Reducing Nest Competition in Birdhouse Nesting Birds

A female bluebird inspects the birdhouse being used by the house wren. Photo: Tom Murray.

We enjoy watching birds build nests and raising their young in our birdhouse aks nest boxes. Nature, however, can be harsh. Competition between cavity-nesting birds can result in physical fights, egg destruction, and the death of the young. 

Nest boxes are in high demand because nesting cavities are scarce. In addition, the populations of the more aggressive and tenacious cavity-nesting European starlings and house sparrows have exploded in numbers, adding competitive pressure to native species. 

All birds that use cavities for nesting, except for woodpeckers, are secondary users; this means they rely on holes dug by woodpeckers and other types of naturally occurring cavities. 

Nesting boxes for bluebirds and other native species can be beneficial. Some cavity nesters may not nest or nest in suboptimal cavities in some breeding seasons due to a lack of suitable nesting cavities. 

This article discusses ways to reduce competition between cavity-nesting birds, allowing bluebirds and other less conspicuous species to breed successfully.

Birdhouses are also known as nest boxes. Both terms are used interchangeably in this article.

How to reduce birdhouse competition among cavity-nesting birds?

In the context of this article, reducing birdhouse competition means encouraging cavity-nesting birds to breed and thus preventing them from fighting, squabbling over, and even killing each other over nest boxes.

Reduced competition for birdhouses leads to higher reproductive productivity in bird populations that have already been decimated.

The effectiveness or success rate of some of these techniques varies widely. Some nest-box landlords claim success while others mixed results. It is often a matter of trial and error. 

Use entrance hole excluders

Bluebird populations have experienced an impressive comeback, but chickadees, wrens, titmice, flycatchers, and nuthatches need a chance to recover as well. 

There is much literature on nest box conflicts between bluebirds, swallows, and house sparrows, but smaller and less conspicuous species also experience intense birdhouse competition.

The entrance holes to a birdhouse should be designed so that European starlings and house sparrows cannot access the nest box. While starlings can easily be excluded if entrance holes are less than 1 ½ inches, house sparrows can fit through holes intended for smaller native species.

Use the list below to design nest boxes with specific entrance hole sizes. The list is given in descending order using bird groups of similar body sizes. Groups of larger birds are on top of the list and are excluded or have difficulty squeezing through entrance holes of bird groups below.

SpeciesIdeal entrance hole size 
* Great Crested Flycatcher
* Ash-throated Flycatcher
* Mountain Bluebird
9/16  inch.
* Eastern Bluebird
* Western Bluebird
* Carolina Wren
* Bewick’s Wren
* (House Sparrow)
1 1/2 inch.
* White-breasted Nuthatch
* Tree Swallow
* Violet-Green Swallow
1 3/8 inch.
* Red-breasted Nuthatch
* Tufted Titmouse
* Prothonotary Warbler
1 1/4 inch.
* All Chickadees
* House Wren
1 1/8 inch. Some recommend 1 inch.

Combine entrance hole excluders with the birdhouse design 

Open-top box with wire netting. Some users cover the top hole using a piece of twin-wall polycarbonate screwed to the roof (insert).

House sparrows can fit through an entrance hole of 1 ½ inch, which is also the recommended size for eastern and western bluebirds, Carolina wrens, and Bewick’s wrens. 

Gilbertson PVC and open-top boxes seem to discourage house sparrows from nesting in them. House sparrows appear to be deterred by the thin walls of the PVC boxes.

Gilbertson PVC nest box.

Open-top birdhouses have proven to be very successful for some birdhouse landlords. Users claim that open-top boxes will keep house sparrows and house wrens away from bluebird nest boxes. 

It appears that house sparrows and house wrens must have an enclosed cavity to consider it suitable for nesting. On the other hand, bluebirds are more flexible in their nesting requirements.

Open top boxes resemble circular tree cavities with an open top that bluebirds regularly use for nesting. 

