A few weeks ago while collecting gypsy moth caterpillars, I grabbed a branch of a cherry tree and narrowly missed putting my hand in a nasty-looking bird dropping. Bird droppings are among the vilest of nature’s creations especially when they catch you by surprise as an unwelcome donation on your shoulder or as a handful of goop as you inspect vegetation for bugs. A second look, however, revealed the dropping slowly escaping along the branch. In reality this bird dropping was a caterpillar rather than the remains of some prior avian feast. Why might a caterpillar resemble a bird dropping?
Caterpillars are one of the most sought after treats for a variety of predators including birds, mammals, and other arthropods. A bird dropping is surely one of the least appealing food items on my list of things to eat. The same is likely true for birds and other predators searching for tasty caterpillars. Brown and white streaks on its skin and gooey-looking blobs on its back probably help the caterpillar go unnoticed by would-be predators searching for a meal.
Larvae of the red-spotted purple are not the only caterpillars to masquerade as bird droppings. The early stages of the spicebush swallowtail, tiger swallowtail, and viceroy also resemble bird droppings. You may recall that in Hans Christian Andersen’s “The Ugly Duckling”, a rather homely gosling grew to be a magnificent swan. Hoping to witness a similar miracle, I brought the wandering bird dropping to my laboratory and fed it cherry leaves. In a few days I was rewarded with a rather scatological chrysalis on a twig.One morning about a week later my watch ended when a swan of a butterfly, a beautiful red-spotted purple, greeted me.
The adult red-spotted purple is thought to be a mimic of the distasteful pipe vine swallowtail, Battus philenor. Red-spotted purples are often seen visiting flowers of viburnum and privets for nectar but they also feed at sap flows and rotting fruit. Red-spotted purples spend the winter in hibernacula, small tubes made of rolled leaves and silk. They emerge in spring and resume eating leaves very early in the season about the time that dogwoods bloom. Caterpillars of the red-spotted purple eat leaves of a variety of woody plants including cherry trees, poplars, and aspens. Two generations of this butterfly occur in Maryland and adults can be seen flying from spring through early autumn. So, next time you see a bird dropping on a branch or leaf have a second look. It may just be an ugly duckling, so to speak.
For more information on red-spotted purples and their kin, please visit the following web sites.