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November / December 2003


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Understanding Tern Limits

 


Understanding Tern Limits
Seven-year study by a UMaine wildlife ecologist looks at the factors endangering Chlidonias niger

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Thunderstorms make researchers of black terns nervous. The problem isn't lightening or downpours out in open marshes where black terns live. It's the sudden rise in water levels.

"In one out of 10 years, a big thunderstorm will dump enough water into a wetland to wash the birds' nests away," says Fred Servello, associate professor of wildlife ecology and leader of a seven-year Maine black tern research project. His goal is to find out what keeps the black tern (Chlidonias niger) population from growing. Their numbers are low but stable in Maine; they have been declining nationally since the 1960s. The migratory bird is listed on the endangered species lists in 21 states and three Canadian provinces. No action has been taken at the federal level, largely because information is lacking.

UMaine's study is one of the first to take a long-term approach to understanding black tern habitat. "Most previous studies focused on nests over two to three years. There are reports of nest success rates, but not much understanding of what's causing them," says Servello.

Floods are just one of the factors that make life precarious for these birds. Not much larger than the common blue jay, black terns build their fragile nests on low spots in wetlands with large pools of open water. In contrast, their cousins, the roseate and least terns (both endangered species) nest by the sea in Maine.

Although their nests tend to be hidden by vegetation, black terns can be seen readily by bird watchers and boaters. Human disturbance can cause the adults to fly off the nest, calling out in alarm while the chicks scatter into the surrounding water where they are vulnerable to a host of predators, including bullfrogs, fish, raccoons and herons.

Tern Nests
Working in waist-deep water, student researchers built enclosures around 31 nests and monitored the welfare of chicks in the absence of predators. With such protection, the birds thrived.

Photo courtesy of Fred Servello
 

"If a flood doesn't get them, predators will pick the chicks off gradually through the summer," says Servello. Less than one-third of the chicks born in any given year survive, he adds, although difficulties in keeping track of the fledglings make that rate uncertain.

"Still, that's not enough to sustain or grow the population in Maine. We know that the species is adaptable. Otherwise it wouldn't be here. But we don't know if Maine's population just hangs on by itself or is really part of a regional population that includes birds in New England, New Brunswick and Quebec."

The species' summer range extends from the Atlantic to the Pacific, centered in Canada's prairie provinces. Birds in the northern U.S. are at their southern extreme at that time. First documented in Maine on Messalonskee Lake near Augusta in 1946, only about 7590 breeding pairs are known in the state, and fewer exist in Vermont and New York. Black terns winter in Central and South America.

"We know from the breeding bird surveys that they have declined throughout their range 23 percent a year from the 1960s to about 1990, and appear to have leveled off since then," says Servello. "That may parallel wetland loss. No one knows for sure. In general, there have been fewer observations of birds.

"It looks like the climate is somewhat difficult for them in New England, because we get these June weather patterns that cause relatively high (water) flows just as their nests are on the ground. It's different from the prairie region, which is drier country," he adds.

Since 1997, 25 undergraduate and graduate students have worked under Servello's guidance to monitor black tern nests from observation towers, to track chicks and returning adults, and to study factors such as food availability, water levels and predation. "We've found that food resources are just excellent. They eat insects and small fish," Servello says.

Neither does there seem to be a problem with reproduction. Black terns can live to be more than 20 years old. Most nesting pairs produced at least three eggs, almost all of which hatched if the nest didn't wash away. About half the nests made it through hatching, which is the average elsewhere.

The major factors keeping Maine's black tern population in check appear to be floods and predators. "The real problem here is that very few of the chicks survive," says Servello.

The UMaine study follows on the heels of black tern observations made by students at Nokomis High School in Newport, Maine. Led by Donald McDougal, a science teacher, students documented the birds' plight and convinced the state legislature to add black terns to the state endangered species list.

In their subsequent studies, UMaine researchers spent hours in observation towers monitoring chick survival. They erected enclosures around 31 nests to see how the birds would fare without predators. Chick survival within the fenced areas increased from about 30 percent to more than 90 percent.

However, building fences around black tern nests is time consuming and expensive, and not a reasonable long-term management strategy. Storms and changes in water levels could cause the fences to shift. Nests located on floating vegetation could even rise above the fence, exposing the chicks to predators.

Nevertheless, the best opportunities may be in water level management. Small dams control water levels in most of the areas where Maine's black tern colonies are located. Adjusting control structures can help to encourage the birds to nest in more secure areas.

It's also possible that black terns are recent immigrants to New England and that their presence is part of an evolutionary process. "It could be that the Maine birds are at the edge of their range and always going to be stressed," Servello says.

by Nick Houtman
November-December, 2003

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