Download Alberta`s Black-throated Green Warbler

Alberta’s
Black-throated
Green Warbler
(Dendroica virens)
special concern
LISA TAKATS
status
Alberta: Recommended
by the Endangered
Species Conservation
Committee (ESCC) as
Species of Special
Concern
Saskatchewan: Sensitive
Manitoba: Secure
British Columbia: At Risk
Canada (COSEWIC):
Not assessed by
COSEWIC; National
General Status is Secure
USA: Secure (N5)
Description
Population
The black-throated green warbler is a
small (11-12 cm long) wood warbler
that inhabits the boreal forest and
foothills of Alberta. The male has a
bright yellow face, a black throat and
upper breast, an olive green crown, back
and wings, a black tail, and a contrasting
white abdomen. The female is duller in
appearance, and her throat and breast
are mottled with black and yellowishwhite, rather than solid black. The blackthroated green warbler’s song is a
distinctive buzzy “zeee-zee-zee-zoozeee”.
In Alberta, the black-throated green
warbler is sparsely distributed over
most of its range, but it can be locally
common in some areas of suitable
habitat. However, no population
estimate exists for the black-throated
green warbler in our province. Canadawide Breeding Bird Surveys indicate that
numbers of the black-throated green
warbler have remained relatively stable
(1966-1996), however, the roadside
counts used for these surveys may not
sample this species well. Biologists
suspect that the population may be in
decline in parts of its range in Alberta,
but have no suitable data to estimate
the rate of decline.
Distribution and Habitat
The black-throated green warbler is a
migrant songbird that breeds across the
boreal forest of Canada and the
northeastern United States, and winters
in Mexico and Central America. It is
found in northern and western Alberta,
where it approaches the northern limits
of its range.
The black-throated green warbler is a
forest specialist that occurs in
mixedwood forests of trembling aspen,
balsam poplar and white spruce. This
small warbler prefers to set up its
territory in older stands (80-130 years),
and tends to avoid disturbed and edge
habitats, as well as small forest patches.
The black-throated green warbler feeds
primarily on insects, which it collects off
leaves and small branches high in the
forest canopy.
Threats
The primary threats to the persistence
of the black-throated green warbler in
our province are habitat loss and
fragmentation. Habitat loss both
reduces and fragments remaining areas
of suitable forest habitat. Small forest
patches may not be large enough to
meet all the habitat requirements of the
black-throated green warbler, and
individuals may be reluctant to cross
areas of unsuitable habitat between
forest patches. In addition, small forest
patches that are surrounded by
unsuitable habitat have a relatively high
amount of edge habitat, which this
species may avoid. Predation of eggs by
small mammals and brood parasitism
by cowbirds may increase at habitat
edges and thus reduce the birds’ ability
special concern
Black-throated Green Warbler
(Dendroica virens)
to reproduce. Habitat fragmentation has been
implicated in the declines of many songbird
populations across North America, and has
contributed to an estimated 50% decline in
black-throated green warbler numbers in
studies in northern Alberta.
Timber harvesting has increased significantly
in recent years in our province, and
regenerating forests will not reach a sufficient
age to support black-throated green warblers
before being harvested again. Oil and gas
development further increases habitat loss
and fragmentation in forested regions of the
province. Loss of wintering habitat in Central
America may also be affecting black-throated
green warbler populations.
Management
Under Alberta’s Wildlife Act, the blackthroated green warbler is currently
designated as a “non-game animal”. The
Endangered Species Conservation
Committee has recommended that it be
recognized as a “Species of Special Concern”.
Conservation of black-throated green warbler
habitat has been recommended, and longterm forest management must focus on
maintaining populations that are well
distributed over the species historic range. In
response to this need, Alberta Sustainable
Resource Development (ASRD) is enhancing
programs that collect information on the
distribution, population size and trend of the
black-throated green warbler in Alberta.
Conservation efforts for Alberta’s blackthroated green warbler population are best
directed towards slowing the rate of direct
habitat loss. Modifications to forest practices
and management policy should be pursued in
order to guarantee continued availability of
black-throated green warbler habitat in our
province. Such strategies could include
maintenance of mixedwood forests through
innovative harvesting and silvicultural
practices, allocation of larger patches of older
forest (> 40 ha) to wildlife reserves in
harvested areas, and increasing the intervals
between subsequent harvests (extended
rotations).
What You Can Do To Help
As a member of the public, a land
manager, or an industrial developer:
• Report any observations of theblack-throated
green warbler to the nearest office of the Fish
and Wildlife Division, Alberta Sustainable
Resource Development.
• Volunteer for the annual Breeding Bird Survey
in northern Alberta [http://www.mp2pwrc.usgs.gov/bbs/participate].
• Contact wildlife management staff at the
nearest Fish and Wildlife Division office for
information on ways that you can minimize
the impacts of human activities on blackthroated green warbler populations.
In 2001, a five-year joint initiative by the
Alberta Bird Atlas Project and the University
of Alberta Remote Areas Program, supported
in part by ASRD, began. This study addresses
the need for population information
throughout the remote northern portion of
the black-throated green warbler range in
Alberta. Other research initiatives related to
forest management are underway, or have
been recently completed.
March 2002
For more information on Alberta’s Species At Risk: http://www3.gov.ab.ca/srd/fw/riskspecies/