The Problem: Bluebirds and Tree Swallows may compete for nest boxes.
For over 40 years the North American public has been led to believe that bluebirds are
in trouble, their very survival in danger unless humans come to their rescue. But this
simply wasn’t, and isn’t, true. In fact you might be surprised to learn that:
Unfortunately, public misunderstanding of bluebird status has caused several problems
- No bluebird species is or has ever been classified endangered or threatened.
Instead, ornithologists rate all three bluebirds species of “least concern.”
- Eastern Bluebird numbers are third highest of the more than 60 species of native
North American cavity-nesting birds.
for Tree Swallows and those of us who enjoy and manage for them. Please read on.
There are three species of bluebirds, the Eastern, Western and Mountain. Like Tree
Swallows, bluebirds depend on cavities for nesting but are unable to make their own.
And although Tree Swallows are significantly smaller than bluebirds in every external
body dimension but wing length, and are outweighed by bluebirds 30 grams to 20,
where their ranges overlap swallows will actively compete with bluebirds for cavities, as
Pat Grantham's photo below of Western Bluebirds and a Tree Swallow contesting a box
in Montana dramatically illustrates.
In recent years there has been a popular and highly publicized movement to erect nest
boxes for bluebirds. Many people are devoted to "bringing the bluebird back" to
supposed former levels of abundance, an impossible task, based on misconceptions and
myths, as we shall see. Some bluebird lovers become irritated when their boxes are
occupied by other species, especially Tree Swallows. Bluebird enthusiasts have been
heard to complain, "Tree Swallows took all my bluebird boxes! They ganged up and
drove my bluebirds away!"
Are these complaints justified? You may meet people, especially in Eastern Bluebird
range, who tell you that encouraging Tree Swallows is wrong. It is not. Bluebirds and
Tree Swallows are both native songbirds, equally worthy of care, consideration, and
conservation, and Tree Swallows aren't preventing bluebirds from becoming abundant.
One species is not more "desirable" than the other. But unfortunately there is still
resentment in some quarters when Tree Swallows compete for bluebird boxes, and a
minority, hopefully very small, of bluebird hobbyists feels free to destroy swallow nests
and eggs they discover in their boxes. This is both sad and unnecessary, to say nothing
of immoral and illegal, since there are accepted management techniques that can
reduce most strife between the species. In the photo below a male Eastern Bluebird
defends its box against a Tree Swallow.
Spacing nest boxes for bluebirds:
Much competition between Tree Swallows and bluebirds would be eliminated if
bluebird hobbyists simply followed accepted box spacing guidelines. Since bluebirds
defend large feeding territories around their nests they don't want to nest close to
other bluebirds. Experts recommend that bluebird boxes should be spaced at least
300' apart (the length of a football field), preferably even farther.
In contrast, unlike bluebirds, Tree Swallows do not defend feeding territories, just a
small area around their nest. For this reason the recommended spacing for swallow
boxes is only 100' apart. This means many pairs of swallows can potentially nest within
an area that just one pair of bluebirds would normally need to claim.
When bluebird boxes are spaced far apart the way they should be, a pair of bluebirds
usually contends with just one pair of Tree Swallows, and in these contests the larger,
stronger bluebirds normally win. It's a myth that bluebirds are "gentler" and "less
aggressive" than Tree Swallows. Bluebirds are very capable of intense, prolonged and
sometimes lethal aggressive behavior; their behavior patterns just differ from the
swallows'. Swallows rely on screaming, persistent approaches, dive-bombing, and pecks
given on the fly-by. A bluebird's usual tactic is to wait at the box, making occasional
hostile displays, then dashing out to intercept the swallow, grappling with it and
tumbling with it to the ground where the bluebird can peck the swallow and beat it
with its wings. In the photo below by Jackie Sills of Alberta a female Mountain Bluebird
has pinned down a Tree Swallow and is striking it.
One pair of bluebirds is normally more than a match for one pair of swallows, but
problems can arise when bluebird boxes are spaced closer than 300'. Clusters of
boxes in one bluebird territory may attract groups of swallows that can mob the lone
pair of bluebirds. However, even then truly fit bluebirds often win out. The proof?
Bluebirds commonly secure and defend boxes, and nest successfully within grids of 50-
100 or more active Tree Swallow nests at ornithology research sites. Here in New York
bluebirds have raised broods in three of our five swallow projects over the years. Our
experience is that if bluebirds really want a box they are very capable of taking and
defending it from swallows. The photo below, showing a Tree Swallow being dragged
from a box by a female Eastern Bluebird, was taken by Jim Gilbert in New Jersey. Note
the species size difference.
The Solution: Reduce bluebird-swallow competition by Pairing Boxes.
