A Tale of Two Boxes: When Pairing Doesn’t Promote Peace

Photo © Holly Faulkner

by Mark Stanback, Professor of Biology, Davidson College

Watching cavity-nesting birds build nests and care for young in our nest boxes is deeply satisfying. However, sometimes nature can be a bit red in tooth and claw. In particular, many nest box landlords can be disheartened or even upset when multiple species compete for a nest box: it’s not always a pretty sight. Sometimes one can observe chases and even violence. But often nest site competition is so subtle it’s practically invisible (to us). If a dominant species is simply able to monopolize the area around a nest box, subordinate species may not even attempt nesting. This is a natural part of the avian life cycle, and one that humans often have little ability to control.

Tree Swallows On The Watch
Tree Swallows On The Watch

Adult Tree Swallows will compete with Eastern Bluebirds for nest boxes.

This doesn’t mean that there is nothing we can do as nest box monitors to reduce nest site competition. Perhaps the simplest way to reduce such competition is to provide multiple nest boxes. Indeed, fans of Eastern Bluebirds in the upper Midwest long ago began to provide paired nest boxes to ensure that bluebirds were able to coexist with the more aggressive Tree Swallows. By installing boxes in pairs (close enough that both boxes are unlikely to fill with Tree Swallows), nest box monitors can ensure that their beloved bluebirds have somewhere to nest.

But what about cases where the bluebird, aided by its bigger body size, is the dominant species? In eastern North America, a standard bluebird box is likely to attract a variety of potential residents, many of them smaller than bluebirds. Body size is often a fairly reliable predictor of interspecific dominance, so it seems likely that larger species (Tree Swallows, bluebirds) might be able to monopolize boxes that smaller species could potentially use. How then, can one assist smaller birds like chickadees and Brown-headed Nuthatches if they are continually facing competition from larger birds like bluebirds? Could paired boxes ensure the coexistence of chickadees and bluebirds the same way that paired boxes allow bluebirds and Tree Swallows to nest peacefully side by side?

On the Tolerance of Bluebirds

My students and I at Davidson College in North Carolina decided to test this. We installed identical nest boxes in pairs 10 meters (~33 feet) apart. Because Eastern Bluebirds are territorial, there was no way that two pairs of bluebirds would nest side-by-side. But would bluebirds allow smaller species, specifically Carolina Chickadees and Brown-headed Nuthatches, to use the box that they were not using themselves? Of course, the absence of a second species nesting beside a bluebird could be due to any number of factors. Consequently, we had two treatments that would allow us to narrow down what was going on. Some of the pairs consisted of two identical nest boxes (both with 1.5-inch entrance holes) while other pairs consisted of one box with a 1.5-inch entrance hole (“Big”) and the other with a 1-inch entrance hole (“Small”). The larger holes could accommodate bluebirds, chickadees, and nuthatches, but the smaller holes effectively excluded bluebirds.

Eastern Bluebirds Scoping Out A Nest Box
Eastern Bluebirds Scoping Out A Nest Box

Eastern Bluebirds can nest in boxes with holes as small as 1 1/2 inches in diameter.

If Eastern Bluebirds are intolerant of subordinate cavity-nesters breeding nearby (the Intolerant Bluebird scenario), we would expect to find similar (and low) occupancy by nuthatches and chickadees in box pairs of either type that contained bluebirds. If, however, bluebirds simply defend their own nest cavity (the Tolerant Bluebird scenario), we would expect to find similar (and high) occupancy by nuthatches and chickadees in box pairs of either type that contained bluebirds. Finally, if bluebirds guard all usable nest boxes in a small area (the Greedy Bluebird scenario), we would expect to find high occupation of “Big/Small” box pairs by nuthatches and chickadees and low occupation by these species of “Big/Big” box pairs.

Black-capped Chickadee Nest Box
Black-capped Chickadee Nest Box

Black-capped Chickadees can nest in boxes with entrance holes as small as 1 1/8 inches in diameter.

Bluebirds Exclude Their Small Neighbors

Combining two years of data, we had a total of 121 “Big/Big” pairs with a successful bluebird nest. We had 100 “Big/Small” box pairs with a successful bluebird nest. Of the 121 Big/Big pairs, 74% contained only a bluebird nest while 26% contained both a bluebird nest and the successful nest of a chickadee or nuthatch. Of the 100 Big/Small pairs, 18% contained only a bluebird nest while 82% contained both a bluebird nest and the successful nest of a chickadee or nuthatch. The difference between these distributions was highly significant. In other words, chickadee and nuthatch nests were significantly more common in Big/Small pairs than in Big/Big pairs. This demonstrates that Eastern Bluebirds tolerate smaller cavity-nesters adjacent to their own nest ONLY if the cavity being used by the smaller species is unusable by the bluebirds. When two identical bluebird-friendly boxes are placed near one another, the bluebirds apparently attempt to monopolize both boxes, even though they could only nest in one.

