Managing Habitat for Eastern Bluebirds
General
Biology
Habitat
Requirements
Management Practices
Bluebird
Nest
Box Essentials
Some
eastern bluebirds are year-round
residents in Pennsylvania, but the majority migrate south in the
winter,
returning to Pennsylvania in early March. The first birds to arrive are
males,
with females arriving a short time later. Adults often return to the
same
territories where they previously bred, and young from the previous
year return
to breed near the area where they were hatched.
Soon
after arriving, bluebirds begin to
look for suitable nesting sites. Pairs begin breeding in early April.
The
female lays 3-6 pale blue eggs, which she incubates. The eggs hatch
within 12
to 14 days, and the young are fed in the nest by both the male and the
female
for 15 to 20 days. The adults feed the young out of the nest for
another ten
days before the young are on their own. Most adults will raise two
broods in a
season.
During
the breeding season, bluebirds use
a variety of habitats, including open fields with scattered trees,
farmlands,
forest edges, swamps, sparsely inhabited residential areas, roadside,
fencelines, orchards, and clearings created by fire and logging. In the
winter,
they use grasslands, shrublands, and forest edges. Bluebirds defend
nesting
territories 2.5 to 7 acres in size. They require cavities for nesting
and
abundant perches near nest boxes for feeding and resting.
Cover
Bluebirds
nest in natural cavities and
nest boxes. Nest are usually 5 to 12 feet above the ground, but have
been found
as high as 20 feet. Because people have removed cavity trees from many
habitats, suitable nest sites often limit the number of bluebirds in an
area.
In addition, many other cavity nesting species, including the European
starling, house sparrow, tree swallow, and house wren, compete with
bluebirds
for the limited number of natural nesting cavities available. Bluebirds
that
overwinter in the region, rather than migrate, require weather-tight
roosting
sites and use both natural cavities and nest boxes.
Food
Insects,
including grasshoppers, crickets,
beetles, and caterpillars, make up 68 percent of a bluebird’s
diet, while the
remaining 32 percent is fruit. Bluebirds gather insects from leaves,
branches,
and the ground. They also catch insects in the air by hunting from
perches. The
lack of perches in the interiors of open habitats may restrict bluebird
use to
the edges, where perches are more abundant.
The
goal of eastern bluebird habitat is to
create and maintain suitable food and cover resources. Since the lack
of nest
sites and perches is the factor that often limits bluebird populations
in an
area, management practices such as the following may be used to
increase the
availability of these resources.
Maintain
open habitat conditions
Bluebirds
prefer open habitats and rarely
are found in forested or dense brushy areas. Open habitats that are
left
unmanaged for several years will become unsuitable for bluebirds as
woody
vegetation invades the site. This process, called succession, will
eventually
result in a loss of habitat unless action is taken to retain the
grassy/herbaceous sites that bluebirds prefer. Grassy fields can be
mowed
annually to prevent the invasion of woody plants.
Provide
perches in open habitats
Because
bluebirds often hunt from and
display on perches, adding perch sites to open habitats with few
existing
perches can improve habitat quality. Natural perches are abundant along
the
edges of large open areas, such as clear-cuts, pastures, and fields,
but the
interiors of these spaces often lack perches. Artificial perches, such
as
garden stakes, fence posts, or tree limbs stuck into the ground, can be
installed in open areas. Scattered shrubs also can be planted to serve
as
perches. Adding perches also increases the amount of available foraging
habitat. To prevent avian predators, such as hawks, from ambushing
bluebirds as
they enter or leave the nest, do not locate perches too close to nest
boxes,
and never put perches on nest boxes.
Install
and maintain nest boxes.
Bluebird
nest boxes have been used in many
areas to increase the availability of suitable nest sites. They are
easy to
make, and building boxes is an enjoyable activity for people of all
ages.
Proper
installation of the box will help
minimize the risk of predation. If possible, mount the box on a fence
post or a
pole made from 8-foot sections of ¾-inch (inside diameter)
galvanized pipe.
Sink the pole 2 feet into the ground and mount the box on the pole 4 to
6 feet
above the ground and facing away from the prevailing winds. If you have
predators in your area attach a predator baffle to the pole below the
box.
Predator baffles usually are conical structures placed beneath the box
to keep
animals like snakes, raccoons, and skunks from climbing up to the box.
A simple
baffle can be made encircling the pole with a 2-foot-wide collar of
metal 3
feet off the ground. Because bluebirds are territorial and will not
allow other
bluebirds to nest close to them, locate next boxes at least 100 yards
apart
(but see next paragraph).
