University of Vermont
Department of Plant and Soil Science
Dr. Leonard Perry, Horticulture Professor
University of Vermont
Spring, as well as fall, are the times that many gardeners and homeowners
see a huge selection of birdhouses at hardware and home supply stores, or in
catalogs. What may be very decorative, may only be that, and not
attract any birds. Build it and they will come doesn’t necessarily
work. Knowing a few birdhouse basics will help you choose ones that
are both decorative as well as functional.
Key points to having birdhouses, sometimes termed “nesting boxes”,
that actually house birds are the sizing of the house and entrance hole,
design for heating and cooling, proper water drainage, safety, mounting, and
placement. About 85 North American bird species are “cavity nesters”,
and of these about three dozen also will nest in birdhouses. These
include bluebirds, house wrens, titmice, chickadees, tree swallows, and
house sparrows. Woodpeckers such as the red-headed and downy, and northern
flickers, also may inhabit birdhouses.
Other species don’t use birdhouses, such as cardinals, cedar waxwing,
red-winged blackbirds, orioles, and goldfinches. Their nests are ones you’ll
find in trees and shrubs. A few common birds such as robins, mourning doves,
barn swallow and Eastern phoebe will build homes on “nesting shelves”.
These platforms you can buy or make have a floor, back, short sides and roof
overhang, with the front open and front sides open—basically a three-sided
birdhouse. Some species will return to them yearly.
Hole size is a key factor to allow the birds you want to enter
birdhouses. The most common is 1-1/2 inch diameter, which songbirds
such as tree swallows and bluebirds to enter, yet keeps out some larger,
aggressive and non-native birds such as starlings. The entrance hole for
house wren and black-capped chickadee homes should be 1-1/8 inch diameter. A
slightly larger hole—1-1/4 inch diameter—is good for tufted titmice,
chickadees, downy woodpeckers, and nuthatches. Red-headed woodpeckers need
2-inch wide holes. Having the minimal hole size for a particular bird
will keep out invaders, such as house sparrows which actually may kill
native bird species.
Birds need a varying amount of floor space, depending on size.
Generally a 4- by 4-inch bottom is sufficient for smaller birds such as
wrens, chickadees, and nuthatches. One slightly larger—5 and
1/2-inches wide on each side—is better for bluebirds. Larger birds
will need even more floor area. Most birds suffice with 5 inches from
the floor to the bottom of the entrance hole, about 8 inches high overall.
Since birdhouses are mainly used during the spring and summer, they need
ventilation to allow heat to escape. There should be a slit or opening
at the top of at least one side so any heat can escape. Wood doesn’t
heat up as much during summer as metal, and retains a bit more heat too in
winter for any birds overwintering. Wooden walls should be at least
3/4-inch thick. Clay or ceramic birdhouses provide adequate summer
insulation, but won’t be usable during winter. If metal, such as some
purple martin houses, they should be painted white to reflect the heat.
To keep water out, birdhouses should have a roof overhang of an inch to two
past the opening. There should be some drain holes in the bottom to
allow any water to drain out. This will help to keep mold and mildew
Having a side that opens makes it easy for you to keep the birdhouse cleaned
out and sanitary between uses, keeping your birds healthy. This is
especially important for species such as wrens and bluebirds that may raise
more than one brood a year. Some birdhouses have clear sides, enabling
you to watch the baby birds inside.
While colors may look nice, make sure birdhouses are not painted with toxic
paints or stains. Best are ones made of natural wood, preferably a
decay-resistant wood such as cedar. This weathers to a natural silvery
gray that blends nicely with natural surroundings.
Another aspect to bird safety, besides cleaning and avoiding toxic paints,
is proper design to deter predators. Since birds that nest in boxes
naturally nest in tree cavities which don’t have perches, neither should
birdhouses. Perches merely serve to give large predators a place to
land. Some birdhouses also have a larger piece of wood, or metal
bracket, around the hole opening, making it harder for predators to get in
or to enlarge the opening.
By installing the right houses you can help bird species in decline in a
region, such as the barn swallow, tree swallow, Northern flicker, American
robin and house wren in the Northeast. One of the most complete guides to
birdhouses, with plans, directions for locations and mounting, and even an
interactive tool for choosing the right house for the right bird, is from
the Cornell Lab of Ornithology (nestwatch.org).
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