THE STORY OF THE GILBERTSON BOX
by Steve Gilbertson
Fairly often I hear or read about individuals that do well with
my design nest box and sometimes I hear they do not so well. While
I feel that a majority of the feedback is accurate, I've discovered
that some people's idea of a Gilbertson box is simply a birdhouse
made from PVC plastic.
Samples of "modified" Gilbertson
boxes contained everything from major flaws in quality to some
with serious flaws in their
design. Problems included:
untreated roofs which would readily split, allowing rainwater
to go into the nest cavity · roofs barely
larger than the body itself, providing no shade or weather
Floors not recessed into the body, allowing water intrusion
by capillary action
of less than 4" from the bottom of the hole to
the top of the floor
Unpainted - while this may not qualify as a flaw, in my opinion
white PVC boxes must be painted. I use brown oil-based solid-color
exterior stain. Spray paint is fine. Avoid latex because it
peels. The inside is painted to lessen light intrusion through
the sidewall and to allow the female to contrast less than
with white. The outside is mottled to look like tree bark.
The Gilbertson nest box system is unique and evolved in 1989
out of my desire to isolate and correct what I felt were shortcomings
in many nest box designs and mountings which could result in failed
nests. Ants, snakes, mice, cats, squirrels and raccoons were problems
connected - literally - to the ground. House sparrow problems could
be related directly to box design. Human nature had to be taken
into consideration as it affects why and how many nest boxes are
put up. To me, this meant creating a lightweight box coupled with
fast, easy and economical installation. Tamper protection and quality
monitoring was yet another challenge.
To act on these concerns was a tall order to be sure, but by
1991 a nest box and mount evolved that to this day has changed
very little. After a brief and foolish trial with treated cardboard
carpet tubes as nest box bodies fastened to a wooden roof. I settled
on 4’ thin wall PVC (polyvinylchloride) pipe, which was lightweight,
strong and easily cut to length. This could be stained or painted
brown on the inside and mottled brown on the outside.
Tamper-protection and quality monitoring is achieved by creating
a two-piece box connected by outward-projecting pins. Squeezing
the body at the roof-line elongates its distance, allowing detachment
by compressing the body. Unwanted human visitors are stymied, as
they look for a protruding screw or nail to open it. When the trail
monitor detaches the body properly, he or she can lower the box
down and have a full view of the neat and contents. Children don't
need to be lifted up to get a quick, elusive glimpse. Field trip
participants can share equally in the delightful view and exposure
was an issue that I honestly felt might not be resolved without
creating bluebird resistance at the same time.
Luckily, sparrows prefer a greater volume of nest material, and
usually create a tunnel-like route to the inner cup. It stood to
reason that the thin-walled, 4" diameter might be too confining
for them, and therefore sparrow-resistant. Experimenting with 4" PVC
boxes of various depths, I discovered that house sparrows lost
interest when the box was only 4&1/2" deep. Bluebirds
were not interested in a PVC box shallower than 4" or less.
A major problem
still existed in protecting the nest box from ground creatures.
I searched for the thinnest, smoothest, strongest
and cheapest mounting system. That came to mean a 5' section of
thin-wall conduit rubbed with coarse steel wool and then waxed.
The conduit can then be screwed or wired to an existing wood or
steel post, or much better, used as a free-standing post: This
requires a 4 or 5 foot section of 1/2" rebar (reinforcing
iron rebar used in concrete structures) driven partially into the
ground. The l/2" conduit is slid over the rebar. (The rebar
is actually l/ 2" exterior size; the conduit is 1/2" interior
size, so the conduit slides easily over the rebar). A conduit clamp
with a slightly longer bolt in the lower hole to firmly contact
the rebar, secures the conduit and rebar together, and keeps the
conduit from twisting on the rebar.
To properly build a conduit-mounted Gilbertson nest box is neither
complicated nor expensive. Its sparrow resistance is unequalled
while it is still highly successful with bluebirds. It is easily
installed, moved around or removed and affords superior protection
from ground-based predators. I've raised nearly 1500 bluebirds
in 7 years, using only this system. I think it is the best.
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