Under the Tolerant Bluebird scenario, bluebirds would be expected to defend only their own cavity and allow other species to use the other cavity—regardless of the hole size of the other box. This is not what we observed. We found high rates of multispecies occupancy only when the “other” box was unusable by bluebirds. This result also allowed us to refute the Intolerant Bluebird scenario, in which bluebirds would attempt to exclude all other cavity-nesters from nesting nearby—regardless of the size of the hole on the other box. Our results instead support the Greedy Bluebird scenario, in which bluebirds attempt to monopolize all the boxes that they could potentially use themselves. Smaller cavity-nesters were generally excluded from larger-hole boxes, but readily nested in boxes with 1-inch holes.

Big/Big vs. Big/Small Pairing
Big/Big vs. Big/Small Pairing

Bluebirds monopolize paired boxes when both entrance holes are large enough to accommodate bluebirds (left), but allow smaller species to nest in paired boxes that have smaller entrance holes (right). Click to enlarge.

Helping the “Little Guys”

Eastern Bluebird numbers have increased dramatically in eastern North America over the last half-century. Although the reasons for this are many, there is little doubt that nest box programs have facilitated this increase. While there may be truth in the saying “one size fits all” (a 1.5-inch hole does indeed accommodate most North American secondary cavity-nesters), we must also realize that competition among birds can prevent less-competitive species from making use of otherwise adequate (and available) nest boxes. Although pairing identical boxes appears to allow bluebirds and Tree Swallows to coexist, our results demonstrate that we cannot generalize this technique to smaller birds. Meaning, pairing identical “bluebird boxes” is not an effective strategy for promoting the coexistence of bluebirds and smaller cavity-nesters such as the Brown-headed Nuthatch—a species threatened by both habitat destruction and climate change. In the case of competition between Eastern Bluebirds, Carolina Chickadees, and Brown-headed Nuthatches, the best way to ensure that these smaller species have access to a nest box is to provide boxes that are “bluebird-proof” (i.e., equipped with a small entrance hole). Eastern Bluebirds undoubtedly represent a conservation success story, but it is time for us bluebird enthusiasts to think beyond this popular species and provide for the “little guys” as well.


Reference:

  • Stanback, M. T., E. Niemasik, D. Millican, and P. McGovern. 2019. Pairing nest boxes does not promote coexistence of Eastern Bluebirds (Sialia sialis) and subordinate cavity-nesters. Wilson Journal of Ornithology 131: 422-427. https://doi.org/10.1676/18-93

27 Comments

  1. Fred Dietrich says:

    For 30+ years I have had 2 small entrance boxes and 1 Bluebird size entrance box about 30’ apart with the small ones on either side. Chickadees and Brown-headed Nuthatches nest without fuss, starting a few weeks before the bluebirds, and then the bluebirds nest.

    Sometimes the nuthatches nest back to back and the bluebirds nest 3 times. Never had problems with nest selection once the small birds realized the big box was off limits to them.

  2. gail herman says:

    Unfortunately, the researchers didn’t look at house wrens, a very aggressive box nester, in my experience. Chickadees have no chance against them. But chickadees are very plentiful, are they not?

    • Holly Faulkner, Project Assistant says:

      Hi Gail, If you’re having issues with House Wrens, adding more boxes is still helpful. House Wrens like scrubby edge habitat, while chickadee prefer more forested habitat. Try placing more boxes in wren habitat to help reduce competition for the boxes.

  3. Thanks for an interesting article. It definitely squares with what I have seen over the years installing thousands of nest boxes. A similar example: Here in North Central Washington, sometimes wrens will monopolize bluebird boxes spaced out at the recommended distance. But by putting bluebird boxes “too close” to each other, wrens will not tolerate another Wren close by but they will allow bluebirds or swallows. In other words we are using proximate placement to encourage species diversity because some birds seem more territorial with their own kind compared to different species. Anyway I think there’s a lot to be learned with this subtle approach to encouraging higher-density nesting and species diversity.

  4. Dan Getman says:

    With regards to pairing boxes so that tree swallows and eastern bluebirds would readily co-exist, what do you find is the optimal distance to separate the 2 boxes, both with 1.5 inch holes?

