Laura Erickson's For the Birds

The end of May is the peak of migration for warblers along Lake Superior. If the weather is pleasant, warblers seem to magically appear on their territories, and we don’t get to see that much of their movement. But rain associated with warm fronts brings them in by the thousands, and under perfect lousy weather conditions, warblers sometimes seem to drip from the trees.

Warblers are Neotropic migrants, most of them traveling thousands of miles between their Central American wintering grounds and their northern breeding range. They stoke up on plenty of fuel down in the tropics, which they store as fat and burn as they go. They migrate by night so that they can spend their days pigging out, replenishing the fat as they go. Migration is timed to coincide with two important phenomena critical for insect eaters. First, trees leaf out, and just as the tiny buds break out, lots of kinds of caterpillars hatch to eat the soft new tissue. Tree leaves, like most plant tissue, have cell walls which make full grown leaves difficult to digest, but in new and growing tissue, the cell walls haven’t developed fully, making the tissue perfect for tiny caterpillars to digest. And those caterpillars, in turn, make a perfect fuel for insectivorous warblers.

Birdwatchers always hope that the first big waves of migrant warblers arrive before the trees leaf out, since these colorful but tiny birds are easier to see without thick foliage blocking the view, and sure enough, many years we do see a substantial migration before leafout. When we do, the best places to find these early birds are virtually always near water, where they can capitalize on the second insect happening of May. Tiny aquatic insects are emerging just in time to provide food for hungry insectivores. Mayflies, caddisflies, midges, and a host of other tiny creatures say goodbye to their watery nursery where they spent their days as nymphs and take wing. Many of these insects will survive as adults only a day or two, long enough to mate and lay eggs, and then their spent bodies simply die. Adult mayflies, whose scientific name Ephemeroptera reflects the ephemeral nature of their adult lives, don’t even have mouthparts since they won’t live long enough to need food. On the right days, when insects are emerging by the bazillions, warblers gather along the edges of rivers or ponds, darting out above the water to snatch insects on the wing. On these days, fishermen in boats often observe more warblers than land-bound birdwatchers.

Warblers are hardly the only birds dependent on these emerging aquatic insects. Many tiny insects swarming through the are spent their early days in water, and sometimes are called aeroplankton. Swifts, martins, swallows, nighthawks, and whip-poor-wills are specialized to sweep up these insects in their huge mouths. As more and more pesticides run off into our waterways, fewer and fewer of these insects reach maturity, and in turn, fewer and fewer of the birds that depend on them return each year. When I was a little girl, I can remember days when bridges had to be closed as slippery piles of dead mayflies made driving treacherous. Those days are gone, but in north country, we still have many reasonably clear waterways where mayflies dance and make merry on the spring breeze. There’s no more pleasant sight than sun-dappled waters abuzz with romantic insects and our most treasured insectivores all aflutter in a dazzling array of spring plumage satisfying their hunger and our visual thirst for beauty.