Install nest boxes at each species preferred habitat

Each cavity nester has a specific nesting habitat and prefers a location and height from the ground for their nests.

Birdhouses at the suitable nesting habitat ensure that target species find them easily.

Establish pairs or sets of nest boxes

Paired Boxes

Paired birdhouses intended for bluebirds and tree swallows. Photo: Virginia State Parks.

Set up two birdhouses at a recommended distance of 5-20 feet apart to reduce the risk of fights and nest takeovers between bluebirds and tree swallows. 

Bluebirds and tree swallows are very territorial and will not allow other pairs of the same species near their birdhouse, but they do not mind nesting close to one another.

In this way, bluebirds keep off other bluebirds from harassing the tree swallows, while tree swallows keep off other tree swallows from harassing bluebirds.

Both bluebird and tree swallows will team up to alert each other and fend off nest predators that they have in common.

Bluebirds and smaller cavity nesters

Stanback and colleagues (2019) installed two separated sets of nest boxes 30 feet apart. Nest boxes in the first set were all the same having an entrance hole of 1 ½ inch (ideal hole for bluebirds). 

The second set had a combination of birdhouses with entrance holes of 1 ½ inch and smaller entrance holes that excluded bluebirds.

Researchers found that in the first set of equal nest boxes nesting bluebirds defended several birdhouses around the one they were using for nesting. Bluebirds kept smaller species such as chickadees, titmice, and nuthatches from nesting in those birdhouses. 

Nesting bluebirds in the second set of birdhouses did not defend the ones whose entrance hole excluded them, therefore allowing chickadees, titmice, and nuthatches to nest in them.

Bluebirds perhaps perceived birdhouses with the right entrance size as having potential for future nesting whereas the smaller entrance-sized boxes were useless therefore were not defended.

The results of this study support the idea that combining birdhouses of different entrance hole sizes reduce competitive pressure, at least among bluebirds and smaller cavity-nesting birds.

Install your birdhouses with a target species in mind 

By trial and error, birdhouse landlords have found arrangements that reduce competition for nesting boxes in their yards. One of the more important findings is combining nest boxes with entrance hole sizes that mutually exclude competing species in your area (see table above).

Consider the temperament of the target species. For species that use the same or similar entrance hole sizes, install nesting boxes as far apart as possible where they are more likely to use them. For instance, install titmouse, chickadee, and Carolina wren boxes close to or even attached to part of your house where these birds do not mind nesting.

Install boxes for shy species such as white-breasted and red-breasted nuthatches at the far corners of your yard where they are more comfortable nesting.  

Carolina chickadee at the entrance hole.

Chickadees and other species. Experienced birdhouse landlords indicate that they can keep house wrens from taking over or attacking chickadee boxes by placing the wren boxes near the woods by the shrub line, away from the house. They install birdhouses for chickadees close to the house or even attached to a wall where house wrens are hesitant to approach.

House sparrows also like to nest close to houses and other man-made structures. However, the entrance hole size in nest boxes for chickadees and titmice excludes house sparrows from accessing them. 

Pay attention to birdhouse arrangements that work for you

Some birdhouse enthusiasts found that allowing house sparrows to nest in boxes close to the one used by bluebirds may help prevent interactions between them. 

Letting the house sparrows breed in a box without removing their nests kept them preoccupied in their nesting activities. When the house sparrow’s nest was removed from the box, the sparrows were more likely to search for another nest and attacked nesting bluebirds nesting in neighboring boxes. 

To keep house sparrows from breeding successfully, their eggs were addled by shaking them vigorously, puncturing them with a needle, oiling, or boiling them to ensure they never hatch. The sparrows continue incubating the eggs without adding new sparrows.

The success of birdhouse arrangements varies according to conditions and species involved in your neck of the woods.

Time the availability of your birdhouses

Ensure that your birdhouses are available when migratory cavity-nesting birds arrive or resident bluebirds and other native birds are about to begin nesting.