It is impossible to design a swallow-proof bluebird box because swallows are so much
smaller and slimmer than bluebirds. However, there is one proven technique that
allows both songbirds to nest together successfully. This is "pairing;" setting up pairs
of boxes, with each box of a pair no more than 5-10 feet apart. Since Tree Swallows
seldom allow another pair of swallows to nest within 10 feet, the second box is usually
free for bluebird use and the two species can co-exist, after some initial squabbling to
sort out who gets which box. Photo below by Andrew Aldrich.
Swallows and bluebirds will even nest in boxes mounted back to back on a single pole as
the pictures below, taken at the ABC Bluebird Trail in Albion, NY, illustrate.
Click HERE to view a YouTube video of Eastern Bluebirds and Tree Swallows bringing
food to their broods at another pair of boxes, and HERE to see inside the boxes.
Does pairing boxes always work? No, of course not. But it is effective often enough
that it should be tried when bluebird vs. swallow competition is a concern.
Remember that if you use pairing to reduce competition between bluebirds and
swallows, bluebird spacing requirements still apply. Each pair of boxes should still be
300' or more from the next pair or the purpose of pairing is defeated.
The example below, at a New York nature center, with one pair of boxes in front and a
second pair just a short distance behind, shows what not to do.
The moral is that people who manage for Tree Swallows and bluebirds need not be at
odds. Bluebirders rightly take pride in how they've increased bluebird numbers
dramatically, and most also realize it's not a tragedy if other native species occupy
some of their boxes. They view these situations objectively as opportunities to enjoy
another species and to learn their ways. They've come to value the diversity found
But aren't bluebirds in trouble? Don't they deserve special treatment?
No, they aren't and they don't! Despite what you may have heard or read there is
nothing unique or inherent about bluebirds that warrants special treatment or
favoritism, and certainly not at the expense of other native species. Contrary to
popular belief not one of the three bluebird species is endangered or threatened now,
and none has been at any time in its recent history. No bluebird species is in peril;
none need to be rescued.
In 2008 the International Union for Conservation of Nature and Natural Resources (IUCN)
rated all three bluebirds species of "Least Concern." A 2012 update to Partners In
Flight's Species Assessment Database, which uses peer-reviewed scientific
methodology, estimated there were 22,000,000 Eastern Bluebirds, 4,600,000
Mountain Bluebirds, and 6,700,000 Western Bluebirds in North America, for a total of
33,300,000 bluebirds. By contrast Tree Swallows numbered 17,000,000. That's
right, there are more Eastern Bluebirds in this world than Tree Swallows. Most
people today are very surprised to learn that, according to the PIF Database, of the 60
plus species of native Canadian and American landbirds that nest exclusively or almost
exclusively in cavities, Eastern Bluebirds are the third most abundant; only House
Wrens and Black-capped Chickadees are more numerous.
On another measure, the Continental Concern Score, Partners in Flight assigned Eastern
Bluebirds a score of 7 on a scale of 4-20, where higher numbers signify species with
greater potential for future vulnerability. Other species receiving a 7 were Yellow
Warbler, Chipping Sparrow, Brown-headed Cowbird, and European Starling. Tree
Swallow received an 8, along with other familiar cavity-nesting birds such as Tufted
Titmouse, Carolina Wren, and Purple Martin. In other words the outlook for Eastern
Bluebirds is considered somewhat better than that for Tree Swallows.
What problems do Tree Swallows face?
Tree Swallows are losing natural nesting habitat as northern forests are cut for
pulpwood and lumber, and lost to large-scale mining. Perhaps more importantly they
are losing crucial migration and wintering habitat as the coastal wetlands they depend
on in the U.S., Mexico, and Central America are degraded, drained, or converted by
humans for use in aquaculture, agriculture, industry, and housing, or lost to rising sea
levels caused by global warming. In fact, in 2011 Birds of North America reported
significant declines since 1980 in the Tree Swallow breeding populations of Canada,
New England, and the Great Lakes states. Even more recently, Audubon's 2014 "Birds
and Climate Change Report" has called the Tree Swallow "Climate Threatened", one of
188 North American species which are "expected to lose more than 50 percent of their
current range by 2080 if global warming continues at its current pace." (Western and
Mountain Bluebirds are also considered "Climate Threatened", but the Eastern Bluebird
is not.) Audubon's scientists predict that Tree Swallow summer and winter ranges will
be forced to shift northward, and that successful utilization of more northerly breeding
areas will require growth of forests where there are none now and/or addition of nest
The rise, fall, and rise of the Eastern Bluebird:
Many bird enthusiasts are becoming aware there are hundreds of species of plants and
animals in North America, including many birds, that are in much worse shape than
either bluebirds or Tree Swallows, and deserve our aid too. With this in mind perhaps
some of the bluebird hobbyists' goals for bluebird "recovery" or "restoration" should be
reconsidered. At this point a bit of historical perspective might help.
