The Problem: Bluebirds and Tree Swallows may compete for nest boxes.
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 though 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 so 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 a Tree Swallow pinned down 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 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 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 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 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 humans arrived.
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 and Tree Swallow Questionnaire.
Learn About Birds at Tree Swallow Nest Box Projects
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