The Problem: Bluebirds and Tree Swallows may compete for nest boxes.
For over 40 years the North American public has been told that bluebirds are in deep
trouble, their very survival in danger unless humans come to their rescue. However, this
wasn't true 40 years ago and isn't true now. You may be surprised to learn that in fact:
- No bluebird species is or has ever been classified endangered or threatened.
- Eastern Bluebirds have the third highest numbers of the more than 60 species of
native North American cavity-nesting land birds, even higher than Tree Swallows.
- Statements that bluebirds used to be as common as robins are utter nonsense.
However, continued public belief that bluebirds need preferential treatment has caused
serious problems for Tree Swallows and those who enjoy them. This is unfortunate
because there are proven methods for reducing competition between bluebirds
and Tree Swallows for nest boxes. You can enjoy both! 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 nesting cavities, as
Pat Grantham's photo below of Western Bluebirds and a Tree Swallow contesting a box
in Montana dramatically illustrates.
For several decades now 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 levels of former abundance, but this is an impossible task, based on
misconceptions and myths, as we shall see. And some bluebird lovers become irritated
when their boxes are occupied by any other species, especially Tree Swallows, and
complain: "Tree Swallows took all my bluebird boxes! They ganged up and drove my
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 justified 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 from an intruding 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 to 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 require.
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 deadly aggressive behavior; but
their behavior patterns differ from the swallows'. Swallows rely on screaming, persistent
intrusions, 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 is pecking and striking a Tree Swallow it has pinned down.
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: Pairing boxes can reduce bluebird-swallow competition.
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. These pairs should be
at least 100 yards or meters apart
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, 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 native species
and to learn its ways. They've come to value the diversity found among birds.
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 none of
the three bluebird species is endangered or threatened now, and none has
been at any time in its recent history. There is nothing unique or inherent
about bluebirds that warrants special treatment or favoritism, certainly not at the
expense of other native species.
In 2013 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. Tree
Swallows were believed to number 17,000,000. That's right, there are more
Eastern Bluebirds in this world than Tree Swallows. And most people 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.
More recently, in 2016 the North American Bird Conservation Initiative's Species
Assessment Summary, which includes data from Canada, the United States and Mexico,
produced Continental Concern Scores for each North American species. On a scale of
4-20, where higher numbers signify species with greater potential for future vulnerability,
Eastern Bluebird received a score of 7 while Tree Swallow received a 10. In
other words, the outlook for Eastern Bluebirds is considered to be better than
that for Tree Swallows. Western Bluebird was rated a 9 and Mountain Bluebird
with a 12, had the highest level of concern of these four species. By NABCI
critera birds with scores of 4-8 (Eastern Bluebird) are species of "Least Concern",
while those with scores of 9-13 (Tree Swallow, Western Bluebird and Mountain
Bluebird) are species of "Moderate Concern".
Click Here to view a chart showing populations and concern scores for 62 native North
American cavity-nesting landbirds.
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 US,
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 boxes.
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 been able to
invade large areas of Canada and the American west where they had not bred
historically. In fact it's very possible Eastern Bluebirds are more abundant now
than when humans first reached their range.
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 regrown 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, buildings and parking lots 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
the Eastern Bluebird is 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 threaten
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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Bluebird Competition, Martin Competition, House Wren Damage, House Sparrow Damage