Bluebird Nest Box Management

Nest Box Construction and Other Best Practices

“Thanks to the concerted efforts of people building nest box trails all across the United States, bluebirds are making a comeback today.”

Bonnie Tull

Eastern Bluebirds are migratory in the north, but here in Central Texas we have a resident population. They are insect and berry eaters and will come to suet feeders. They prefer mostly insects in the summer and will eat more berries in the winter. Their favorite habitat involves open fields or meadows with bordering trees, where they may find shelter and nest sites. Unfortunately, bluebirds have to compete for natural nest holes with invasive species (such as European Starlings and English House Sparrows) as well as with some native species (such as woodpeckers and other cavity nesting natives).

Bluebirds used to use old woodpecker holes in fence posts until the fence posts started disappearing. Thus the program to construct nest box trails to improve the habitat options for bluebirds was born.

A nest box trail consists of a number of boxes spaced at least 100 yards apart in bluebird habitat. Obtaining landowner permission to set up a nest box trail is usually not difficult, as most people are eager to participate—especially if you are doing the work. Community sites like cemeteries, churches, fire stations, and golf courses present opportunities with the necessary permissions. It’s important to establish access to the property for monitoring. I usually try to site the boxes near the edge of properties, so that I don’t have to enter far into them to monitor. Permission is usually enhanced by being able to present Master Naturalist credentials and assuring the landowner that having nest boxes on their property does not threaten government oversight.

Bluebirds will begin checking out nest sites for their first brood of the season in January. So the ideal time to ready existing nest boxes and to replace old ones that are in disrepair is late fall through December. It’s also an excellent time to install a new bluebird nest box on your own property or to launch a new nest box management project.

Plans for bluebird nest boxes are available on websites for the North American Bluebird Society (NABS), Texas Bluebird Society, and Cornell NestWatch with all the detailed information you could ever want.

You can also find my own NABS-approved plans made from PVC fence posts here along with detailed instructions. All of my boxes are specially designed for Texas’ brutal summer heat by having added insulation and shade.

Here are some best practice tips for nest box construction and nest box management.

Nest Box Construction

The first thing to consider when constructing an Eastern Bluebird nest box is the size of the entrance hole. It must be 1½ inches in diameter if circular and 2¼ inches by 1 3/8 inches if oval. The reason for these dimensions is to prevent predators and larger species like European Starlings from entering. What I have done is to leave a half-inch gap across the top of the box and cut the entrance hole one inch out of the gap. I believe the less obvious the hole is, the better. Cutting the entrance hole close to the top of the box also conserves building materials. It is important to smooth the edges of the entrance hole with a file in order to prevent excessive wear damage to adult bird wings. There is some speculation that oval holes deter house sparrows, but I do not find that to be the case. However, I do have evidence that sparrows do not like small floor diameters (4 inches) or round PVC pipe boxes.

Optimal floor space for bluebirds is about five inches in diameter, either square or round, but they can use a smaller space, which might be more effective in deterring the larger species. The bottom of the entrance hole should be at least 6-7 inches from the floor to deter certain predators. It is also important to allow for drain holes in the floor of the box. This is most easily done by cutting off the corners of a square or rectangle (as shown in the photo below). Alternatively, you can drill holes with at least a 1/4-inch drill bit.

In PVC boxes, I glue wood ladders with notches or grooves cut into them inside the front of the box below the opening in order to allow nestlings to climb up to the opening and have a place to perch before fledging. It is helpful to do the same inside wood boxes or at least to cut grooves into the inside front.
And one of the most important features of a good nest box is ease of opening for observation and cleaning. If a box cannot be opened or is difficult to open, monitoring and clean out is challenging. I prefer a design that opens both from the top and the front or back. Top openings make for very easy monitoring. Front, side, or back openings make clean out easy.

With the extra heat of Texas in mind, I’ve designed all my boxes with a lot of ventilation. I think the best design leaves half-inch gaps in the top of the back and front.

