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Thunder grumbled to the east as the heavy blackness of the afternoon storm churned away toward the horizon. Already the rain was ebbing, the dark clouds overhead dissolving quickly as the fast-moving storm drifted eastward. The heat of the mid-afternoon sun quickly returned and with it came the dragonflies. Rising from their sheltering roosts on the undersides of reeds and marsh vegetation, swarms of skimmers, darters and damselflies took wing over the open waters here at Rat River Wildlife Area in Winnebago County. It didn't take long for the black terns to mobilize and take advantage of such a bountiful feast.
They are beautiful birds, silver and smoky black feathers with a flight at once graceful yet frantic, purposeful, playful, innocent, yet deadly accurate. Patrolling the open water back and forth, diving to the surface with lightning speed then cartwheeling into flight once again. The air was soon filled with the soft muted voices of these sociable birds as they greeted one another calmly after the heavy rains, and the sharp, piercing cries of those with more at stake. As far as the woodland edge at the far side of the marsh, the skies were clouded with foraging black terns taking wing after the storm.
"The black tern is a restless waif of the air," notes renowned naturalist Arthur Bent, "flitting about hither and thither with a wayward, desultory flight, light and buoyant as a butterfly. Its darting zigzag flight as it mounts into the air to chase a fluttering moth is suggestive of a flycatcher or a nighthawk; as it skims swiftly over the surface of the water, it reminds me of a swallow; and its true relationship to the terns is shown as it hovers along over the billowy tops of a great sea of tall, waving grasses, dipping down occasionally to snatch an insect from the slender swaying tops."
Tight-knit neighbors in the marsh
Often black terns share the same wetlands and feeding areas as common terns and Forster's terns, and the three appear to coexist peacefully for the most part as they arrow to and fro, bouncing along in masterful flight over the still waters of marshes and lakeshores. Most terns are coastal birds, but the black (Chlidonias niger) breeds in freshwater marshes and flies over land hunting down insects and other food.
A nesting colony of black terns is a fascinating and bewildering place. Few birds are as bold or aggressive as the black tern in defense of its eggs, which are usually laid on floating nests of reeds, algae and other marsh vegetation. Colonies vary greatly in size, as does quality of nest construction. Some nests are elaborate and ornate, others are simply a pile of loosely placed plant material. Shrill, sharp cries and angry piercing rattles erupt from the colony as the tiny defenders attack en masse any invader of their precious floating nursery. The excited birds will literally strike an intruder again and again, lining up in flight formation to dive-bomb the uninvited "guest" until the danger has been driven off, quite an impressive and convincing display for such a small, graceful bird!
A rare find
During recent decades, black tern numbers have decreased in many areas due to loss of marsh habitat. Since the 1960s, the North American population has declined sharply as wetlands have been drained. Farm chemicals and urban pollutants running off into nesting areas may also affect hatching success of black terns and other marsh species. In 1978, the black tern made the Blue List, the watch list of the National Audubon Society and a valuable conservation tool for state and federal wildlife agencies. While it appears the key factors that cause declining black tern populations have been identified, their populations remain at low levels.
Here in Wisconsin, the black tern is officially listed as a Species of Special Concern reflecting uncertainty about its status. This designation does not carry any state legal authority, but the species is protected under the federal Migratory Bird Treaty Act of 1918, making it a federal crime to kill the bird.
Wisconsin is fortunate to host some of the largest black tern colonies in the Midwest region, attracting researchers and biologists from other states to study, band and record observations. Well-established colonies include those located at Horicon Marsh (Dodge County), Navarino Wildlife Area (Shawano County), Rat River Wildlife Area (Winnebago County), Oconto Marsh (Oconto County), Grassy Lake (Columbia County) and the Winneconne/Lake Poygan area (Winnebago/Waushara County).
The season of change
It's late summer at Rat River and the smoke birds show signs of restlessness. The young of the year are flying now and often the entire colony is in the air at once, swirling and darting as a massive school, moving as one. They bunch together, pull apart, cyclone back together, then suddenly rise up and disappear into the distance. Birdwatchers up for a challenge delight in the fascinating variations of black tern plumage as young and old transform into alternate winter coats. Feathers on their head and underparts change to white with dusky smudges on the eyes and back of the neck. The winter colors are so strikingly different from the familiar black and silver breeding colors that many wonder if they are really seeing the same birds.
Their restless behavior and the plumage change signal that another breeding season has come and gone. Starting in early fall, the smoke birds will be gone from these vital marshlands, making long migrations to the sunny, warm northern coasts of South America. They'll stay until May when spring breezes once again bring black terns back to Wisconsin.
Robert J. Zimmer lives in Neenah.