Mounting
nest boxes on pole instead of
trees enables you to place them in locations that will decrease the
chance that
they will be used by birds other than bluebirds, To reduce competition
from
house wrens, which prefer to nest near brushy cover, locate boxes at
least 100
feet from treelines and shrubs. If wrens nest in the boxes, do not
harass them
to try to make them leave. Not only are they protected by law, as are
all
native species, but they also may respond to harassment by destroying
other
nearby nests. If tree swallows use the boxes, put up boxes in pairs 15
to 25
feet apart. The swallows will defend both boxes fro other swallows, but
will
tolerate the presence of bluebirds in the second box.
If
starlings or house sparrows use the
boxes, you may remove their nests, because they are non-native species
and are
not protected by law. House sparrows and starlings will try to rebuild
their
nests several times, so it is likely that you will need to watch these
boxes
and periodically remove new nesting materials. Prevent starlings from
using
boxes by making the entrance hole less than 1-9/16 inches in diameter.
House
sparrows are smaller than bluebirds, so they cannot physically be
excluded from
the boxes. Some evidence suggests that house sparrows are less likely
to nest
in boxes mounted low on a pole (below 5 feet). House sparrows often are
found
in residential areas and where animals are fed grain; so do not locate
nest
boxes near such areas. Chickadees, titmice, nuthatches, woodpeckers,
wrens, and
flycatchers also might use bluebird nest boxes.
To
attract migrating bluebirds, prepare
the boxes for use by February. Leave them up year-round, since
overwintering
bluebirds use them for roosting. To ensure that they are weather-tight
and
clean, open the boxes after each nesting season and remove all nesting
materials. Leave the boxes open for several days in good weather to air
them
out and dry them, if necessary. This will help reduce problems with
insect
infestations. Do not use pesticides in or near the boxes.
Provide
water for bathing
Bluebirds
are attracted to shallow water
for bathing. They prefer water that is less than 2 inches deep. Place
flat
rocks at varying water depths to provide secure footing and allow
bluebirds to
select the depths that they prefer. Bluebirds also are attracted to
dripping
water. Bluebirds are more likely to use water that has perches nearby.
Locate
water sources near existing perches, or plant shrubs and trees near the
water
source.
Plant
fruiting trees and shrubs
Bluebirds
rely on fruit for more than 30
percent of their diet. In the winter, they depend on persistent fruits
more
than at any other time of year. Tree and shrub species that provide
fruit
include black cherry, serviceberry, sumac, dogwood, grape, honeysuckle,
bittersweet, viburnum, greenbrier, American holly, mountain ash, spice
bush,
and pokeweed. Planting these and other trees and shrubs that retain
their fruit
through the winter, or encouraging the growth of such trees and shrubs
already
present on the site, increases the quality of year-round habitat.
Minimize
the use of pesticides
Bluebirds
are very susceptible to
pesticides, which are thought to be one of the main causes of their
historic
population declines. Insecticides reduce the amount of food (insects)
available
to bluebirds. In addition, many insecticides and herbicides are toxic
to
bluebirds, especially young nestlings. Therefore, minimize the amount
of pesticides
used in bluebird habitats.
Whether
you buy or build these points are
essential.
Material: Exterior woods
such as redwood,
cedar, and cypress, ¾ inch or thicker, provides the best
insulation from heat
and cold. Pressure treated wood contains copper arsenate and should not
be
used. Other woods should be protected from the weather by applying a
light
stain or varnish.
Preparation: Nails or
screws, not staples,
should be used to hold the box together.
Access: Side or front
should swing open for
monitoring and cleaning, and should be secured at the bottom with
something to
prevent tampering. Top-opening boxes are hard to clean and must be
mounted to
low to be safe from predators. They also have a better chance of
leaking.
Dimensions: Entry hole
should be 1 ½ in. for
eastern bluebirds. Floor: Preferably 4 ½ to 5 ½
in. square. The floor should be
8 inches below the entry hole. Perches are favored by house sparrows
and should
be avoided.
Roof: Should be
slanted, with back higher
than front, and overhanging 2 inches or more to keep rain and sun out
of entry
hole.
Interior: Inside of
front should be deeply
scored below hole to give toehold to emerging birds.
Ventilation/Drainage: Should have
drainage holes in floor
and a way for ventilation to get into the house.
Floor: Should be
recessed and completely
covered by the sides and front of the box. Rain will seep into the
seams of a
floor nailed flush to the box sides.