There is a large overhang on the roof of the PVC boxes as well as the tin roofs of the wooden boxes. These overhangs provide protection from both the sun and inclement weather. The wood lasts longer when protected from UV damage too. You can build your own, or you can modify store bought boxes. It’s best that any perch material does not protrude much from the box, as that can attract bird predators. If a larger bird, such as a jay, tries to predate a nest, you can quickly staple a modified Noel guard in the shape of a tube made of hardware cloth around the entrance hole.

Wooden boxes also benefit from hole protectors of metal (which can be purchased) or PVC to prevent damage by woodpeckers. As long as they have baffles, woodpeckers are the only species capable of damaging the box.

I highly recommend that you install stovepipe baffles on all of your boxes, and plans are available online. In addition, I mount a 20-inch square of hardware cloth above the baffle to try to deter snakes.

Nest Box Management

Bluebirds in more northern climes will typically hatch up to three broods in one season, sometimes fledging as many as 15 birds from one set of parents. In Texas, I find only marginal success with third broods. Usually, the first brood will consist of 5 eggs laid in March over a 5-day period. A second brood in late April or May will typically be 4 eggs, and if there is a third try, it will usually be only 2 to 4 eggs. Many of the eggs in my Texas experience do not hatch, and I can only speculate as to the reasons.

Bluebirds are known to “egg dump” if a female’s nest has been destroyed, or for other reasons. One year one of my boxes had over 20 eggs laid in layers of nests, none of which hatched. That one was a real puzzle. There had been one successful brood from that box that season before the multiple egg dumping occurred.

The nests themselves are easy to recognize, being composed almost entirely of grasses in a neat cup—not to be confused with English House Sparrow nests, which have messy grasses and trash forming a loose hood over the nest itself.

Bluebird eggs are— surprise—pale blue! Though it’s not out of the ordinary to have an individual who lays white eggs.

If you would like to become a nest box monitor, it’s easy! Just go onto the NestWatch web site and sign up for a permit or certificate after a short study. After mounting the boxes, all you have to do is check them twice a week or so during the season. Ideally, you should keep records and report your findings to the NestWatch site.

The NestWatch program involves registering all of your box locations and names using GPS coordinates—much easier than it sounds. They have forms to use for data collection. I make index cards for field data to carry around in a special pouch with a telescoping mirror allowing easier and quicker nest checking, but you can also download the NestWatch App and record your observations in the field.

All this information is explained on the Cornell NestWatch website. It is a citizen scientist program. Of course, you can also monitor other native birds that might choose to use one of the bluebird nest boxes, such as Ash Throated Flycatchers, Titmice, Chickadees, Wrens, and House Finches. However, please remove House Sparrow nests. This requires close attention. House sparrows will kill both adult and baby bluebirds in the box—very tragic. I now provide bolt holes on many of my “at risk” boxes, hoping that an adult bluebird will be able to escape a sparrow attack.

Hays County Master Naturalist Bob Adkins checking nest boxes on the Dripping Springs Ranch Park bluebird trail.

Nest boxes are used year round by bluebirds for roosting, often by multiple birds at the same time, which is a good reason to insulate them in winter.

During the nesting season, which runs a full six months for Texas bluebirds, the boxes should be cleaned out soon after each brood fledges. If the box has been successful for the first brood, the females will usually begin building a second nest within a week of fledging the first brood. It is best to have her build in an empty box for several reasons. Deleterious insects can be removed, and the new nest will not be built too high up in the box, where predators may more easily reach it.

I do find that here in the hot Texas summers, the birds tend to use less nesting material for subsequent broods, saving themselves energy. I’ve also observed that in extremely hot weather the female tends to incubate less. Since incubation temperature cannot exceed 107 degrees F without lethal results, I conclude that our hot weather almost incubates the eggs by itself. The high heat danger may also come into play when the eggs hatch and the nestlings may become too hot and dehydrated as they are squished together. However, that is pure conjecture on my part. A small telescoping and rotating mirror commonly available in hardware stores is a very good tool for monitoring, and of course a cell phone picture can document very well. I also carry a small brush for clean-outs.

