BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Dendroica kirtlandii | Kirtland's Warbler 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
Breeding - The male arrives at the breeding habitat a few days before
the female, sometime between May 3 and May 20.  The male immediately
establishes a territory.  When the female arrives the birds pair off and
courtship begins [21].  One-year-old males select their first
territories in younger jack pine stands than those in which they were
fledged.  Once they have established a territory, most males return to
it every year [22].

Nesting - Most Kirtland's warblers nest for the first time when they are
1 year old [22].  Females generally lay five to six eggs in late May
[21].  They lay one egg per day until the clutch is complete [22].  Egg
laying is sometimes stimulated by warm temperatures 5 to 6 days before
the laying of the first egg.  One study showed that when the temperature
exceeded 68 degrees Fahrenheit (20 deg C) for a prolonged period of time
egg-laying started as early as May 23 [22].  If for some reason the
first nest is lost, another nest is started shortly thereafter.
Incubation requires 13 to 16 days.  Eggs hatch sometime between June 12
and June 26.  Both adults feed the young, but the female does all the
brooding and most of the defending [11,21].  Some pairs raise two broods
a season.  Second clutches are generally laid between June 28 and July 1
[22].  The average production per pair is three or more young each year.
Occasionally, pairs may raise 9 or 10 fledglings in a summer [11].
       
Fledging - Young develop rapidly and are out of the nest within 9 days.
By the third week they begin to gather most of their own food and by the
fifth week parental feeding has ceased [21].

Migration - In late August some of the Kirtland's warblers start
returning to the Bahamas, and by mid-September all have left their
breeding grounds [21,22].

Life span - The average lifespan of the Kirtland's warbler is about 2
years, and the longest known lifespan is 9 years [11].


PREFERRED HABITAT : 
Breeding and nesting habitat - During the nesting season, the Kirtland's
warbler favors a narrow and distinctive habitat of jack pine barrens in
the lower Peninsula of Michigan [15,22,26].  The soil is nearly always
Grayling sand and the terrain is flat or gently rolling [3,11,15].
Grayling soils are extremely well drained and, therefore, prevent nests
from becoming flooded [12,15].  The climate on the nesting grounds can
be as low as 20 degrees Fahrenheit (-7 deg C) in late May, and snow has
fallen in early June.  On the other extreme, summer temperatures up to
112 degrees Fahrenheit (44 deg C) have been reported in this area [15].
The predominate overstory vegetation consists of 8- to 20-year-old jack
pines that are 6 to 20 feet (1.8-6 m) tall with living branches near the
ground.  The stands must be 80 acres (32 ha) or larger and Kirtland's
warblers seem to produce best in stands of 200 acres (81 ha) or more.
The ground vegetation must be low and dense [11].

The nests are built in or under ground vegetation approximately 3.1 to
7.9 inches (8-20 cm) high.  Most nests are sunken so as to be flush with
the ground surface.  Nests are usually located in areas where they are
shaded by small jack pines and well concealed by lush ground cover.
Nests are often found near or at the edge of a fairly dense growth of
jack pines.  Out of a group of 126 nests found in a naturally burned
area, 22 nests were located on the edge of an opening while 40 nests
were located within the pine growth [22].

The crucial requirement for suitable nesting habitat appears to be the
presence of living pine-branch thickets near the ground.  Trees are not
large enough to produce such thickets until adjacent trees touch each
other, and are too large when the lower limbs die, opening a gap between
the foliage and the ground cover [15].  Occasionally, the Kirtland's
warbler has been found nesting in artifical plantings of Christmas tree
size red pines which duplicate these conditions [14].

Winter habitat - The Kirtland's warbler spends 44 percent of the year on
its wintering grounds in the Bahamas.  Here, these warblers mostly
inhabit the low scrub, prefering vegetation which is only 3 to 4 feet
(0.9-1.2 m) high.  At night they roost in the higher, more dense
shrubbery near the spots which they frequent during the day [15,20].


COVER REQUIREMENTS : 
The Kirtland's warbler requires dense ground vegetation for nesting
cover.  This cover provides protection from nest predation and provides
concealment for parents as they approach the nest [15,22].