    • Holly Faulkner, Project Assistant says:

      Hi Dan, I’m not sure of any research that’s determined an optimal distance, as it likely depends on the species and habitat, but you can place boxes in pairs on poles 15 to 25 feet apart or even back to back on a single pole. Birds have been known to nest in boxes in both situations.

  5. Janet Kirklen says:

    Great article. We have 8 bluebird size boxes around our property. We used to get a mix of chickadees and bluebirds. In recent years it has only been bluebirds. I wondered if perhaps the bluebird had established a “claim” on the boxes. Looks like putting up some small hold boxes would be a good move. Thanks for your research.

  6. Richard Poznysz says:

    I was a cranberry farmer for 35 years. Think 50 acres of open fields with scores of boxes set up and well tended by neighbors who are birders. I never observed close habitation between bluebirds and tree swallows. Invariably, two boxes with any proximity would both be populated by tree swallows, with the earlier arriving bluebirds driven from their nested boxes as soon as the later swallows arrived and took over. In one 50 acre area, I saw only one apparently very tough bluebird successfully fledging young in a fifteen year period. This was in an area of about 20 boxes. All the other bluebirds were driven away from the boxes by swallows.

  7. Van D Kleiner says:

    For several years, we have had one bluebird house and two small entrance boxes. We had bluebirds every year until 2019 and sporadically had Carolina chickadees and Bewick wrens in the small entry houses.

  8. Bridget says:

    This finding is very helpful. I wonder how this might be practically applied to managing other dominant/subordinate bird situations. I live in a wooded area, so there are no Bluebirds around and the “bully” birds in my neighborhood tend to be House Wrens and House Sparrows. I’m not sure about the minimum hole size for House Sparrows, but the House Wrens would likely fit in 1″ diameter holes, right? Not sure then how to “save space” for Chickadees, who in this case are the subordinate species. Otherwise, one strategy that I have found helpful in providing for multiple bird species is placing some nest boxes very close to or on my house itself (which attracts the more human-friendly birds) and others situated away from the house (which attracts the more human-shy birds). Generally, the House Wrens, Chickadees and House Sparrows compete for the boxes situated close to or on the house. Nuthatches will use the nest boxes farthest away from the house. At first, House Sparrows were more likely to choose the house attic vents and other parts of the house’s structure over the nest boxes I had made available, but as the original nesting pair’s extended family grew over the years, the expanding flock started using the nest boxes. It’s possible that eventually the House Sparrows will take over and both the House Wrens and Chickadees might be shut out.

    • Robyn Bailey says:

      Hi Bridget,

      If you use an entrance hole that is 1 1/8″, that will allow the chickadees and wrens to nest but exclude the House Sparrows. It would unfortunately also exclude the nuthatches and titmice, but they don’t use my boxes anyway.
      I also live in a wooded area and mostly get chickadees and wrens. I have found that the House Wrens prefer the boxes furthest from the house, while the chickadees will nest in the boxes near/on the house/garage. So you may be right about “saving space” for the chickadees by providing small boxes on or near your home and some further away. And by making the entrance hole smaller, you can exclude the sparrows from the competition altogether.

  9. Savannagal says:

    I haven’t had bluebirds in any of my bluebird boxes for several years. One is always taken by European Finches/Sparrows. I remove the nesting material every week, but it always comes back. Another is always occupied by wrens. Once they are in, I leave them alone regardless of whether there are eggs yet. There have always eventually been eggs in the box. The third box is way out on my property, and sadly I don’t often get over to monitor it. I suspect it will have sparrows as well. I wish I could rid of them. Back in the day when I did get bluebirds, I’d often find that European sparrows would destroy the bluebird eggs or kill the chicks. I’ve even had adult bluebirds killed.

  10. Bridget says:

    Thanks Robyn Bailey for the suggestion. I’ll keep 1 1/8″ in mind with the next nest box I place and see what happens 🙂

  11. Bridget says:

    Robyn Bailey, just another thought: From observing the two or three pairs of (White-breasted) Nuthatches that nested on my property, I got the impression they are “edge” nesters. One pair nested in a box mounted on a large tree at the edge of the lot our house is built on, at the border between our backyard and our wooded lot. Another pair nested in a natural tree cavity in a tree along the road at the other end of our wooded lot. Just FYI if you wanted to experiment with nest box placement.

