Species profile: Black Terns in view

There’s always been a soft spot in my heart for the odd guy out. Whether human or avian, I go for the one that’s different.

That’s why on each spring and summer day for the past two years, I’ve paddled deep into a New York marsh at sunrise. I wanted to watch and photograph the latest object of my affection —  the Black Tern.

If you think “tern,” I bet the image that comes to mind is a mostly white bird that nests on beaches in marine habitats, often in dense colonies, and that captures fish by plunge-diving into the water. A breeding-plumage Black Tern, however, is sooty black with pale gray wings. It nests in freshwater wetlands, not on beaches, and is only loosely colonial. And while it does eat fish, much of its breeding-season diet is insects, which it captures in flight, either by hawking above the marsh or by gleaning from vegetation.

In the 15 years since my first fling with the Black Tern, the species’ status in New York has slid from special concern to endangered, a pattern being repeated in the Great Lakes Region with worrisome frequency.

Special access to secluded habitat

What is happening to these lovely, graceful birds? Hoping to find out, I teamed up with state biologists to help survey a Black Tern colony. In exchange, I was granted special access to the bird’s secluded breeding habitat to pursue my photographic goal: updating old film images with action shots made possible by modern digital equipment.

I hadn’t paddled my lightweight boat far into the marsh one morning in mid-May when a dark bird winged its way past me. Arthur Cleveland Bent, the renowned early-20th-century naturalist, aptly described the Black Tern as a “restless waif of the air, flitting about hither and thither with a wayward, desultory flight, light and buoyant as a butterfly.”

To me, its flight style resembles the erratic movement of a nighthawk, rowing through the air on long, floppy wings. At times the tern seems flycatcher-like as it darts in pursuit of a dragonfly. Other times it acts like a typical tern, hovering first, then swooping to snatch up prey.

In this one’s bill was a silvery fish. As the bird flew deeper into the marsh, it began giving a shrill call. Although the sexes look alike, I knew that this was a male, performing his aerial courtship display —  the fish flight —  advertising for prospective mates.

While a displaying male circles the marsh, one or more females may follow him at first, but over time he is accompanied only by his eventual mate. Soon the pair is prospecting for a nest site, but courtship behavior continues —  either at the nest site or on a perch nearby. Calling excitedly, the two birds display to each other with up-raised bills. Mate feeding occurs, and as the pair bond strengthens, copulation becomes frequent.

Ideal nesting habitat is a healthy, shallow freshwater wetland, with a mix of equal parts emergent vegetation and open water. The tern prefers large wetlands or wetland complexes of 50 acres or more. In good habitat, its nests are often clustered, sometimes 15-60 feet apart but occasionally closer together. Both the male and the female contribute to nest building. Over a few days they pull dead vegetation from the immediate surroundings into a small heap located on a floating mat of dead, flattened vegetation, a detached root clump, a patch of mud, or even an old muskrat lodge.

My New York marsh had 15 nests in the space of a few acres, but my initial attempts to find them were thwarted. I began looking in late May, relying for my search pattern on the nests I had photographed elsewhere in the early 1990s —  each a three-inch-diameter cup atop a shallow, floating mound of dead cattails and other vegetation. But to my surprise, I couldn’t find one nest, even though I was looking precisely where each pair of terns was landing regularly.

Only when egg-laying started did I realize why. Most of the nests were stunningly flimsy —  scanty cups of plant material often barely an inch above the water’s surface holding two or three buff-colored, heavily marked eggs. No wonder I had missed them at first! I quickly understood one of the factors that cause Black Tern nests to fail: the fragile nests are easily washed over by motorboat wakes or waves caused by strong winds, or flooded by heavy rainstorms.

Perilously close to water

Already a few eggs seemed perilously close to sitting in water. Would the developing embryos survive? In fact, Black Tern eggs are adapted to a moist nest environment. Their shells have half again as many pores as typical eggs, providing extra pathways for gas exchange as the embryo grows. Studies have demonstrated that being in water-soaked nests does not reduce the probability that the eggs will hatch.

Like most breeding terns, Black Terns defend their nests fearlessly. They gang up to mob Great Blue Herons and Black-crowned Night-Herons, birds known to prey on their eggs and chicks. They’ll also attack American Bitterns, gulls, snapping turtles and snakes, and various birds of prey. I was amused to watch two particularly feisty terns pursue an Osprey, a bird that of course eats fish, until it had disappeared over the horizon. Mammalian predators include such terrestrial creatures as mink and raccoon, especially if dry conditions leave nests accessible.

New York marks the southern edge of the tern’s breeding range in the Northeast, so we might presume that it was never common here. But Montezuma National Wildlife Refuge alone hosted more than 1,000 breeding pairs in the 1950s. Irene Mazzocchi, a wildlife biologist with the New York State Department of Environmental Conservation, tells me that, statewide, the number of breeding pairs is down by a third since 1989, now numbering only around 200. The Black Tern was designated an endangered species in New York in 1999 and has similar status in Pennsylvania, Ohio, and Illinois. In Ontario, it’s listed as threatened. In Michigan, it’s a species of special concern.