FOOD HABITS : 
On its summer range, the Kirtland's warbler has been observed eating the
following invertebrate species:  Gall midges, sawflies, grasshoppers,
spanworms, deer flies, spanworm moth (Diastictic inceptata).  Winged
insects reported fed to young birds include damsel flies, white and
cream-colored moths, tabanid flies, and beelike insects.  When the
blueberries ripen in early August the Kirtland's warbler feeds
extensively on them, selecting only the ripe soft fruit.  In the
Bahamas, Kirtland's warblers have been observed feeding on the berries
of low sage bushes [15].  Sykes [20] found that, in the Bahamas, the
Kirtland's warbler food items consisted of 59 percent small fruits, 20
percent arthropods, 1 percent seeds, and 20 percent undetermined.


PREDATORS : 
Common predators of adult and young Kirtland's warblers are blue jays
(Cyanocitta cristata), American crows (Corvus brachyrhynchos), American
kestrels (Falco sparverius), sharp-shinned hawks (Accipiter striatus),
thirteen-lined ground squirrels (Spermophilus tridecemlineatus), red
squirrels (Tamiasciurus hudsonicus), snakes, and house cats (Felix
domesticus). Blue jays are considered by some to be the worst predator
of the Kirtland's warbler [11,22].  Thirteen-lined ground squirrels are
probably the next most serious predator.  This squirrel has been
observed dragging nestlings from the nest.  Predation is now the single
most important mortality factor on Kirtland's warbler breeding grounds
and the activities of blue jays and other potential predators should be
closely monitored [22].


MANAGEMENT CONSIDERATIONS : 
Brood parasitism - Brood parasitism by brown-headed cowbirds (Molothrus
ater) has been a major threat to the Kirtland's warbler survival.
Studies have shown that up to 78 percent of warbler eggs in parasitized
nests fail to produce fledlings.  In the 1940's and 1950's the
brown-headed cowbird was depressing the production of fledgling
Kirtland's warblers by 50 percent and in the late 1960's the toll had
risen to 60 percent.  In 1972 the United States Forest Service and the
United States Fish and Wildlife Service in cooperation with the Michigan
Department of Natural Resources and the Michigan Audobon Society
launched a brown-headed cowbird control project [21].  Through 1988
almost 60,000 brown-headed cowbirds have been removed from Kirtland's
warbler nesting habitat.  Brood parasitism has declined dramatically
from about 60 percent in the early 1970's to less than 5 percent today
[17].

Global warming - Jack pine forests that Kirtland's warblers rely on may
respond rapidly to the projected global warming.  The trees are
extremely sensitive to changes in temperature and soil moisture.  If the
climate warms as predicted, in about 50 years quaking aspen and oaks
(Quercus spp.) will replace the pines along the southern margin of their
range.  The Kirtland's warbler can not gradually shift their nest sites
northward because they are unlikely to find jack pines growing in sandy
soil.  Unless extraordinary measures are taken to maintain the Kirtland
warbler habitat or to transplant them to similar habitats farther north,
the species may become extinct [4,7,24].

Plantations - Plantations of 100 acres (40.5 ha) have been used
successfully by nesting Kirtland's warblers.  In these management areas,
the jack pines were planted 3.9 feet (1.2 m) apart in rows spaced 5.9
feet (1.8 m) apart.  Ten rows of jack pines were alternated with grassy
clearings 15 rows or 90 feet (27.4 m) wide.  At Mack Lake, Michigan, red
pine plantations planted specifically for the Kirtland's warbler have
also attracted these birds [22].  It appears that plantations of other
needle-bearing trees might be equally acceptable to the Kirtland's
warbler if planted so as to meet the specific habitat requirements of
this warbler [17].

Management areas - Kirtland warbler management areas should be at least
80 acres (32.3 ha) in size on which even-aged conifer stands can be
managed on a commercial rotation.  The area should be managed to provide
a continuous supply of suitable Kirtland's warbler habitat [16,17].

Human disturbance - The use of forest trails and roads by people
produces some disturbance to the Kirtland's warbler.  Additionally,
pets, especially cats reduce the Kirtland's warbler population [22].

Foster parents - Consideration has been given to the use of foster
parents to Kirtland's warbler eggs.  Either the Vesper sparrow
(Pooecetes gramineus) or the palm warbler (Dendroica palmarum) would be
a good choice as a foster parent species [22].


REFERENCES : 
NO-ENTRY

Related categories for Wildlife Species: Dendroica kirtlandii | Kirtland's Warbler