A study that used experimental sets of nesting boxes found that among tree swallows and bluebirds, the timing of claiming ownership of a nesting box was critical in keeping the nesting box for the season.

However, even after one species claimed ownership, the other wanted to usurp an occupied birdhouse, mainly when nest sites were scarce in the area.

To time the availability of your birdhouses, use each species’  nesting period and migration patterns to make your birdhouses available when migratory birds arrive.

Starlings and sparrows do not migrate and take over the best nesting boxes before migratory birds arrive, or native species are ready to breed.

Plug birdhouses. Some birdhouse landlords plug their nest boxes’ entrance holes until migratory birds arrive. By precluding access to birdhouses, starlings and house sparrows, that breed earlier than most native birds, are forced to find nest sites elsewhere.

Making your birdhouses available on time gives native birds a chance to find unoccupied birdhouses. 

A disadvantage of the timing technique is that resident bluebirds and native non-migratory cavity-nesting birds will not have access to plugged boxes either. You would need to find a date to unplug your boxes that benefit native birds to the maximum extent possible.

Birdhouses whose entrance holes exclude starlings and sparrows do not need to be plugged.

Manage the tenant in your birdhouses

Active management implies having a monitoring plan of your nest boxes.

Adopt an active management approach for birds using your nesting boxes.

House sparrows: House sparrows are not protected by law. Therefore their nests and content can be removed. Whether you remove them or manage them in place is a personal choice.

The approach to manage house sparrows varies considerably.

Some birdhouse enthusiasts allow house sparrows to take one box so that they do not harass other occupied birdhouses. As indicated above, if you remove the entire nest every time, house sparrows may try to usurp other occupied boxes.

Others use a more invasive method that consists of plugging the entrance hole with an incubating sparrow inside. Then, two to three days later, they remove the dead sparrow and nest content.

Open top and Gilbertson nest boxes (see above) work rather well to discourage house sparrows from nesting in them.

Some birdhouse landlords share that they use pellet guns to eliminate house sparrows.

House Wrens

House wrens are native species, and their nests are protected by law. Management measures adopted cannot include any direct harm and harassment of the birds.

House wrens are notorious for filling up alternate birdhouses other than the one they use for nesting. They also puncture and remove eggs from other nest boxes, remove very young nestlings, and kill well-feathered baby chickadees.

To manage house wren, some birdhouse landlords first identify alternate boxes that wrens have filled with nesting materials. Then, they remove part of the nesting material, leaving nearly half the amount inside. 

House wren at the entrance hole.

Leaving nesting material inside the box will not prompt the wrens to refill the nest and allow other birds to build their nest on top of the existing material in the box.

Alternatively, install chickadee and titmouse birdhouses near your house or attached to a wall. If there is enough open space between the shrub line around your yard and the birdhouse, house wrens may be kept from attacking those boxes. House wrens are generally reluctant to fly across open spaces. 

If there is not enough open space between the shrub line and your house adopt a different approach.

Instead of having your chickadee boxes near your house, install them inside the woods, approximately 60 feet between the forest edge (shrub line) and the chickadee box. House wrens do not generally like foraging or nesting inside the forest and stick to the shrubby edges.

Avoid attracting house sparrows.

Attracting house sparrows to your yard will make it more likely for them to find your nest boxes. 

Keeping sparrows from your feeders is nearly impossible, but there are ways to discourage them from coming back. The trouble is that the food you offer your favorite birds is the same as what house sparrows like to eat.

House sparrows and starlings are ground feeders and favor platform feeders. Instead, use a tube feeder with small ports and short (5/8 of an inch) perches where they can not feed comfortably. 

House sparrows appear no to like safflower and thistle, which may also discourage them from staying in your yard.

Some bird enthusiasts trap them and transport them a long distance for release. Others euthanize them. Yet, others don’t mind the sparrows and use some of the techniques listed in this article to control and coexist with them. 

References:

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.

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