Pairing Boxes

By Steve Eno


The issue of pairing boxes (one box for bluebirds and one box for tree swallows) has recently come into discussion. Does the pairing of boxes on a bluebird trail benefit the bluebird or can it result in a decline in their numbers? I recently mailed a letter to 17 different bluebird organizations asking what their experience has been with paired boxes, what recommendations they may have, and what if any studies or research they may be aware of. I was very pleased with the results – having received 10 responses. Following are the responses I received in alphabetical order:


California Bluebird Recovery Program – Don Yoder

As a Program and collector of nesting results I don’t think we can refer to any figures that will be informative. Our 1997 monitoring record has been revised to include information about pairing of boxes, but prior years won’t reveal those details. My own roster of boxes indicates which ones are paired but, again, our Program can’t respond in that way.

Personally, I favor pairing and believe that total fledglings have benefited from the availability of alternate quarters. I’ve read of alternate suggestions, from back-to-back on the same post, to 23 to 25 feet apart. I don’t know how those values were determined. It is something that can be extensively researched and your efforts should produce some more interesting results.

Best wishes for the new nesting season!



Abernethy, Saskatchewan – Ron Bittner

Using paired nestboxes reduces competition between bluebirds and other species such as Tree Swallows. Usually a bluebird will use one box or a par and a swallow will use the other box.

I use one standard box and one experimental box in each pair. Bluebirds usually nest in the experimental boxes and swallows in the standard boxes. The experimental boxes are probably raccoon-resistant.

I recommend the boxes be placed 10 yards apart. I prefer a site-spacing of 300 to 400 yards. I think that the closer spacing of sites, especially closer than 200 yards, decreases the bluebird/swallow ratio. This might be due to bluebirds having a greater territorial defense than swallows, or to bluebirds being less abundant than swallows.


Alberta – Ellis Bird Farm – Myrna Pearman

Thank you for your letter regarding nestbox pairing. We first started pairing the boxes on our 450-box trail in 1991, and I am including a summary of our productivity data since 1981. As you can see, pairing has not had an effect – one way or another – on Mountain Bluebird productivity. I feel that the greatest benefit of pairing is to reduce nest site competition between swallows and bluebirds. I believe that local populations of swallows would increase, at the expense of bluebirds, if we did not pair the boxes. Pairing, in my opinion, provides an opportunity for both species to coexist quite peacefully. I look forward to receiving a summary of your survey.


Ellis Bird Farm Productivity Data

Year 1981 1982 1983 1984 1986 1987 1988 1989 1990 1991* 1992 1993 1994 1995 1996
Percent of bluebird eggs that hatched 64 73 71 79 70 77 83 77 82 79 63 82 79 85 87
Average young per nest hatching at least one 4.1 5.0 5.1 4.9 4.4 4.8 4.9 4.8 4.75 4.81 4.41 4.94 4.69 4.63 5.4
Average young fledged per nest hatching at least one 3.4 4.6 4.0 4.5 4.0 3.9 4.1 4.2 3.88 4.35 3.16 3.95 3.89 4.4 3.93
Average young fledged per nest fledging at least one 3.7 4.8 5.0 4.8 5.0 4.2 4.6. 4.6 4.58 3.96 4.54 4.43 4.65 4.54
Percent young hatching resulting in young fledging 82 91 77 93 90 89 83 88 81 90 81 80 83 95 81
N Mort 19 9 23 7 10 11 19 12 19 10 19 20 17 5 19
Percent eggs laid resulting in young fledging 53 66 63 73 63 68 68 68 67 71 52 63 66 81 70

* Year started pairing boxes



Johnson County Songbird Project – Jim Walters

Jim wrote that he has not worked with paired boxes on a bluebird trail, but he sent along a copy of an article relating to two-box sites. The following is from the study “Eastern Bluebirds Are Attracted to Two-Box Sites” by Jonathan H. Plissner & Patricia Adair Gowaty, published in the Wilson Bulletin, Vol. 107, No. 2, June 1995:

“ABSTRACT – In early March, just prior to the onset of the breeding season, responses by Eastern Bluebirds to playbacks of the territorial song of conspecific are more likely on site with two nest boxes rather than one. One or two boxes were placed randomly at potential territorial sites during early winter; so responsiveness is not explained by territoriality of the previous season. Explanations for our observations include that bluebirds prefer potential territorial sites with two boxes because of increased habitat quality or that bluebirds locate two-box sites more readily than one-box sites. We infer from our result that potential territorial sites with two nesting boxes are more attractive to bluebirds than sites with only one nesting box.”

