DAKOTA NATURAL HERITAGE

THE BLACK TERN

By Eileen Dowd Stukel

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For some people, the robin is their special vanguard of spring; for others, it is the meadowlark. For me, a symbol of spring’s arrival is the sight of black terns gracefully coursing over small wetlands. Black terns are also a spring favorite of mine because they are the one member of the gull and tern family I can identify with no doubt or hesitation.

We usually think of gulls and terns as birds of marine environments. The black tern is no exception during winter, but favors inland marshes for nesting. During mid-May, birds arrive in nesting areas of the northern United States and central Canada. Their overall range has shifted northward somewhat. Breeding areas are dynamic because of the extreme habitat changes possible from droughts or floods.

Like most members of the family Laridae, black terns are gregarious and social during most of the year. Although they may migrate singly or in small groups, they also form flocks of thousands in coastal or marine areas with concentrated food sources.

After spending a week or two gathered at feeding and roosting areas, courtship begins and pairs disperse to nest in wetlands that provide a mixture of emergent vegetation and shallow open water. Black terns probably change mates from one year to the next, but a couple may pair again if both return to the same nesting area.

A pair may build its nest on a floating or nonfloating substrate. Possibilities include a floating mass of dead matted vegetation, detached floating root masses, or anchored sites, such as muskrat lodges or used nests of other birds. The pair gathers dead plant material from nearby and builds a nest that sits just a few inches above water level.

Black terns are semicolonial nesters. They may cluster nests in groups of a dozen to several dozen, although some pairs nest apart from the clusters. Even within a nest cluster, a black tern pair defends a territory of about two yards around its nest. Pairs learn to recognize and tolerate their closest neighbors. A pair usually sites its nest away from shore and close to open water. Wetlands used for nesting are usually at least 50 acres in size, although black terns may nest on wetlands as small as 13 acres.

The female lays 2-4 eggs, which have many pores as an adaptation to their damp nest environment. Both parents incubate the eggs for at least three weeks. After hatching, adults share brooding duties for their semiprecocial chicks. The shared workload continues as adults bring food to the nest, at first delivering it to the chicks and later dropping food from the air. As chicks reach a fast-growing www when nearly two weeks old, parents may forage up to 16 hours per day.

Chicks leave the nest on short excursions when only a few days old and probably make their first flights at about four weeks of age. The family group then moves from the nest site to open water. Parents may continue feeding their chicks, with fish being an important diet item during this www. On an average, black tern pairs rear less than one chick per nest, due to loss from wind and wave action, changing water levels, and bad weather. Potential predators include the great horned owl, black-crowned night-heron, great blue heron, raccoon, and mink.

As chicks learn to fend for themselves, they demonstrate characteristic black tern foraging methods. This species rarely dives into the water when pursuing prey, but rather flies low, circling flights with slow, shallow wingbeats and a downturned bill. The bird may dip its bill into the water to snatch a small fish or aquatic insect or pluck insects from plants. Before widespread pesticide applications, black tern flocks were a common sight as they followed plows, seizing grasshoppers or crickets. In the Southern states, black terns capture cotton bollworm moths over young cotton plants.

The black tern differs from many of its gull and tern relatives in having a predominately insect diet during a large part of the year. Although the species has webbed feet, it is a poor swimmer and seldom rests on the water. Instead, it may rest on fenceposts or, in winter, on drifting coconuts or floating logs.

By the end of summer, black terns begin gathering at bountiful feeding locales. Most leave breeding areas by mid- to late-August to begin their southward migration. They seek productive feeding sites on land and water enroute to wintering areas, found mainly along the Pacific Coast of Central America and both coasts of northern South America. These winter habitats are usually 18 miles or less from land. Winter plumage is much less distinctive than its breeding appearance, as the black tern assumes a white head and underparts and various dark areas around the eyes, ears, and on the back of its neck.

Based on results of the recent South Dakota Breeding Bird Atlas project, the black tern is considered "uncommon and limited" in the state. Nesting colonies are clustered mainly in northcentral and northeastern South Dakota. Across its breeding range, the black tern has experienced a continual breeding population decline during the last 30 years. Potential culprits include wetland loss, particularly of shallow wetlands, and indirect pesticide impacts to its aquatic prey.

In its favor, the black tern responds well to wetland management that provides stable water levels and preserves emergent vegetation. The species adapts to artificial wetland sites and often accepts artificial nest platforms. But artificial habitat and nesting sites aren’t panaceas for this symbol of our diverse prairie wetlands. We must conserve the natural components to sustain this "sea swallow" that Arthur Bent described in this way: "a restless waif of the air, flitting about hither and thither with a wayward, desultory flight, light and buoyant as a butterfly."

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