As far as box construction and structure goes, there are a number of aspects to keep in mind for any bluebird box. These are listed on the Cornell website (http://nestwatch.org/learn/all-about-birdhouses/features-of-a-good-birdhouse/), and include no perch, ventilation available, correct hole size for western bluebirds (1 9/16"), ample depth to reduce predation, a door for cleaning out the inside of the structure (not located on the underside of the structure), a roof that will shed rain and reduce rain from getting into the hole, and a design on the inside of the box to allow young to leave the box. On this last point, as the young are just learning to maneuver and fly, there should be interior grooves to allow young to grip onto the inside of the box enough to be able to exit. As we use wooden boxes on base for our bluebirds, we use the rough side of the wood board for the inside of the door leading to the hole, which allows enough grip for the young to leave. We have seen designs where very small sections of mesh-hardware were attached to the inside of the door leading to the hole, however, in one instance an adult bluebird died after her tongue (which is forked in the back of the mouth) somehow got caught on the wire. This was a freak accident, but still something to consider, as some birds tend to get their tongues stuck (especially woodpeckers).
For box materials, I have not heard of anyone making boxes out of
metal, nor do I know about success rate correlated with specific materials. I
would note, though, that metal boxes pose a number of concerns, including
insulation and rust potential. We have seen PVC pipe designs, and those seem to
work fine. As for coloring the boxes, I don't necessarily think that this would
deter birds from nesting in them, as long as the paint is free of any toxins.
One concern I would have with having a very colorful box is the increased
potential for them to be disturbed by people - this could always be remedied by
having the boxes high enough on trees, but that makes checking the boxes or
cleaning them at the end of the season more challenging.
For the grouping of boxes, bluebirds are not colony nesting birds
(like purple martins, where you would have at least 24 boxes within one colony
site). We tend to have about a 100-200 meter distance between each bluebird
box, as this significantly reduces competition between individuals. If you are
going to have boxes near each other, I would suggest only two within a 100
meter space, minimum of 10-15 feet apart. You may get bluebirds nesting in one
of these and swallows nesting in the other. If they are too close, the birds
will spend more time defending their territory than is necessary. It looks like
you would be able to fit about 6 boxes maximum within the space you're working
in.
A bluebird box can be placed on an oak tree trunk or on a pole that
is cemented into the ground. For orientation of the boxes, we have seen success
in boxes that are oriented any direction; however, we tend to place ours in a
Southeasterly direction, as they get more insolation from the sun facing south
and are not facing directly into our wet, Westerly winds we often get in the
area.
As Jerry mentioned, depending on the structure and which species are
drawn to the habitat, you may get other birds nesting in your boxes. For your
initially suggested design, those longer structures are typically used for
purple martins, though swallows will nest in them too. The hole you would want
to use to attract bluebirds would be too small (typically) for purple martins
to nest in them. Thus, height is a big consideration here, as more birds will
be able to nest in a deeper box.
Christa LeGrande-Rolls, Biologist, DPW/ED
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