Observations on Pairing Bluebird Nest Boxes
It has been a common practice to pair bluebird nest boxes in areas
with high Tree Swallow (Tachycineta bicolor) densities. The theory is
that by supplying a second nest site, competition between the two
species is reduced. Because both species have a higher level of
intraspecific than interspecific competition, the expectation is that a
box pair would support both bluebirds and swallows. Box pairing is
widely accepted, but little controlled research has been published
documenting its benefits. Recently, the Bluebird Association of
Wisconsin (BRAW) has published data indicating that box pairing actually
favors swallows over bluebirds and may even reduce bluebird populations
What Is a Box Pair?
How individuals 'pair' boxes varies widely. The BRAW study considers
a pair to be boxes that are within 1 00 ft. (30.4 m) of each other. Most
literature would consider boxes at this distance to be independent, not
paired. Tree Swallows readily nest simultaneously at sites 1 00 feet
apart. A more common practice is pairing boxes from 5-25 ft. (1.5-7.6 m)
apart. Boxes can even be paired on the same mounting post such as a pipe
or a telephone pole. I have most of my boxes paired at 5-1 0 ft.
(1.5-3.0 m) intervals, a few back to back on telephone poles, and
typically have Tree Swallows in nearly every pair. My bluebird pairs
rarely nest in a site without swallows nesting adjacent to them. have
also occasionally observed swallows nesting in both boxes within a pair.
Pairing vs. Spacing
I believe that it is important to clarify the difference between
pairing of boxes and spacing between boxes. I would only consider a
"pair" to be boxes up to 25 ft. apart from each other, beyond
which point they would function as independent "sites."
Bluebirds rarely nest within 100 yards (91.0 m) of other bluebirds,
while swallows are much more accepting of other swallows. In sites where
I have supersaturated an area with boxes, swallow numbers have increased
while bluebird numbers have not. This is a result of the swallows'
greater tolerance of other swallows and the smaller territories that
they defend. The swallow could almost be considered a colonial nester
due to its tolerance of other swallows at high densities. At any sign of
disturbance at a swallow nest site, every swallow within hearing range
comes to investigate and harass the intruder. This mobbing could cause a
bluebird pair to abandon a potential nest site. As a result, high
densities of boxes and, thus, high densities of swallows, appear to
favor swallows over bluebirds. In my opinion, box spacing is far more
critical to bluebirds than box pairing.
To maximize bluebird productivity, box densities should be minimized.
I have consistently observed higher box occupancy rates by bluebirds in
areas with lower box densities. However, if only one box is at a
"site,' swallows may often occupy that site first making it
unavailable for bluebird use. With paired boxes, bluebirds are nearly
guaranteed an available nest box even if swallows are already present. I
believe that favorable 'sites' should be identified for bluebird use.
Placing box pairs at those sites will allow a higher percentage of those
sites to produce bluebirds than if only one box were present. My goal
would be to maximize the number of .sites' which could potentially be
occupied by bluebirds, not to maximize rates of box usage as suggested
by O'Halloran (1997). Without pairing, many sites where swallows
establish territories would be completely unavailable to bluebirds and
of no value to them.
Do Tree Swallows and Bluebirds Compete?
Competition exists when the niches of two species overlap. Both
swallows and bluebirds obviously are dependent on cavities for nesting,
but otherwise they are different in many of their survival strategies.
Bluebirds feed on ground-dwelling insects while swallows feed on insects
in the air. Given places to nest, they can coexist within an area quite
effectively. It may even be beneficial for bluebirds to have swallows
nearby to warn them of potential predators or danger. Box pairing allows
the advantages of coexistence while reducing competition for nest
Other Advantages of Pairing
In areas with House Sparrows (Passer domesticus) box pairing may
reduce the chance of sparrows destroying a bluebird nest. A sparrow may
be more likely to kill established bluebirds (or swallows) at a single
box than if an empty paired box exists nearby. If a sparrow begins a
nest in an empty paired box, a trail monitor could remove the sparrow
before it interferes with the nest of a native species.
1 believe that in areas of high Tree Swallow densities close pairing
of nest boxes with the pairs widely spaced is the best management
strategy for bluebirds. This strategy should result in the highest rates
of 'site' occupancy. While pairing obviously reduces the potential rate
of box occupancy, this is less critical than having the site used. With
unpaired boxes, if swallows occupy the site before bluebirds locate it,
the site is then completely unavailable to bluebirds. Box pairing is not
a threat to bluebirds; instead, it maximizes the potential for them to
use a site. I would recommend pairing boxes 5-10 ft. apart to minimize,
but not eliminate, swallows' use of both boxes within a pair
simultaneously. In 1977, 1 had swallows simultaneously using both boxes
of a pair in five of my 48 sites. In one study area I greatly reduced
the number of boxes at the site from previous years. Large numbers of
birds returning to this site from previous years found an unexpectedly
low number of nest sites with the result that in several cases swallows
nested in both boxes within a pair. I would expect that increasing
distances between boxes would increase the probability of swallows using
both, certainly boxes placed at 100 ft. intervals should be expected to
frequently result in increased swallow use of both boxes. Bluebirds on
my trails do not hesitate to nest on the opposite side of a telephone
pole from a Tree Swallow, so they can be very tolerant of a close
neighbor, particularly with the visual barrier of the pole.
Regional variations may exist in how bluebirds and swallows interact.
If so, different management strategies may be needed. In New York state,
Tree Swallows are abundant. In 1977, 43 of 48 box pairs on my trail were
used (nest built and at least one egg laid) by swallows. Without
pairing, many of those sites would be unavailable for bluebirds. In
areas of few or no swallows, box pairing may indeed be a waste" of
O'Halloran, J. 1977. BRAW's 1996 season box-pairing update.
Associate Professor Kevin Berner, SUNY-Cobleskill, is the Research
Committee Chairman for the North American Bluebird Society.
Reprinted, with permission, from "Sialia/Bluebird"
Journal of the North American Bluebird Society. NABS is a membership
organization for persons interested in bluebirds and other North
American birds which use cavities for nesting. For membership
information, send a message to
or go to the NABS web site at