Eastern Bluebirds typically take 12-14 days to incubate their eggs and hatch the nestlings, and 17-21 days to fledge after hatching. They will often try three broods, beginning in January or February and extending through July if the weather is not too hot. The female will lay one egg a day, usually in the morning, until she has a full clutch of usually 5 eggs (but can be 3-7). She will begin incubating as soon as she lays the last egg. Typically, the eggs will all hatch on the same day, and the nestlings will all fledge on the same day.

I love Bluebirds and find this a very satisfying independent project. If you have any questions, I’d love to help answer them, and I do encourage you to try attracting these charming birds—even if it is only in your own backyard.

Quick Reference Guide

Bluebird Nest Box Specifications

Round entrance hole openings: 1.5 inches in diameter with edges well smoothed
Oval entrance hole openings: 1.375 x 2.25 inches with edges well smoothed
Bolt holes in the side opposite the entrance hole, allowing escape from predators
Entrance hole height above floor: at least 7 inches
Floor area: at least 4x4 inches
Height of box above ground: at least 5 feet
Box opening: easily opens without tools for both monitoring and cleaning
Roofs: at least a 3-inch overhang for sun and weather protection, ½-inch ventilation space at the top front and back or sides
Sun shades can be made from a variety of materials. Corrugated plastic works well either screwed or glued on, with spacers of at least 1/2 inch. Scrap tin with spacers is very durable and helps prolong the life of wood.
Floor: Cut corners off for drainage and floor ventilation.

Tools for Monitoring

Unscented bar soap for soaping the ceiling and top sides against wasps
Small brush for cleaning
Small telescoping mirror, or use cell phone
Small notebook, index cards, or nesting app in your portable device
Pen or pencil for recording

Data Collection and Reporting

- Certify on the NestWatch site for permit to monitor.
- Use the Cornell NestWatch form or digital app.
- Make up your own list to include: date, state of nest (unfinished, complete, or used and flattened), whether or not adults are present, how many eggs if present, how many hatchlings, nestlings, or fledglings are present and if dead or alive; any evidence of parasitism or predation.
- If you want to be a helpful citizen scientist, open an account on Nestwatch and report your findings.
- Do not open nest boxes after nestlings are about 12 days old, as there is a danger that they could fledge prematurely.

Predator Control

- Do not mount nest boxes on fences or trees.
- Mount them at least five feet away from fences and overhanging branches.
- Always use a predator guard on the post. Stovepipe or squirrel baffles are probably the most effective. 20-inch squares of ¼-inch hardware cloth on top of the stovepipe baffles may provide extra protection. The baffle needs to move freely. Larger hardware cloth wired together is more effective. If fire ants are in the vicinity, grease the pole under the baffle with a non-toxic lubricant, or dust the inside and bottom outside of the box with diatomaceous earth.

Cleaning

Clean by brushing or scraping the interior of the box after each brood fledges, and wash the box with water and Clorox and let dry thoroughly at the end of the season. If very cold weather is predicted, winterize the box by stuffing the openings (all except entrance holes) with something like foam for roosting protection.

House Sparrows

Become familiar with nest and egg identification. Remove sparrow nests and/or eggs as necessary. Try not to let any sparrow eggs hatch.

Bluebird Websites

Texas Bluebird Society https://texasbluebirdsociety.org/
Cornell Lab of Ornithology https://nestwatch.org/
Sialis https://sialis.org/
North American Bluebird Society nabluebirdsociety.org

Birding Apps

iBird Pro or Ultimate
Sibleys
eBird (free from Cornell)
Merlin (free from Cornell)
Cornell NestWatch Field app from Apple App Store or Google Play