The Problem: Bluebirds and Tree Swallows may compete for nest boxes.
For many years it has been suggested to the North American public that bluebirds are in
deep trouble, their very survival in danger unless humans come to their rescue.
However, this simply isn't true. 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 major
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 the
scarce nesting cavities both species need, 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, and some bluebird lovers become irritated
when their boxes are occupied by any other species, especially Tree Swallows. The
complaint may be heard: "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 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.
Proper spacing of nest boxes for bluebirds:
Much competition between Tree Swallows and bluebirds could 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
sweet, gentle and less aggressive than Tree Swallows. In fact, speaking of
Eastern Bluebirds, Birds of North America states "Not only males but also females fight
among themselves, and sometimes females wound and kill each other over access to
Bluebirds are very capable of intense, prolonged and sometimes deadly
aggressive behavior; but their aggressive behavior patterns differ from those
of 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 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, as the
female Mountain Bluebird below is doing. Photo by Jackie Sills of Alberta.
One pair of bluebirds is normally more than a match for a single 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
to 100 or more active Tree Swallow nests at ornithology research sites. Here in New
York bluebirds have claimed boxes and 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.
Pairing boxes can also 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 considered 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
have been located at least 100 yards 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 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. Some evidence is described below.
In 2017 Partners In Flight's Avian Conservation Assessment Database, using peer-
reviewed scientific methodology, estimated there were 24,000,000 Eastern Bluebirds,
6,000,000 Mountain Bluebirds, and 7,300,000 Western Bluebirds in North America.
Tree Swallows were believed to number 20,000,000. Yes, there are more
Eastern Bluebirds in this world than Tree Swallows. The database also reports
that, while Tree Swallow numbers have declined 40% since 1970, the Eastern
Bluebird has increased by 178% during the same period! And most people are
very surprised to learn that 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.
Furthermore, 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
criteria 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 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 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. In 2014,
Audubon's "Birds and Climate Change Report" 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 much worse off than either
bluebirds or Tree Swallows, and which 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 some historical perspective might help us see why.
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 largely gone, replaced by
small farms with cropland, pastures and orchards. It's hard for us to appreciate how
completely deforested the land was unless we 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 open
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. The
advent of 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 scarcer. Then in the 1900's indiscriminate application of
chemical pesticides became common, poisoning bluebird food and bluebirds that ate it.
Bluebird numbers declined further still. But luckily, by the 1930's and 1940's observant
people realized this species had become rare in parts of its range relative to the golden
days of the 1800's, and the movement to "bring back the bluebirds" began.
In recent decades 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 year-round, especially
during the winter months. 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, with the dramatic 178% increase in
their numbers 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.
It's clear, Eastern Bluebirds have faced many challenges in the last 150 years. House
Sparrows and Starlings are here to stay. Forests have regrown in many places,
although a mere shadow of their former size. 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. But despite these changes, and
with some human help, the Eastern Bluebird has proven its adaptability and is thriving.
In fact, the Birds of North America's Eastern Bluebird species report states
"despite concerns about 20th-century fluctuations in numbers, populations may
have been more abundant in the 20th and early 21st centuries than in any other
period of North American history since the Pleistocene."
What does all this have to do with concerns about Tree Swallows using bluebird boxes?
These concerns are mostly based on the invalid notion that the Eastern 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 other birds. 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 be as abundant as
robins, and that having bluebird boxes occupied by Tree Swallows or other native
species does not in any way threaten bluebird existence. Rather, it's our own species'
activities, for better or worse, that will determine 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
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