Summer 1999

[TEXT ARCHIVE WEB-PUBLISHED MARCH 2002.
ORIGINAL PRINT PUBLICATION DATE: SUMMER 1999.]

Black Tern:
A Tern for the Better?

By Sheryl De Vore

I used to think of terns as elegant, migratory birds that skimmed the surface of a huge body of water — Lake Michigan, for example, or the Gulf of Mexico.

Then, while doing a breeding bird atlas 10 years ago, I saw my first black tern (Chlidonias niger) several miles away from Lake Michigan. It hovered with shallow wingbeats over a Lake County marsh. Then, with its bill turned down, it swooped to snatch an insect off the vegetation like a large, black swallow.

I soon learned that the black tern, as well as the Forster's tern I had seen along the lakeshore during migration, breed at inland wetlands. Both are endangered breeders in Illinois and confined to the Chicago Wilderness region.

Though it may have once bred throughout Illinois, the black tern, which winters from Panama south to Peru, probably now breeds only in Lake and McHenry counties. The species abandoned a Cook County breeding spot several years ago. Degradation and loss of habitat and changes in water flows have created problems for the black tern; only in the region's highest-quality, protected wetlands will you find breeding black terns.

According to Mike Ward of the Illinois Natural History Survey, black terns probably nested only in three places in Illinois last summer. While conducting research on yellow-headed blackbirds, Ward counted approximately 20 black tern nests in Broberg Marsh in Lake County. Chicago birder Eric Walters found terns nesting at a private marsh near Moraine Hills State Park in McHenry County. Brad Semel, a natural heritage biologist with the Illinois Department of Natural Resources, said black terns also nested with Forster's terns at Lake Elizabeth in McHenry County.

Carolyn Fields annually checks Deer Grove East Forest Preserve in Cook County, an historical breeding place for this species, but she found none breeding there the past few years.

The black tern nests where marsh vegetation is locally low and thin, most commonly near open water. Terns need floating vegetation on which to breed; when a wetland deteriorates, that type of vegetation disappears. The Illinois Department of Natural Resources is working on a plan to erect nesting platforms in wetlands where the terns might breed.

It's also possible that tern numbers are declining due to lack of prey. For example, the tern probably eats the state-endangered banded killifish, a species that thrives at Broberg Marsh. Terns eat mostly insects, but feed their young small fish for extra protein.

In some marshes the muskrat helps create the kind of vegetation the tern needs for breeding; in fact, the black tern may even build its nest atop an inactive muskrat lodge. Forster's terns and black terns often breed together, with little competition, because each requires different food and nesting sites. Forster's terns choose slightly higher and drier sites over water than do the black terns. The Forster's terns prefer active muskrat lodges, while black terns choose dead or older and flatter ones upon which to build a nest. Forster's terns also eat more fish, while black terns eat mostly insects.

The black tern nests in loose colonies beginning in late May. The female builds a nest of old weed stems, dead rushes, and wet and decaying plant materials. She lays two to three olive eggs spotted with brown. She may also lay her eggs on floating driftwood anchored by surrounding vegetation. Both sexes incubate and the young hatch about three weeks after the last egg was laid. Chicks leave the nest when they are two days old and beg for food for at least another three weeks.

Black terns glean most insect prey off marsh vegetation or from the air, making them interesting to watch.

A prime spot for black tern watching in summer is the Lakewood Forest Preserve in Wauconda, where they forage on a few of the more easily accessed ponds and marshes. A favorite delicacy in August is dragonflies. As summer wanes, their jet black bellies start to turn white for their trip south. If suitable habitat within the Chicago Wilderness region is maintained, they'll return black-bellied to breed again next year.