The Problem:  Tree Swallows compete with Bluebirds for nest boxes.
There are three species of bluebirds, the Eastern (below left), Mountain, and
Western.  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 gm. to  20 gm., where their ranges overlap swallows will compete with
bluebirds for cavities, something they were doing long before humans ever came on
the scene.













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
former levels of abundance, an impossible task, based on misconceptions and myths,
as we shall see.  Some bluebird lovers become very 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" (we hate this term) than the other.  
But unfortunately there is still resentment in some quarters when Tree Swallows
compete for bluebird boxes, and an ignorant minority of bluebird hobbyists feels free
to destroy any 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.  














Spacing nest boxes for bluebirds:
Much competition between Tree Swallows and bluebirds would be eliminated if
bluebird hobbyists simply followed 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 be spaced at least 300' apart
(the length of a football field!), preferably even more.  

In contrast Tree Swallows only defend a small area around their nest and, unlike
bluebirds, swallows usually leave their defended area to feed.  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 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 both intense and
prolonged 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 such as Wing-Flicks (below left), 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 strike it with its wings.  
























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 who 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 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.  Photo below by Jim Gilbert.  Note size difference.

















The Solution: Reduce bluebird-swallow competition by Pairing Boxes.
There is no way 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 10-20 feet apart.  Since
Tree Swallows will not allow another pair of swallows to nest within 20', the second
box is 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 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 on the ABC Bluebird Trail.

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.

However, please remember that if you use pairing to reduce competition between
bluebirds and swallows, bluebird spacing requirements still apply.  This means that
each pair of boxes must still be 300' from the next pair, or the whole purpose of
pairing is defeated.  The example below, at a 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
needs to be rescued.  In 2008 the International Union for Conservation of Nature and
Natural Resources (IUCN) rated all three bluebirds as species of "Least Concern"!  In
2004 Partners in Flight's authoritative North American Landbird Conservation Plan
estimated there were 10,000,000 Eastern Bluebirds, 5,200,000 Mountain Bluebirds,
and 1,400,000 Western Bluebirds, for a total of 16,600,000 bluebirds.  (Tree Swallows
numbered 20,000,000).  On another measure, PIF assigned Eastern and Mountain
Bluebirds a score of 8 on a scale of 1-20, where higher numbers signify species with
greater potential for future vulnerability.  (Other familiar cavity-nesting birds scoring
8 were Tufted Titmouse, Carolina Wren, Purple Martin and, surprise, Tree Swallow).  
Western Bluebirds' situation was judged more problematic, rating a 13.  More
recently Partners in Flight upgraded Eastern Bluebird's "Continental Combined Score"
to a 7, on a par with such common species as Yellow Warbler, Chipping Sparrow,
Brown-headed Cowbird, and European Starling.  In other words the prospects of the
Eastern Bluebird were actually rated better than those of the Tree Swallow!  What
problems do Tree Swallows have?  They are losing nesting habitat as boreal forests
are cut for pulpwood and lumber, and 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, converted for
aquaculture and housing, or lost to rising sea levels caused by global warming.

Most bird enthusiasts are 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.












The rise, fall, and rise of the Eastern Bluebird:
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 old burns, and they were certainly never the abundant species
that folklore and some popular literature would have you believe.

Then, Native Americans arrived.  Their clearings for farms and villages increased
bluebird habitat.  Fires they set to create and maintain feeding openings for deer,
elk, and bison would also have benefited bluebirds.  

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 absolutely perfect
habitat, and bluebird numbers skyrocketed.

But then things changed again.  In 1851 House Sparrows were introduced from
Europe, followed in 1890 by European Starlings.  As these aggressive cavity nesters
spread 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.  Bluebird numbers
plummeted.  Luckily, by the 1930's and 1940's observant people began to realize how
scarce they had become 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 suburban
backyards, parks, and cemeteries are now commonly utilized for bluebird habitat.  
More responsible use of pesticides has certainly helped.  And although it can't be
proven, competition with House Sparrows and Starlings may have selected for
bluebirds that are genetically rougher and tougher.  Let's hope it's true.  Whatever
the reasons, Eastern Bluebirds are now common nesters again across much of eastern
and central North America.  It's very possible Eastern Bluebirds are more abundant
now than when the first humans arrived!














However, to expect bluebird numbers can (or should) ever return to the highs of
the 1800's is extremely 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".  Actually, far from being a species in peril Eastern
Bluebirds are doing extremely well today compared to most birds, aided by their
toughness as a species, their devoted followers, some new habitat, and the growing
popularity of winter 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.



































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