Over the years I’ve come to know and appreciate a very interesting species of tern that I’ve only encountered on shallow lakes, large marshes and certain other wetland basins. Almost entirely black and dark gray in color, the small tern is aptly named, black tern.
Black terns are a unique tern common in Minnesota, but my first introduction to the species occurred in the Prairie Pothole Region of east-central North Dakota during the summers of 1997-99 while conducting wildlife research work. Some wetland basins that I studied were teeming with these colonial nesting birds.
Known to be aggressive birds that boldly defend their nesting territories against all threats, human included, yours truly didn’t know this during my first encounter with the species all those years ago.
Indeed, as I waded into the brackish water along the edge of one of my study wetlands while carrying research gear in both hands, I was shocked when the entire colony of resident black terns flew to my side of the wetland within feet above my head in a chaotic swarm of screeching and dive-bombing madness.
My more recent observations of black terns occur regularly on Assawa Lake, the small shallow lake behind my house. With nowhere near the numbers I encountered in North Dakota, Assawa Lake is nevertheless the home to about three nesting pairs of black terns each summer. These birds will fly overhead to curiously and non-aggressively investigate me whenever I’m near the shore or out in the canoe, but I’ve watched the terns exhibit their aggressive behaviors toward other birds.
Out for a paddle one evening this July, I was alerted to the agitated vocalizations of the terns coming from the far side of the lake. Knowing that the area of the lake to be where the birds’ nests are located, I wasn’t surprised to see the pairs of “terns taking turns” flying and diving at a trumpeter swan’s head.
The hapless swan, evidently swimming too close to the terns’ nests, trumpeted loudly as terns took turns screeching and diving at the swan as they effectively drove the giant swan from their nesting area. Mr. Trumpeter was no match for the terns, even ducking its head in comical looking attempts of evasion.
Black terns belong to the same large avian family that gulls belong to, Laridae. Though named black tern, the species is not all-black in color. Black headed and black-bodied, black terns have dark gray wings and white vents. Non-breeding adults and juveniles are basically all gray and white.
Widespread across most of North America, black terns occur throughout all of Minnesota, especially in marshes and other secluded shallow lakes and wetlands. About 10 inches in length, these relatively small terns have about a two-foot wingspan. Flying in buoyant and bouncy flight-patterns a few feet above the surface of the water, black terns will suddenly dip, dive, and then skim insects or small fish from the water with their beaks, sometimes lightly touching the water with their breasts and wings, too.
Colonial nesters, black terns migrate, roost, and nest in colonies of just a few pairs to as large as in the hundreds, sometimes numbering in the thousands. Often choosing nest sites on top of muskrat lodges or building their own nests out of dead vegetation in very shallow water or on top of other floating vegetation, black terns lay anywhere from two to four eggs. The chicks hatch in about three weeks and are able to leave the nest by foot about two to three days after hatching. Black tern chicks fledge about two to three weeks after they’ve hatched.
Though noisy and sometimes overly assertive in defending their nesting territories, black terns are delightful wetland dependent species of birds to observe. I consider it a privilege to have a few of them nesting on Assawa Lake each summer. Only with us a few short months each year, black terns are nonetheless at home in Minnesota as we get out and enjoy the great outdoors.
Blane Klemek is a Minnesota DNR wildlife manager. He can be reached at email@example.com.