Another Reason Birders Wear Hats

tent-caterpillar_sanpedro-sierravistaaz_20100510_lah_1212Plop, plop, plip plop… it sounded as if it was raining, but the sky was blue and the sun was shining. I was happily prowling a row of tall cottonwoods growing thickly along a creek, searching for flycatchers, migrating warblers, and anything else of interest. Now, something was falling out of the tree tops, but it couldn’t possibly be rain.

Blop. Something landed on my hat brim. Something else hit my shoulder, and another falling object landed on my arm. What in the world? I put down the binoculars and focused more closely.

tent-caterpillars_sanpedro-sierravistaaz_20100510_lah_1219Yikes! I was being pelted with small, very fuzzy, falling caterpillars! I hastily brushed them off my clothing, but more and more of the insects were landing on me, my camera, and accumulating on the ground. I couldn’t walk without squishing their soft bodies. My imagination quickly conjured up the next day’s headlines: Insect Larvae Smother Birder!

Somehow, a more logical section of my brain got a message through my growing hysteria: get out from under the trees! I hastily shoved my way through the underbrush, heading for a clearing.

tent-caterpillars_sanpedro-sierravistaaz_20100510_lah_1196Once I was safely out of harm’s way, I could assess the situation more calmly. I’d noticed a large number of dark objects up in the tree branches, but had dismissed them as last year’s bird nests, perhaps from the orioles I’d seem flitting among the branches. Now I realized that they were “nests” of a sort—the webby enclosures created by tent caterpillars. Apparently, it was time for the tenants to leave their safe shelters and venture out into the world.

Tent caterpillars build their silken sanctuaries as protection from hungry birds. As I watched, a Hooded Oriole suddenly noticed that one nearby tent was torn open. He stuck his head in and started gorging on the nutritious food hidden inside. I wished the other birds would do the same, although it would take an awful lot of birds to make a dent in the caterpillar population. For the most part, however, the birds ignored the potential feast. Maybe the bristles covering their bodies deterred most predators.

Later on, I learned some more about these prolific insects. Since I was visiting Arizona, the species that assailed me was Malacosoma incurvatum discoloratum. (A number of other species are found in various parts of the country.) The adult moths are drab and unlikely to be noticed as they a mass of eggs on a tree trunk or branch of a host tree. The larvae form inside these eggs, and spend the winter snugly encased.

In the spring, the larvae hatch and begin to grow… and grow… and grow. They spin their silk tents, where masses of caterpillars go to rest between feeding forays into the surrounding foliage. An immense quantity of frass (biologist-talk for insect poop) accumulates in the tents, turning them dark—or even overflows, adding to the detritus falling onto the beleaguered birder. Finally, in late spring, the caterpillars form their cocoons and metamorphose into adult moths. These mate and start the cycle all over again. They only have one life cycle per year.

Even though they were being munched on by tremendous hordes of caterpillars, the cottonwoods looked healthy. Sometimes trees are defoliated, but it’s only a temporary setback. They grow new leaves, while the caterpillars turn into non-leaf-eating moths. It’s one more natural cycle, with populations waxing and waning from year to year.

Meanwhile, I have yet another good reason why birders wear hats.

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