What has happened? Historically, numbers undoubtedly suffered from the extensive drainage of wetlands for agriculture following European settlement. But the recent decline in the Great Lakes population has a more insidious cause —  lakeshore marsh vegetation has changed due to human activities and is no longer suitable tern habitat.

Mazzocchi explains that the problem in New York stems from the artificial stabilization of Lake Ontario’s water level by a massive hydroelectric dam built in 1958 by the New York Power Authority and Ontario Power Generation. “Because water levels no longer fluctuate as widely as they did naturally, a lot of the lakeshore marshes don’t get flushed, and the vegetation is getting too dense,” she says.

Seasonal water-level changes once kept the marshes in a constant state of regeneration, forming the patchwork-like vegetation structure that Black Terns need. New York has lost half its active breeding sites since surveys began, most of them Great Lakes coastal marshes that are now filled with dense stands of cattail. The remaining 14 sites are inland, managed wetlands.

Better out west

Out west, especially in the Prairie Pothole Region of the Great Plains, the situation is far better. USFWS biologist Neal Niemuth tells me that Black Terns are common and abundant breeders in North Dakota, especially in the central part of the state. They’re common too in South Dakota and Minnesota and throughout the Canadian provinces of Alberta, Saskatchewan, and Manitoba.

Water-level changes influence western terns too, but here the issue is drought. A good breeding site one year may dry up and be unavailable the next. As Niemuth describes it, from year to year, there’s “a phenomenal variation in Black Tern breeding population and location of breeding sites due to fluctuations in precipitation.” He doesn’t see the variation as a bad thing. Rather, he explains that the wetlands thrive on the dry-wet cycle, which releases nutrients into the water, stimulating a flush of insects on which the terns and other birds depend.

By June, pairs of Black Terns in New York were incubating their eggs. It would be 21 days before any chicks hatched, so while I waited, I turned my attention to tern foraging behavior. They certainly had plenty of food. Paddling my boat through the reeds at sundown one evening, I was amazed at the clouds of damselflies fluttering up on gauzy wings. Experts agree that large numbers of dragonflies and damselflies signify healthy, unpolluted wetlands. My marsh seemed to be a damselfly paradise.

Next morning —  calm, warm, and humid —  dozens of terns swirled over that very spot, swooping and zigzagging through the air, chasing the scores of damselflies taking wing from the reeds below. When it’s overcast or windy, fewer insects fly, so Black Terns switch to a different strategy —  gleaning insects from vegetation. They search for prey by coursing slowly and low over the marsh in a harrier-like manner, dipping down to seize prey. They also take small creatures from the water’s surface, including fish.

If my math was correct, the first hatchlings would appear in the third week of June. At that time, I cautiously approached one of the nests suitable for photography and peered through my binoculars. Last week’s two eggs were gone! But the adults were hovering overhead, giving sharp alarm calls —  a sign that the nest was still active. I quickly backed off, wedged my boat in the reeds at a pre-selected spot with a clear view of the nest, and scrambled to set up my camera gear and conceal myself under a camouflage blind. Then I waited. After several anxiety-provoking minutes, one of the parents landed on the nest and peered around, giving low growling calls. I heard faint peeping. Then a tiny chick, only a few hours old, popped up and toddled over to its parent to be brooded.

For the next hour, I enjoyed the culmination of my summer’s efforts: photographing a Black Tern family. The parents alternated brooding and feeding the chick at the nest as well as a second youngster that had moved a short distance away out of sight. Once an adult brought a large dragonfly, which took the little chick many gulps to swallow. Then, seemingly exhausted by the effort, it promptly fell asleep. That was my cue to leave the new family in peace.

Black Tern chicks stay at the nest for several days, fed and protected by their parents, but then often move away, remaining hidden in vegetation nearby. They take their first flight at about three weeks old. I witnessed fledging only once, by chance, in mid-July, when the marsh was replete with purple pickerelweed flowers. Little did I know that an adult I saw hovering continuously at one spot was actually encouraging its youngster to take wing. Without warning, a pale-colored fledgling launched itself into the air and flew off with its parent in tow!

It was a bittersweet moment for me. Within a month, I knew, that youngster and its now heavily molting parents would join other restless waifs in large flocks and then make a long migration to winter grounds in coastal Central or South America. I was sad to think that my time with them had come to an end but elated to watch them take flight, each as “light and buoyant as a butterfly.”

Marie Read is a wildlife photographer and a longtime contributor to BirdWatching. She is the author of the book Secret Lives of Common Birds (Houghton Mifflin, 2005) and the co-author, with Lang Elliott, of the ebook Music of the Birds, Vol. 1. She wrote about the birds of California’s Mono Lake in our October 2012 issue.

Originally Published February 1, 2010