  12. Robert Veach says:

    I made a EBB nestbox out of eastern cedar with a 1 1/2″ entrance hole but then I put on a large piece of bark from a fallen tree over the hole – the bark has a 1 1/4″ hole in it. I have a picture of an EBB flying by the entrance looking in but the box was eventually nested by a (black crested titmouse or maybe carolina chickadee?) Eggs were white with reddish brown spots – but did not see the nesting bird. The EBB did nest in another nest box about 200′ away and somewhat out of the way due to trees and cedar. I would upload the pics of the EBB and the eggs but I do not know how to upload at this blog.

  13. Chaz says:

    I’ve heard that the 4 inch bluebird boxes are detrimental and kill off the swallows, yet assume those are the box sizes you refer to for swallow nesting. Are you seeing healthy swallows coming from the blue bird houses, or is it better to use 5×5 houses in case swallows live there?

  14. John says:

    Hello. Our problem is sparrows. Has anyone discovered a strategy favoring bluebirds over sparrows, besides evicting the sparrows every single morning/evening? Thank you.

  15. Tim says:

    Nothing was mentioned about the Baudry bluebird box that was popular in the Green Bay Wisconsin area about thirty years ago. It was unique in that it had an open top which eliminated competing birds and many parasites. The opening was further protected by wire mesh over the top opening. My Baudry bluebird boxes were successful. It was my understanding that the success of these houses was ignored by EBB associations for unknown reasons.

  16. Christy Horton says:

    What besides cats eat your birds? I have fed and kept birdhouses of all kinds for ten years at this home and all of a sudden I am finding injured birds and feathers all over my yard. I don’t see cats, so I wondered what could be getting my birds?😡😡😡😡

    • Holly Faulkner, Project Assistant says:

      Hi Christy, Many mammal species and larger birds eat small songbirds. Hawks, owls, raccoons, rodents, weasels, and many others could be possible culprits, however they generally eat their prey, unlike cats. And, cats can be very sneaky and avoid being seen. However, if you’re finding these birds near windows, it’s very possible they are striking the glass. If so, try putting up vinyl clings, paper, cardboard or netting to help break up the reflection .

  17. Scott says:

    One thing that I’ve learned and witnessed over the years is that each individual bird has its own personality. We had one eastern bluebird defend every single box on the property to the extreme. As soon as a chickadee or tree swallow landed on one of the boxes intended for them well away from the bluebird house he would immediately fly over and chase them off even though he had his own box. He eventually died and the next male bluebird couldn’t care less about the other boxes.

    We also had a chickadee nest in a wren box with a 1 inch hole. Originally the wrens weren’t around but eventually migrated up when the chickadee was still nesting. I was expecting the house wren to destroy the nest and eggs but they never touched it. Later on they ended up nesting in the box after the chickadees fledged.

    I’ve also tried pairing bluebird boxes with tree swallow boxes and the tree swallows were totally dominated. A lot of people claim the tree swallows will dominate the bluebirds but that was not the case with our birds.

    Just goes to show you every bird is different some are much more aggressive than others at defending territories.

  18. Tim Bray says:

    Savannagal, and others who have trouble with House Sparrows:
    Experimentation has shown that a slot opening, or a two-hole box, will allow the Bluebirds to exit when attacked by House Sparrows, and then turn the tables on them by re-entering and attacking the Sparrow. A two-hole box is the same as a regular bluebird nest box, but you drill two holes in one side. A slot opening is easier, just leave a 1-3/16″ slot at the top.

  19. Richard Vedder says:

    Wrens drove out the chickadees and it seems most other birds also. We used to have both purple and yellow finches at our feeder and since we put up the wren box we never see them anymore.

  20. Forrest Hudspeth says:

    I have 7 houses, 2 with hole restricters for Wrens and Chickadees. My success with Bluebirds had always been depending controlling the House Sparrows. The Bluebirds and Tree Swallows usually have few spats in the Spring but settle on a house and peacefully co-exit for the season. House Wrens are an occasional problem but usually stick the smaller hole houses.
    I clean my houses in the Fall and start checking them long about January onward for House Sparrows. They don’t make their typical chirping at this time of the year but start early “camping out” in their house of choice. I make a visit around 9PM with an inspection mirror and an 8 inch long piece of wood closet bar. If I discover a sparrow, I plug the hole with the wood and remove a dead male sparrow 2 days later.
    In the Spring, if the Bluebirds have at least one house uncontested, I let the sparrows work on the others. I check the sparrows every few days and remove any eggs. If I rip out the nests, I’ve had the sparrows attack the Bluebirds in their house, destroying eggs, killing young and adult alike. This method ties up some houses but leads to zero population growth of sparrows and keeps them preoccupied. Last resort – a very accurate pellet gun.

  21. Harper price says:

    I have had never had bluebirds. iv’e been trying for years. all i get is wrens.

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