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in dense woods where chickadees would feel at home, and in that enclosure we set up two compartments: one was to hold our birds (an eventual six), and in the other we installed ten small birch or cherry trees every other day. Our results were clear—all of our subjects quickly learned to search preferentially on trees (whose locations we kept switching around) with leaf holes, if such trees had yielded food before. In subsequent tests we also determined that they could be trained to search at specific kinds of trees (such as birch versus maple or cherry), and also to use real caterpillar feeding damage as opposed to our experimentally damaged leaves, which had holes made with a paper punch.

Birds in the wild hunting for caterpillars face a considerably more difficult problem than our chickadees faced in our enclosure. If the natural situation in the surrounding woods were as simple as we had to make ours in order to answer one specific question, then birds would probably be behaviorally hardwired to be attracted to caterpillar feeding damage. They aren’t. Our chickadees learned to associate leaf damage on specific kinds of trees with food. However, to be indiscriminately attracted to feeding damage, as such, even on the proper species of tree, could be a liability in the field, because trees accumulate much leaf damage throughout the summer (or over a period of up to six years in the tropics), and that damage eventually ceases to be a clue to whether or not a caterpillar is still in residence. Early in the summer, when all the leaves are fresh, leaf damage can suggest that a caterpillar fed recently nearby; but in the fall a damaged leaf could mean that a caterpillar was there three months ago.

There is a second potential problem with using leaf damage as a tracking cue: the caterpillars that are the least palatable leave the most leaf damage. As already mentioned, caterpillars that are bristly or spiny (or both) and caterpillars that are poisonous, which are not routinely eaten by birds, are “messy” feeders—they make no attempt to hide their feeding tracks. They feed on the softest leaf tissues and often leave the tough leaf veins and the rest of the leaf hanging. Using leaf damage, as such, could therefore be a very misleading clue in hunting for palatable caterpillars. The contrasting behavior of the specific caterpillars that are less favored by birds thus provides independent evidence that parasitoids are probably not searching primarily by visual cuing on leaf damage.

I could distinguish whether a leaf had been fed on by a palatable or an unpalatable caterpillar, since unpalatable caterpillars ate a leaf into tatters, and palatable ones pared it down gradually to reduce tatters. I wondered if birds, to whom being able to make the distinction would matter, could also learn to differentiate leaves eaten by palatable versus unpalatable caterpillars. I talked about this problem with the animal psychologist Alan Kamil, who had recently used blue jays in lab studies to test these birds’ visual acuity in discrimination in finding cryptic moths. In his lab, jays undergo individual choice tests, after they are trained to peck a screen in response to specific pictures projected onto it. They get a food reward if they respond to the “correct” picture. I sent his lab a series of photographs of leaves eaten by palatable versus unpalatable caterpillars, and Pamela Real et al. conducted the experiments. To my satisfaction, but not to my great surprise, they reported that “the jays exhibited little or no difficulty” in distinguishing the pictures of leaves that these two kinds of caterpillars had fed from. Not only that: they generalized. The lab jays learned to peck at pictures on a screen of a leaf partially eaten by a palatable caterpillar, and to ignore pictures showing unchewed leaves or leaves fed on by unpalatable caterpillars.

29 MAY 1985. I WAS WALKING UP THE PATH TO MY CAMP IN Maine. The aspen leaves had unfurled pea-green leaves about a week earlier, and as I walked under their canopy I again found an interesting fresh