[The Plissner/Gowaty study considered several hypotheses and was designed to examine the “attractiveness” of two-box sites to bluebirds as opposed to the fledgling numbers resulting from their use of two-box sites.] The study concluded with the following:

“These hypotheses to explain our observations predict that territories on two-box sites will suffer higher intrusion rates by conspecifics than territories on one-box sites. Harris… reported higher rates of territorial aggression by breeding resident Tree Swallows when nesting boxes were 1 m apart than when they were separated by 30 m. Also, Meek and Robertson… observed more intrusions of Eastern Bluebird territories by Tree Swallows at territories containing two nesting boxes than at those with single boxes. In a study of nest guarding, Gowaty… found that both males and females remained closer to boxes during nest-building and egg-laying periods, when territories are most susceptible to usurpation and conspecific nest parasitism, but not during incubation or while nestlings were present.

Extrapolation from our observations also suggests that bluebirds may initially appear in greater numbers in areas with many rather than fewer nesting boxes. A test of this idea would vary the density of nesting boxes randomly over areas that currently have few or not bluebirds. They hypothesis would be supported if significant differences in the numbers of bluebirds initially nesting were found in areas with higher densities of nesting boxes. Finally, if bluebirds are more attracted to two-box sites than one-box sites, managers interested in attracting bluebirds might have greater success associated with potential territories containing two nesting boxes.



Minnesota Bluebird Recovery Program (BBRP) – Dorene Scriven

I hope you get good feed back on the pairing discussion. I think distance between boxes is important (as well as terrain, distance to water, etc.). For instance, I do not consider 50′ to 100′ as pairing. MBRP also recommends placing nest boxes 20 to 25 feet apart, 22 feet appears to work best. They also recommend to start pairing when tree swallows inhabit 50% of the boxes on your trail.

From Bluebird News, Vol. 9, No. 1: Pairing Revisited

Despite some very strong arguments based on statistics which Joe O’Halloran of Bluebird Restoration Association of Wisconsin (BRAW) has put forth in the Wisconsin newsletter, BBRP is still convinced that pairing of nest boxes can have real advantages where tree swallow populations tend to exceed bluebird populations. To further our position, we cite some of our own statistics from the 1996 reports; the research done by BBRP Research grantee Jason Smith sent in late October; and a communiqué from Professor Raleigh Robertson, Biology Department, Queen’s University of Ontario, Canada.

The study of Jason Smith, on the spacing preferences at Nest-Sites between Bluebirds and Tree Swallows in Bemidji, was influenced by the very late spring in which tree swallows arrived before bluebirds. he had nest boxes spaced 7 yards, 15 yards, and 30 yards apart. Initially, tree swallows selected boxes spaced at 15 yard, then 30 yards, and then a couple chose 7 yard spacing, though in this spacing actually chose 15-yard between pairs. The later-arriving bluebirds nested in the 7-yard spacing area. From his study, Jason concluded that tree swallows probably prefer 15-yard spacing and that a 7-yard spacing would be successful between bluebirds and tree swallows, and keep tree swallows from taking over a trail or area.

Professor Raleigh Robertson and his graduate students focused mainly on tree swallow spacing, but by nature also included eastern bluebirds. The nest boxes, arranged in grids, have been studied by Professor Robertson and his students for many years, and he was kind enough to send us many of his published papers. “There are many factors. . .in analyzing the effects of “pairing,” such as habitat, distance between boxes in a pair, overall nest box density in an area, etc. It is a fairly complex issue, which must be examined with a careful experimental approach, including appropriate controls. “For expressing nesting success, it seems, as some of the BBRP information points out, that you need to consider a measure other than simply number fledged per box. For a solitary species like eastern bluebirds, once you reach habitat saturation, increasing the number of boxes will not increase your number of pairs of bluebirds. So, if you add boxes beyond the habitat saturation point, you will effectively lower your success, as measured by fledgling per box. A better measure would be fledglings per unit area, or perhaps [by length] of trail.”

