf petioles still attached to a twig of the tree where the leaves had come from. The twig with its many remaining leaves looked un-grazed, and I would normally have passed by without a second look. But now I looked closely, and as expected found a large caterpillar (a big brown Catocala moth larva that was nearly invisible as it rested tightly pressed onto a nearby branch, mimicking the irregularities of the bark). I later watched and photographed the caterpillar and learned that it spent all day motionless in its disguise on the branch. In the evening it moved quickly out onto the branch, fed on a leaf that was far too large to consume in one meal, and after eating a portion of this leaf, backed down the petiole and chewed through it so that the leaf remnant dropped off. The caterpillar then walked away and resumed its position in its hiding place on a twig. I observed and photographed similar behavior in many species of caterpillars, but these were only those species that are “invisible” and hence have evolved to avoid predators that hunt by vision instead of scent.

These caterpillars were also the same ones that, while still feeding and before clipping, pared the leaves down so that they looked smaller rather than conspicuous because of tatters or holes. Some of these species, like the prominents (Heterocampidae), disguised their feeding damage by fitting their own bodies into the area at the leaf edge that they had consumed, and their bodies mimicked the leaf edge they had removed even to the extent of having fake leaf blemishes and leaf-edge patterns resembling those of the leaves of the tree they fed on.

Only those caterpillars that were routinely predated by birds had cryptic body markings, practiced leaf paring, held specific feeding positions on the leaf, and engaged in leaf clipping. The bristly or brightly marked ones that were not eaten by birds (but were still parasitized by wasps and flies) were “messy” feeders who did not clip leaves. These observations suggest that leaf clipping is part of a behavioral repertoire in the game of hide-and-seek with birds.

Although it seemed like a safe bet that birds hunt for the palatable “invisible” caterpillars, as I did, using leaf damage as a cue, nothing can be taken for granted. Any such idea needs to be tested, generally through long, tedious work that may take months or years to carry out, and that almost invariably leads to surprises. Here was a project that needed to be done, and I invited a friend and colleague, Scott Collins, to join me at my cabin in Maine for a summer’s work and fun, to do the critical tests and determine whether a bird might be capable of learning to hunt for caterpillars by using leaf damage as a tracking cue.

Scott and I decided to use chickadees as our subjects. They were abundant, tame, and easy to catch in mist nets we set out in the woods. We erected a screen-cloth aviary in a clearing we made