Kirtland's warbler

              The first Kirtland’s warbler in North
America was identified in 1851 from a
specimen collected on Dr. Jared Kirtland’s
farm near Cleveland, Ohio. Biologists did not
learn where it nested until 1903 when they
found a warbler nest in Michigan. Today,
Kirtland’s warbler faces two significant
threats: lack of crucial young jack pine
forest habitat and the parasitic cowbird.
A pair of Kirtland’s warblers requires at
least eight acres of young jack pine forest to
nest, but usually needs 30 to 40 acres to raise
a nest of young. Its exacting nesting habitat
requirements, as well as cowbird parasitism,
caused a drastic decline in its numbers and
led the U.S. Fish and Wildlife Service to list
the Kirtland’s warbler as an endangered
species in 1973. Endangered means a species
is in danger of extinction throughout all or a
portion of its range, while the less dire
threatened designation means a species is
likely to become endangered within the
foreseeable future.
Today, Kirtland’s warblers are found only in
ten counties on Michigan’s northern Lower
Peninsula and four counties in the Upper
Peninsula. Until 1995 they had never been
known to nest on the Upper Peninsula.
Kirtland’s warblers migrate from Michigan
to the southeastern coast of the United
States on their way to wintering grounds in
the Bahamas.
Bluish-gray with black streaks on its back,
Kirtland’s warblers are the only gray-backed
warblers that persistently bob their
tails. Yellow breasts, black side streaks and
split white eye rings further distinguish
Kirtland’s warblers, which measure about
six inches in length. Males are not as
brightly colored as females.
Primarily insect eaters, Kirtland’s warblers
forage for insects and larvae near the
ground and in lower parts of pines and oaks.
They also eats blueberries and pine sap;
adults feed soft berries to their young.
Kirtland’s warblers nest only on the ground
near the lower branches of large stands of
young jack pines that are five to 20 feet tall
and six to 22 years old. The tree’s age is
crucial, although biologists are not sure why.
It is possible that the birds need low
branches near the ground to help conceal
their nests. Before the trees are six years
old, the lower branches are not large enough
to hide the nest. After 15 years, these lower
branches begin to die.
Concealed by the branches, overhanging
grass and low shrubs, the warbler’s cup-shaped
nest is made of grasses. Females
incubate four to five eggs for about 14 days
while males feed them. Once hatched, the
birds remain in the nest for another nine or
ten days before fledging, or leaving the nest.
It was once believed that destructive forest
fires were bad for the environment. However,
scientists now understand that fires play an
important role in forest ecosystems. For
example, jack pines need fire to release their
seeds. Preventing forest fires prevented new
jack pines from growing. Kirtland’s warblers
will not nest without young jack pines, and
the population dwindled dramatically before
scientists realized that forest fires were
actually good for the environment—and for
Kirtland’s warblers.
The second greatest threat to the survival of
the Kirtland’s warbler is the brown-headed
cowbird. Cowbirds lay eggs in another bird’s
nest, leaving the unsuspecting owner of the
nest, called the host, to incubate and care for
the young cowbird. When the female
cowbird lays its egg in a nest, it removes one
of the host’s eggs. The cowbird egg hatches
a day before the others, getting a head start
on growth. The young cowbird is bigger and
able to claim more food than the other
nestlings, and it often crowds or pushes the
other baby birds out of the nest.
Some bird species have developed ways to
combat the cowbird. They may abandon
their nest and lay more eggs elsewhere or
build another nest on top of the cowbird egg.
However, Kirtland’s warblers never
developed such defenses. Because of the
cowbird’s habits and the Kirtland’s warbler’s
inability to protect its nest and its young,
less than a third of warbler nests produced
any young warblers in 1971.
The Fish and Wildlife Service, in
cooperation with the Michigan Department
of Natural Resources, the U.S. Forest
Service and the Michigan Audubon Society,
Kirtland’s warbler
Dendroica kirtlandii
Forest fires, once thought to be bad for the
environment, are crucial to the survival
of the Kirtland’s warbler because young
jack pine trees, the only trees in which
warblers will nest, need fire to
release their seeds.
U.S. Fish & Wildlife Service
initiated an aggressive cowbird removal
program in 1972. From 1972 through 1997,
they removed more than 105,000 cowbirds
from warbler nesting areas. As a result,
Kirtland’s warblers now have very good
nesting success and enough young are being
produced to maintain or increase the
population.
Biologists and bird counters began to
recognize the dire plight of the Kirtland’s
warbler in the 1950s. To keep track of the
dwindling numbers of Kirtland’s warblers,
birders counted the number of singing males
every 10 years starting in 1951. Females do
not sing, and therefore are almost impossible
to count accurately, but studies indicate
there is one female for each male. In 1961,
the total population of males and females
was more than 1,000. By 1971 the population
had plummeted to about 400 birds. At that
time, biologists began counting singing male
warblers every year.
In 1973, the Fish and Wildlife Service
appointed the Kirtland’s Warbler Recovery
Team, the first endangered species recovery
team established by the Service. This team
includes representatives from the Michigan
Department of Natural Resources, the Fish
and Wildlife Service, U.S. Geological Survey,
Forest Service and interested citizens. The
team’s job is to determine how to save the
warbler from extinction.
The recovery team’s goal is to establish 1,000
breeding pairs of Kirtland’s warblers. In
1995, 765 male warblers were counted, the
highest number recorded since the first
count in 1951. Since then, the number has
remained high, with 692 singing males in
1996, and 733 in 1997. The number of singing
males in the Michigan Upper Peninsula
increased to 19 in 1997, and five unmated
males were found that year in northern
Wisconsin.
Today, warbler conservation measures seem
to be working. About 140,000 acres of public
lands have been set aside by the Michigan
Department of Natural Resources, the U.S.
Forest Service and Fish and Wildlife Service
specifically for Kirtland’s warbler
management.
The recovery team has recommended that
30,000 acres of warbler nesting habitat
always be available—enough to reach the
recovery goal. Since the trees continuously
grow older and warblers cannot nest in
forests older than about 22 years, land
managers must create new habitat every
year. Two thousand acres of forest are clear-cut
and two-year-old jack pine seedlings
planted each year. The cut trees are chopped
and used for fuel or particle board—nothing
is wasted. Sixty-nine percent of the warblers
counted in the 1997 census were on these
managed land areas.
A portion of the Michigan Department of
Natural Resources annual habitat
management is funded through Endangered
Species Act grant money from the U.S. Fish
and Wildlife Service. In recent years, the
amount of these grants has decreased, along
with other funding for similar work by the
U.S. Forest Service and the State of
Michigan.
Kirtland’s warbler populations have yet to
meet the recovery goal, but they have
certainly stabilized and are increasing each
year thanks to many extremely dedicated
people. With their continued help Kirtland’s
warbler will continue to search out young
jack pine forests each spring for generations
to come.
Thanks to protection under the
Endangered Species Act and
conservation measures by the U.S. Fish
and Wildlife Service, U.S. Forest Service
and the Michigan Department of
Natural Resources, Kirtland’s warbler
populations have rebounded. Biologists
counted more than 700 singing male
Kirtland’s warblers in 1997.
U.S. Fish & Wildlife Service
1 800/344 WILD
http://www.fws.gov
August 1998
Kirtland’s warblers forage for adult insects
and insect larvae in pine and oak trees.
They also eat berries and pine sap.