“For tree swallows, which are quite colonial, at a certain spatial scale, the habitat saturation point is much higher. So by adding boxes, you will keep attracting tree swallows, to a much higher level. While this may be detrimental to bluebirds, I doubt it.”

Dr. Robertson’s former students, Linda Whittingham and Peter Dunn, are establishing a tree swallow research program in Wisconsin. We hope they will share their findings and enlighten us further on pairing.

418 of the 478 reports BBRP received last fail, indicated whether nest boxes were paired. Results are interesting, if not necessarily significant:

Of the 323 trails with paired boxes:

  • 136 had more bluebird occupied boxes than tree swallows
  • 139 had more tree swallow occupied boxes than bluebirds
  • 48 had equal number of occupied boxes
    Of 95 reports on unpaired trails:
  • 53 had more bluebird occupied boxes than tree swallows
  • 32 had more tree swallow occupied boxes than bluebirds
  • 10 had equal number of occupied boxes
    As Professor Robertson states, there are many parameters, including maximum saturation levels for each species, which make pairing a complex issue.



Project Bluebird – Harold & June Cox

We do not have Tree Swallows in our area but we will read with interest your results. We hope, over the coming years, to work in areas beyond Missouri. Our [bluebird] work is moving along very well and we are doing it virtually full-time. Thanks for your writings and support. Wish we could make your conference. Maybe 1998.



Art Aylesworth, “Bluebird Man of Ronan”

I received your letter today regarding paired bluebird boxes. For the last 12 years I’ve had 7 miles of nest boxes involving 70 boxes. These are all along a large river bottom that has a big population of swallows. They are along a railroad track on railroad telephone poles with very low wires which create excellent perching habitat for both bluebirds and swallows as far as the nest box is concerned. Swallows eliminated the bluebirds almost entirely through this area until I paired the boxes. Now we have swallows and bluebirds both in probably 50% of the paired boxes. Six of the paired boxes are 10 feet apart and at that distance there have been two times that the swallows have claimed both nest boxes. The balance of the boxes are back to back on the telephone poles under that telephone line. I have never had two pairs of bluebirds and I’ve never had two nests of tree swallows in any of those boxes that are back to back. One year I had a Mountain Bluebird use one nest box and a Western Bluebird build a nest and laid one egg in the opposite box but then abandoned it, so I question whether even that difference in species will overcome the territorial experience.

In addition to this, I have probably 30 people with smaller nest box trails using paired boxes and it works beautifully. There is no question in my mind but what nest box placement of 20 to 50 feet apart on a nest box trail will eventually be solid tree swallow nesting. This is so because as the number of swallows increase, they simply gang up on the male bluebird until they drive him away and then the female is gone also. I encourage my people to place their nest boxes in open country not more than 5 to the mile. With this box placement, the bluebirds will pretty well occupy all the boxes. In some cases we have 250 boxes without a swallow but this is on open range land where there are no trees for at least a half a mile. As soon as we get into some open pine forest area, the boxes can be placed closer together but again, probably not closer than 250 yards. The nest box trail on which I have the most nest boxes is 13 miles long with 95 boxes. However, some of these are on little spurs back away from the main trail.



“New Hampshire Bluebird Conspiracy” – Bruce Burdett

If you mount your Bluebird houses in pairs about 10 to 15 feet from each other, you will find that Tree Swallow competition will be much reduced, even eliminated. Neither species will nest that close to its own kind, and each seems capable of living side by side with the other without fighting. Houses mounted singly will almost always be taken by the swallows, which are more numerous and aggressive. Remember, too, that Bluebirds are territorial, and will not normally nest closer than about 400 feet from other Bluebirds. . .

