by Dr. 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.
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.
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.
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.
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.
- 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