The Problem: Tree Swallows compete with Bluebirds for boxes:
There are three species of bluebirds, the Eastern, Mountain, and Western.  
Like Tree Swallows, bluebirds are unable to make their own nest cavities.  
Though the swallows are significantly smaller than bluebirds in every external
body dimension but wing length, and are outweighed by the bluebirds 30 gm.
to  20 gm., where their ranges overlap they may compete with bluebirds for
cavities, something they were doing long before humans 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, as we shall see).  Some bluebird lovers get 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 encouraging Tree Swallows is wrong.  It is not!  
Bluebirds and Tree Swallows are native songbirds that are equally worthy of
care, consideration, and conservation, and Tree Swallows aren't what's
preventing bluebirds from becoming abundant.  One is not more "desirable"
(and we hate to use this term) than the other.  But unfortunately there is
still resentment in some quarters when Tree Swallow compete for bluebird
boxes.  This is both sad and really unnecessary, 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 simple box spacing guidelines were followed.  Bluebirds defend large
feeding territories around their nests.  They don't want other bluebirds
nesting nearby.  Bluebird experts recommend that bluebird boxes be at least
300' (the length of a football field) apart, preferably more.  

In contrast Tree Swallows only defend a small area around the nest and,
unlike bluebirds, swallows usually leave their defended area to forage.  This
means many pairs of swallows can potentially nest in an area that would
support just one pair of bluebirds.   

When bluebird boxes are spaced far apart a pair of bluebirds usually contends
with just a single pair of Tree Swallows, and the larger, stronger bluebirds
normally win.  (It's a myth that bluebirds are "gentler" and "less aggressive"
than Tree Swallows.  Bluebirds are capable of both intense and prolonged
aggressive behavior; their behavior patterns just differ from the swallows').
But bluebird boxes placed closer than 300' may attract groups of swallows able
to mob a lone pair of bluebirds.  However, even then truly fit bluebirds can
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.

The Solution: Reduce competition by pairing boxes.
Since Tree Swallow bodies are smaller and slimmer than bluebirds' there is no
way to design a bluebird box that will prevent swallows from trying to use it.  
However, there is one proven technique that allows both songbirds to nest
together successfully.  This is to set up pairs of boxes, no more than 10'-20'
apart.  They can even be mounted back to back on the same pole.  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.  







Although it isn't foolproof, pairing boxes works.  However, when pairing is
used to reduce bluebird-swallow competition, bluebird spacing requirements
still apply.  This means each pair must still be 300' from the next pair, or the
whole purpose of pairing is defeated.  The boxes below, with a pair in front
and a second pair a short distance behind, show what not to do.









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.  Instead, they view these situations objectively as opportunities
to enjoy other species, learn their ways, and value their diversity.

But don't bluebirds deserve special treatment?
No, not really.  There is nothing unique or inherent about bluebirds that
warrants special treatment, certainly not at the expense of other native
species.  Contrary to popular belief no bluebird species is threatened or
endangered.  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 more problems.  
(Other familiar cavity-nesting birds scoring 8 were Tufted Titmouse, Carolina
Wren, Purple Martin and, surprise, Tree Swallow).  Western Bluebirds were
cause for more concern, scoring 13.  More recently Partners in Flight
upgraded Eastern Bluebird's "Continental Combined Score" to a 7, on a par
with such species as Yellow Warbler, Chipping Sparrow, Brown-headed
Cowbird, and European Starling.  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 losing crucial migration and wintering habitat as
coastal wetlands in the U.S., Mexico, and Central America are degraded,
drained, converted for aquaculture and housing, or lost to rising sea levels.

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 goals for bluebird "recovery" or "restoration" should
be reconsidered.  Some historical perspective might help.

The rise, fall, and rise of the Eastern Bluebird:
Before dramatic alteration by humans most of eastern North America was
dense forest.  Since Eastern Bluebirds prefer open to 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 conditions Eastern Bluebirds were never likely to
have been common breeding birds except perhaps locally on old burns, and
certainly never abundant.

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 until you see
photos from the time.  However, this was a bonanza for bluebirds!  The small
farms of 19th century rural North America offered 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.  These aggressive cavity
nesters could out-compete bluebirds for nest sites.  The small farm was also
changing.  Marginal, unprofitable farms were abandoned and began the 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.  More
responsible use of pesticides has certainly helped.  And though it's hard to
prove, competition with House Sparrows and Starlings may have selected for
bluebirds that are genetically rougher and tougher.  Whatever the reasons,
Eastern Bluebirds are now common nesters again across much of eastern and
central North America.  It's very possible they are more abundant now than
before the first humans arrived!

However, to expect bluebird numbers could ever return to the 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 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 are mostly based on hopes and expectations that a
restoration to the bluebird population levels of the 1800's is possible, and
that the bluebird must be "saved".  Actually, far from being a species in peril
Eastern Bluebirds are doing just fine today, aided by their toughness as a
species, their devoted followers, some new suburban habitat, and the
growing popularity of winter feeding.  Bluebirds are among the lucky few bird
species whose future seems assured.  But it needs to be recognized that
bluebirds are never 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.  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.

                                                          Top
Learn About Birds at Tree Swallow Nest Box Projects
Bluebird Competition