With paired houses, Bluebirds and Tree Swallows work hand in hand to fight off intruders in the vicinity of their nests. . . .

Let me say briefly that without pairing Tree Swallows would take 95% of my houses. This was my experience before I started pairing 4 years ago. Furthermore, of the 5% that Bluebirds managed to occupy, most were harassed by the swallows until they abandoned their nest and eggs. It was a no-win situation. Perhaps my experience is typical of New Hampshire but not of Wisconsin, Iowa, or Nebraska. In most areas of New Hampshire, Vermont, and Maine that I have heard about, the swallows out number the Bluebirds by a wide margin. Many letters to me confirm this. With paired houses the two species live side by side without strife. Our Bluebird population density in N.E. is still quite thin (sparse) though it is improving steadily.



Robert E. Orthwein

I do not consider anything over 7 yards to be a properly paired box site. Actually, when I change a location or add a box, I am now using a 5-yard spacing. On rare occasions, I have had Tree Swallows nest side by side 7 yards apart. This has occurred when a late-nesting Swallow pair moved in next to an early-nesting Swallow pair with young.

Over the years, on a few rare occasions, I have had Bluebirds build nests in both paired boxes but lay eggs in only one. I also had Titmice do this on one occasion. One Bluebird did lay 3 white eggs in one box and two white eggs in the other box 7 yards away. The three hatched and fledged. In all cases the boxes were identical. I have painted some of my boxes an earth-tone tan and left the others green. No double nesting has occurred at these sites but it is so rare, it doesn’t matter much. I suspect that these confused double nesters were first year, first time birds.

Paired and triple-box sites do reduce Bluebird production per box and this should be expected. In 1996, one of my best trails of 10 boxes on a 140-acre farm produced 39 Bluebirds and 6 Tree Swallows. This trail was in ideal, low-grass, almost wren-free habitat. Locations were 200 to 250 yards apart and not saturated. There were no ponds or streams to attract swallows. All locations were 40 to 45 yards in the open. Three locations had 5-foot to 8-foot perching posts near the boxes. There were no Bluebird or Swallow casualties due to House Sparrows and no Wren damage. A perfect Bluebird trail.

The 10 boxes were at 4 locations – a 2-Gilbertson box location, a 3-Gilbertson box location, a 3-raised roof box location, and a 2-conventional box location. Bluebird production per box was 3.9, but 9.75 per location. Production per location and House Sparrow protection is all that I am interested in.

My paired and tripled boxes have not produced hoards of Tree Swallows. In the past 17 years I fledged 2,212 Bluebirds and 690 Tree Swallows. Properly monitored paired boxes provide some protection from House Sparrow attacks and triple boxes provide a lot of protection.



Bluebird Restoration Association of Wisconsin, Inc. (BRAW) – Tom Whalley

I’m enclosing our last newsletter which has some of our data analysis for the last year. Up until this point, I have paired most of our boxes but the data right now is bringing out some questions. We have a virgin trail where we are still doing a pairing study with Olsen & Peterson boxes. This data is not consistent with the generalizations that Joe O’Halloran had. I’m sure Joe O’Halloran would be glad to share our data.

From Wisconsin Bluebird, Winter 1996, Vol. 11, No. 4


Pairing – Good or Bad for Bluebird Production? Bad!

by Joe O’Halloran, Chair, BRAW Data Analysis Committee

Congratulations to all BRAW monitors, whether you monitored a paired-box trail, or singles-box trail. Over 90% of you took part in the 1996 study by simply specifying whether the boxes you reported were singles or paired. The massive study is a huge success.

We are still getting more late-filed monitor reports at the time of this writing. However, Carol McDaniel and I wanted to shared the preview of the 1996 results with you, for three reasons. First, because the results are of vital importance to bluebirders. Secondly, because the pattern seen now with the database at over 4,600 boxes (about 82% of the /size of last year) is the same pattern found at 75%, 60%, and at 40%. Thirdly, by getting this news out now, it may help you in “Helping Bluebirds Through Wise Management,” as the BRAW motto says, while you are getting ready for the spring 1997 bluebird season opening.

