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Photo by Ron Austing |
What
is the Kirtland's warbler?
The Kirtland's
warbler is a tiny bluish-gray songbird with a yellow breast and black
streaks on its back. The male's plumage is brighter than the female's,
and the male also has a black mask. Both sexes have a distinct whitish
eye-ring split in front and behind. The Kirtland's warbler is yellow below
with white undertail coverts, the sides and flanks are spotted. In autumn,
the warbler's gray plumage becomes mixed with brown.
The bird
has a habit of constantly bobbing its tail up and down. Except for singing
males, most activities of the Kirtland's warbler are concentrated low
in the pines or near or on the ground.
Feeding: The
Kirtland's warbler eats primarily insects such as ants and caterpillars,
but also eats some plant matter such as blueberries. Because biologists
have not observed the warbler drinking water in the wild, they believe
the bird receives all the moisture it needs from its food.
Habitat
The
Kirtland's warbler breeds in the northern part of the lower peninsula
of Michigan and winters in the Bahamas and other Caribbean islands.
Reproduction
The
warbler has several reproductive characteristics that make it unique.
During the mating and nesting seasons, the male sings almost constantly,
under all kinds of weather conditions, from dawn until dark. Its distinctive
song is loud and clear, and can be heard as far away as 1/4 of a mile.
Another reproductive
trait that makes this bird unique is that it only nests in jack-pine stands
that are at least 80 acres in area, preferably much larger, and consists
of young trees from 5 to 20 feet in height. The warbler nests directly
on the ground under low branches of jack pines. These stands naturally
occur about 6 years after a fire destroys the original trees. After 6
to 12 years of use, when low branches in contact with the ground begin
to die, warblers abandon the habitat and seek younger stands nearby.
In May and
June, the warbler lays four to five eggs in its nest. Incubation takes
about 14 days. The nestlings develop rapidly and leave the nest by the
ninth day. Both parents care for the brood, but some Kirtland's may initiate
a second nest. In that case, the female incubates the second nest while
the male cares for the first brood. By September, the chicks are strong
enough to begin the annual migration to the Caribbean. Plants associated
with the bird's winter home are pine forests and broad-leafed scrub.
Why
is the Kirtland's warbler threatened?
Limited
Habitat
Nesting
Areas:The
warbler's specific nesting habitat requirements and other natural factors
have limited the population to a small size. Because the warbler only
nests in large stands of young jack pines created by dynamic forces,
such as forest fires, any factors that curtail those forces may reduce
the birds' reproductive success.
Fire
Control:
Forest
fire control measures have contributed to reduction of the bird's habitat.
Fire suppression is necessary in Northern Michigan forest land management,
especially to protect human life and property. But, fire control prevents
the natural regeneration of jack pine because the heat of fire is necessary
to open their cones and release their seeds. Fire also prepares the
ground for seed germination by temporarily clearing away competing vegetation
such as sedge and grass. If suitable nesting areas were not available
for several years, warblers likely would die out without reproducing,
and the species would become extinct.
Brown-headed
Cowbird Parasitism
A
second factor affects warbler reproduction even if suitable nesting
habitat is available. The brown-headed cowbird is a blackbird species
native to North America, but not found in northern Michigan prior to
European settlement. The cowbird invaded warbler nesting grounds around
the beginning of the 20th century after logging cleared the original
forests. Cowbirds are called "nest parasites" or "brood
parasites". Instead of building their own nests, cowbirds lay their
eggs in the nests of other species of birds which are called "hosts".
Kirtland's warbler did not evolve with the cowbird, and so never developed
natural defenses against its parasitism. The warbler parents do not
recognize that a cowbird chick in its nest is not its own, and so will
naively care for the cowbirds. Cowbird chicks hatch a day before any
warbler eggs, and are much larger than warbler chicks. Consequently,
they get most of the food brought to the nest by warbler parents. About
70% of warbler nests were parasitized by cowbirds before 1972 and warbler
pairs were producing less than 1 young each year. Biologists believed
that Kirtland's warbler was likely to become extinct if that rate of
parasitism was allowed to continue.
Natural
and Human Intervention
Events
occurring during migration or at the bird's winter habitat also may be
harmful. Biologists know little about the warbler's Caribbean home. Droughts
and logging on the islands may have contributed to the decline of the
warbler. Recent publications discuss possible correlations between heavy
logging in the Northern Bahamas during the 1960s and 1970s and the warbler
population decline between 1961 and 1971. Recovery of the Caribbean pine
forest now coincides with improvements in the warbler population. The
warbler migrates south during the hurricane season, storms may kill some
of the birds. The warbler's diet of insects and vegetation may expose
the birds to pesticides. However, recent studies in Michigan and the population
increase indicate that overwinter survival is healthy in spite of the
many factors that could have adverse effects.
What
is being done to prevent the extinction of the Kirtland's warbler?
Endangered
Species Act
Listing
and Management Areas:
The
Kirtland's warbler was one of the first species listed as endangered
under the Endangered Species Act of 1973. However, measures
to save the songbird began long before the Act was passed. U.S.
Forest Service and Michigan Conservation Department (now Michigan Department of Natural Resources)
foresters dedicated public forest land to warbler management and planted
jack pines so that there were always jack-pine stands of the age and
size required by the Kirtland's warbler. Portions of forest lands also
were cut and burned so that new jack pines could grow. The U.S. Fish
and Wildlife Service began trapping and removing cowbirds from warbler
nesting areas in 1972.
Land
Acquisitions: The
Endangered Species Act helps further protect the warbler in many ways.
The Act provides for acquisition of land to be used as warbler habitat
and for funding for additional management programs. In addition, the
Act requires Federal agencies to ensure that their activities do not
harm the species or its habitat, and requires that a recovery plan be
developed that outlines management steps to be taken to protect and
increase the numbers of the endangered species. A Recovery Plan was
completed in 1976 and revised in 1985. The warbler's Michigan nesting
areas are posted and closed to human entry - except
on escorted tours provided by the U. S. Forest Service and U. S. Fish
and Wildlife Service.
Recovery
Plan: The
Kirtland's warbler recovery plan goal is to establish a self-sustaining
population of 1,000 pairs of warblers throughout the bird's known habitats.
Intensive habitat management must be conducted annually on dedicated
state and federal forest management units in order to provide sufficient
nesting habitat for 1000 pairs. Management consists of commercial logging
of 50 year old jack pine stands followed by planting or seeding to regenerate
the stand. Site preparation by controlled burning is occasionally used
between logging and planting or seeding. Safety and cost considerations
preclude routine use of prescribed fire. The objective is to manage
a minimum of 127,600 acres of habitat for the Kirtland's warbler.
Cowbird
Control
Brown-headed
cowbirds return each spring to northern lower Michigan. Annual cowbird
control on Kirtland's nesting areas will continue in order to facilitate
normal warbler reproduction.
Emergency
Measures
The
recovery plan calls for the development of emergency measures that would
be initiated if the population size falls below 100 pairs of birds. Such
measures could include attempting to breed Kirtland's warblers in captivity,
or holding warblers in captivity with the hope of avoiding a heavy over-winter
mortality. If the other activities described by the recovery plan are
successful, such emergency measures will not be necessary and the Kirtland's
warbler will be saved from extinction.
Fact Sheet Revised January 2007
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