Understanding Tern Limits
Seven-year study by a UMaine wildlife ecologist looks at the factors
endangering Chlidonias niger
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.
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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
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"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 75–90 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 2–3 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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