If a box was within 100 feet of another box, this study counted it as “paired.” Hence, the monitors reported all their paired boxes, be they paired within 10, 20, 30, 40, 50 feet, etc. and up to 100 feet from another box, as in the “paired-box” category. The conclusions are not statistics-based speculations or extrapolations, and they don’t required esoteric hypotheses. This study reports actual bird production results, for singles-boxes and for paired-boxes.

With box pairing, the bluebird fledged per box numbers dropped to about half, and the tree swallow fledged per box increased by about a fourth, when compared to the singles results. Hence, in paired situations, tree swallow production, per box site, more than doubled, generally speaking.

The drop in bluebird production with pairing should be emphasized? Another impact of paring for bluebirders, shown by this BRAW study, is that box pairing shifted the bluebird/tree production balance downward, in favor of tree swallow production. Moreover, the 1996 BRAW data clearly suggests that in your area of Wisconsin, if you did not produce an excess of tree swallows using singles, you probably would produce an excess of tree swallows using paired boxes. The BRAW data also clearly suggests that in your area of Wisconsin, if you started out by producing an excess of tree swallows using singles, your situation would have gotten even worse by pairing, and much worse is you use Hill Lake boxes.

The production numbers appear to me to be saying that, as far as the bluebirds are concerned, the second box in the box pair causes the box site to be supersaturated with boxes. But, as far as the tree swallows are concerned, the second box does not cause the box site to be supersaturated with boxes.

BRAW members and monitors should be proud that in the BRAW paired-box study, monitors reported the actual numbers of bluebirds and tree swallows fledged, and hence the actual production of bluebirds and tree swallows was determined. To our knowledge, no other state or provincial bluebird organization does that, not even the fine large organization in Minnesota.

DISCUSSION – Some pairing partisans, with no data that we know of to support it, have expressed “common sense” positions that if they don’t pair, they don’t keep a box “open” for bluebirds, etc. There seems to be no end of the hypotheses in favor of pairing in the popular literature, but none of it has any data to support it, to the best of our knowledge.

Quite to the contrary, thanks to the 1996 BRAW monitors, we now have the actual bluebird and tree swallow production numbers for both singles and paired boxes from the entire State of Wisconsin. And these actual bird production numbers clearly showed (a) that pairing did not enhance bluebird production, and in fact dropped it, and (b) that pairing caused tree swallow production to overrun bluebird production.

And, the production results say to me that if more bluebird production is wanted, single box trail management is the choice that will do it, and that box pairing is the choice that will work against it.


Here in NEBRASKA, I personally have had great success with paired boxes. I submitted the following article to NABS which appeared in Sialia (Spring 1997, Vol. 19, No. 2.)


To Pair or Not to Pair

To pair, or not to pair, that is the question. Whether ’tis wiser on the trail to suffer the slings and arrows of swallow overpopulation, or to take arms against a sea of swallows and by pairing, end them.

Do I have a computer full of figures, statistics, and data analysis? No. Am I an expert statistician? No. Did I pass my basic high school math courses? Barely. What I want to share with you is four years of trail experiences and observations of tree swallows with paired boxes on Wachiska Audubon’s Bluebird trails in southeast Nebraska. Please keep in mind that this is not, by any means, any type of research project nor do I consider myself to be an “expert” bluebirder.

These trails were originally put up in 1986-1987 by Nebraska Game & Parks Commission on wildlife areas in the Salt Valley recreational areas around Lincoln, Nebraska. Because of a lack of staff by Game & Parks, Wachiska Audubon (the local Audubon chapter) took over the monitoring of these trails.

I became chairperson of Wachiska’s Bluebird Recovery Committee in 1993 and was in charge of their trails through the 1995 season. Sanford Downs took over my position in 1996, but I have remained actively involved with the trails.

The original boxes on these trails were all NABS boxes mounted on fence posts 3 to 4 feet from the ground. After taking over the trails, reviewing their past records, and seeing the decline in the bluebird numbers in recent years, it became obvious that a major trail renovation was in order.