Before its dramatic alteration by humans most of North America east of the Great Plains
was dense forest. Since Eastern Bluebirds prefer open and semi-open areas with
scattered woody vegetation, they were probably restricted to areas burned over by
forest fires and in early stages of regeneration. Burns would have dead trees with
cavities, perches for hawking down on insects, and fruit-bearing shrubs typical of early
plant succession. However, as burns returned to forest, bluebirds would have needed
to relocate. In any given year most of the eastern forest would have been unsuitable
habitat for them. Therefore, under natural, pre-human conditions Eastern Bluebirds
were never likely to have been common breeding birds except perhaps locally on
burns, and they were certainly never the abundant species that some hobbyist
literature and web sites would have you believe.
Then, Native Americans arrived. The clearings they created for farms and villages
increased bluebird habitat, at the expense of woodland bird species. Fires Native
Americans set to create and maintain feeding openings for deer, elk, and bison would
also have benefited bluebirds by thinning undergrowth.
But it was when North America was colonized by Europeans that the forests began to
be cleared in earnest. By the mid 1800's the eastern forests were almost gone,
replaced by small farms with cropland, pastures and orchards. It's hard to appreciate
how completely deforested the land was unless you see photos from the time. Though
this was devastating for woodland wildlife, it was a bonanza for bluebirds! The small
farms of 19th century rural North America offered vast areas of new and absolutely
perfect habitat. Bluebirds were able to occupy wide areas where they had never lived
before and their numbers skyrocketed.
But then things changed once again. In 1851 House Sparrows were introduced from
Europe, followed in 1890 by European Starlings. As these aggressive non-native cavity
nesters multiplied they out-competed bluebirds for many nest sites. The small farm,
once so inviting, also changed. Marginal, unprofitable farms were abandoned and
began the slow but inevitable return to forest. On better farms mechanized equipment
replaced horses and mules, and allowed small fields to be combined into large.
Automobiles meant pasturage was not needed for horse-drawn vehicles. "Clean
orchards" and dwarf varieties of fruit trees became the rule. Suitable cavities and
habitat for bluebirds became scarce. Then in the 1900's indiscriminate application of
chemical pesticides became common, poisoning bluebird food and bluebirds that ate
it. Bluebird numbers plummeted. Luckily, by the 1930's and 1940's observant people
began to realize they had become scarce relative to the golden days of the 1800's, and
started the movement to "bring back the bluebirds."
Since then bluebird nest box programs have been very successful, and the mowing of
lawns, parks, and cemeteries has created vast new areas of short-grass habitat that
bluebirds have been quick to utilize. More responsible use of pesticides has certainly
helped. Supplemental feeding now aids bluebird survival during cold snaps. And
although it can't be proven, competition with House Sparrows and Starlings may be
selecting for bluebirds that are genetically rougher and tougher. Let's hope it's true.
Whatever the reasons, Eastern Bluebirds are common nesters again across much of
eastern and central North America, and have actually expanded their nesting range into
large areas of Canada and the American west where they had not historically bred. In
fact it's very possible Eastern Bluebirds are more abundant now than when the first
However, to expect Eastern Bluebird numbers could ever return to the extreme
highs of the 1800's is unrealistic. House Sparrows and Starlings are here to stay.
Forests have returned in many places, although a mere shadow of their former size.
And the small farm of yesteryear will not return. A couple hundred million more people
with houses, cars, highways, and buildings now occupy bluebird habitat of the 1800's.
The clock cannot be turned back. The Eastern Bluebird cannot reattain the abnormally
high numbers of the 1800's. Those levels are gone forever because the conditions that
allowed them are gone forever.
What does all this have to do with concerns about Tree Swallows using bluebird boxes?
These concerns are mostly based on unrealistic hopes and expectations that a
restoration to the bluebird population levels of the 1800's is possible, and that the
bluebird needs to be "saved." But as we've seen, far from being a species in peril
Eastern Bluebirds are actually doing extremely well today compared to most birds,
aided by their toughness, their devoted followers, new habitat, and the growing
popularity of year-round supplemental feeding. Bluebirds are among the lucky few bird
species whose future seems assured. Bluebird hobbyists must recognize that bluebirds
never were and never are going to become truly abundant, and that having bluebird
boxes occupied by Tree Swallows or other native species does not in any way
jeopardize bluebirds' existence. Rather, it's our own species' activities, for better or
worse, that will determine the population swings of bluebirds, swallows, and most
other species on earth, at least in the short run.
TEST YOURSELF: Click HERE for an EASTERN BLUEBIRD - TREE SWALLOW QUIZ
Learn About Birds at Tree Swallow Nest Box Projects
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Box Location, Pole Options, Mounting Boxes, Nest Box Grids, Predator Protection,
Bluebird Competition, Martin Competition, House Wren Damage, House Sparrow Damage