In early March of 1993, the renovation began. Most of the old NABS boxes were in poor condition and had to be replaced. The majority of the boxes removed were replaced with Peterson boxes with some being replaced with Gilbertson PVC boxes. The Peterson boxes were mounted on 1-inch pipe and the PVC boxes were mounted on a 1/2-inch conduit placed over 1/2″ rebar. Many of the original boxes on the trails were already paired; but after the renovation, all boxes on the trails were paired, approximately 15 to 20 feet apart. Some of the box locations were relocated because of an overgrowth of cedars and/or sumacs.

Before the 1993 trail renovation began, there were 278 boxes on 10 trails. After three years of work and expansion, there are now 355 boxes on 14 trails. Wachiska Audubon is very lucky to have a good number of volunteer trail monitors. There is a standard form filled out and mailed to the chairperson after each weekly trail walk. I set up a record system where each box has a separate sheet with that box’s data recorded on a weekly basis. This became a tremendous educational experience for me, as it allowed me to follow the progression of each box on a weekly basis without having to actually walk each trail myself. (The weekly report form also provides a space for the trail monitor to indicate any problems or “unusual activity” happening in or around a specific box.) I do walk some of the trails on a regular basis and occasionally substitute on some of the rest, so I do manage to stay very familiar with these trails. The 52 boxes on my own trail are also paired (with a high occupancy by tree swallows).

As the chart below shows, there was a dramatic increase in both bluebirds and tree swallows from 1993 through 1996. Unfortunately, there is no reliable information on tree swallows fledged prior to 1993.

The Tree Swallows had a 109% increase, compared to the 68% increase in Bluebirds in that same four-year period. The largest yearly jump for both birds was 1993-1994, with a 42% increase in bluebirds, compared to a 54% increase in Tree Swallows.

I believe that if your trail consists of single boxes and your tree swallow numbers increase to where they are in direct competition with bluebirds for a box, the only solution is to pair the boxes on your trail. Referring back to Sialia (Vol. 13, No. 1), Richard Tuttle’s article on tree swallows and paired boxes talks about how the bluebirds actually benefit from having tree swallows in the neighboring paired box. His theory is that tree swallows will defend the territory around both of the boxes, protecting them from usurpation from House Wrens and House Sparrows. When I am walking a bluebird trail and find a pair of boxes occupied by tree swallows and bluebirds, I feel good knowing that there are two pair of birds defending the nesting territory.

In the past four years, I know of only three times that tree swallows nested in both boxes, and in one instance, one of the tree swallow nests had been abandoned. In order to discourage tree swallows from nesting in both boxes, I believe that 25 feet should be the maximum distance between paired boxes. On the other hand, I would keep a minimum distance of 15 feet between paired boxes because of the natural competition between the two species.

I believe there are numerous benefits to pairing boxes. But there are certain circumstances when box pairing won’t eliminate conflicts and competition between these two birds. When the bluebirds’ first nesting attempt is delayed due to weather conditions, the bluebirds’ and tree swallows’ first nesting attempts coincide. If the the bluebird hasn’t laid claim to a nesting box before the tree swallows arrive, there may be direct competition for a particular box. Nest failure by one or both birds can also affect the compatibility of these two birds, as well as nest usurpation by both species.

As pointed out earlier, what I have shared here are simply four years of observations from established bluebird trails in Nebraska. For more detailed information on the compatibility of the bluebird and the tree swallow, I would refer you to two excellent reports written by Richard Tuttle which are found in Volume 9, No. 1 and Volume 13, No. 1 of Sialia.

Remember, also, that we should be helping other cavity-nesting birds such as the tree swallow. They are always a pleasure and add an extra dimension to walking a bluebird trail.

Number of Bluebirds & Tree Swallows Fledged from Wachiska Audubon’s Bluebird Trails


Year 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996
Total bluebirds fledged 234 270 194 199 131 160 148 225 321 309 379
Total tree swallows fledged Not Available Not Available Not Available Not Available Not Available Not Available Not Available 317 491 627 665

Originally printed as a supplement in the Spring 1997 Bluebirds Across Nebraska Newsletter BANner