If you are in any online bluebird groups, then you probably have been seeing a lot of people posting about two-holed bluebird nest boxes. One of the big features marketed about this nest box is that the second hole enables the bluebird to escape if a house sparrow attacks.
That alone, might have you really thinking about getting one of these nest boxes. But is it worth the money? Does it really work? Besides sparrow escape, are there any other benefits? Are there any disadvantages? IS THIS JUST A FAD?
Because house sparrows pin birds down while attacking in a nest box, two-holed bluebird nest boxes don’t always allow escape. A second hole may make it easier for predators to invade. But there are other advantages to these boxes such as ventilation, light availability, and feeding efficiency.
This article is going to talk all about the 2-hole nest box so you can determine if it’s right for you.
What are Two-holed bluebird nest boxes?
The 2-hole nest box is a different type of bluebird nest box. Classically, the design features two holes on the front of the nest box, both the appropriate size for a bluebird. Conversely 1-hole nest boxes, as you would guess, only have one hole.
2-Hole nest boxes may provide escape routes, but escape can be limited…
One of the hypothesized benefits of the 2-hole nest box is that if one hole is blocked by a competing bird, the nesting bird inside still has a chance for escape. Now, this only works when the competing bird is entering through one hole. During entry, the nesting bird inside has a split second to get out of the nest box before its attacked.
Unfortunately, with house sparrows attacking bluebirds, tree swallows and chickadees, once the house sparrow is inside, it will pin the bird down and attack. If the bird is pinned down, it doesn’t matter how many holes are in the box.
Note: In the spring of 2024, there were 2 reports of bluebird fatalities resulting from house sparrow attacks inside a 2-hole nest box (reports in Facebook bluebird groups).
If you want to see just how these attacks go inside a nest box, this is a very graphic video of a Eurasian tree sparrow attacking a bluebird in a single-hole nest box. She still manages to escape through the hole. But you will see from the video that event his smaller non-native sparrow is capable of holding her down during the attack.
Beware – this video may be hard to watch.
Advantages to two-holed bluebird nest boxes
Feeding advantages of a 2-hole nest box
2-hole nest boxes offer two feeding advantages: first, both parents can feed babies simultaneously due to the 2 holes. Second, when a nestling is being a hole-hog (typical of tree swallows), parents can access the second hole to feed their other young.
If you’ve ever watched your bluebirds or chickadees or tree swallows feeding their young, the parents do this one at a time. The mother will go in, feed the nestlings, sometimes remove the fecal sac, meanwhile the father is waiting his turn. Once mom is out, he goes in for his turn.
2-hole nest boxes are not only built with 2 holes, but they’re usually larger. This combination means that parent birds don’t have to take turns when feeding their young. They can both enter through a hole and get right back out.
Along with the fact that both parents can feed simultaneously, sometimes a particular nestling will crowd the feeding hole. He or she will cling to the wall of the entry panel and poke its head outside, waiting for all the food. This is particularly true for tree swallows. In fact, some specialized tree swallow nest box variations have multiple feeding holes so other babies have a chance for food.
If you have historically gotten tree swallows, the 2-hole nest box, or a specialized tree swallow box might be an option to consider for this reason.
Potential advantage for cowbird situations (hypothesis only)
Though this is a hypothesis, another potential benefit is dealing with cowbirds. Cowbirds are parasitic nesting birds. They will lay an egg in another bird’s nest, and leave the other bird species to raise the young nestling. Unfortunately, cowbird nestlings aggressively compete with the natural young of the nest, sometimes pushing it out or hogging all the food.
Because cowbirds are native and protected by the Migratory Bird Treaty Act, they are federally protected. This means you cannot remove eggs, nestlings, feathers, etc.
Fortunately, some bluebirders have had success by keeping an abundant supply of live mealworms nearby so parent bluebirds could keep up with feeding the young. By having a 2-hole nest box, which already eases feeding, this may help protect the natural young inside the nest from being starved out by the adopted cowbird.
Additional light may be advantageous for preference
One of the purposes behind the Gilwood nest box was to address the amount of light coming into the nest box when a bluebird is looking inside. Once they poke their heads inside to look down, they block a lot of the natural light that would come in. The inverted mouse-hole design of the Gilwood box along with its larger size, allows more light to get in.
Because 2-hole nest boxes have that second hole, bluebirds won’t block out all the light coming in when they buy up one one to look inside. That second hole still lets more light in.
Steve Gilbertson (Gilwood box designer), found that after testing different designs and entry hole sizes, bluebirds seemed to like having more light available in the box.
The fact that 2-hole nest boxes let in extra light by the very nature of its design is not an advantage that is commonly discussed among bluebirders, but it still is an advantage, and maybe advantageous when trying to attract bluebirds.
2-Hole nest boxes may allow better ventilation on hot days
Many variations of the 2-hole nest box are referred to as the two-holed mansion, meaning it’s a bigger bluebird nest box. These designs usually call for better vent holes as well.
The combination of its bigger size and more ventilation means that these nest boxes may stay slightly cooler on very hot summer days than traditional nest boxes.
However, this has not been field tested. So we cannot say for certain if this is the case. What we do know is anecdotal evidence form some bluebirders reporting in find that larger nest boxes with better ventilation provide better nestling outcomes compared to smaller nest boxes.
Should I really be concerned about heat?
When temperatures hit 100 F or hotter, things get a little scary for bluebirders. At 100 F, things are go, but at about 107 F, eggs are cooked, and nestlings can die.
Some bluebird monitors put up umbrellas or Styrofoam heat shields to protect boxes.
And on some occasions, a box that is too hot can lead to premature fledging (when babies jump out of the nest too early).
Two-hole mansion bluebird nest boxes can fit nest box cameras
One of the most popular nest box cameras bluebirders use is the Blink Outdoor series cameras* from Amazon. They go on sale all the time, and are easy to install.
The trouble is, they are not suited for all nest boxes. You need a bigger nest box to support this camera. The Woodlink BB3 model is about the smallest nest box out there that fits the Blink camera on its native mount without conflicting with a Van Ert trap.
Fortunately, with a larger interior floor of at least 5×5 inches (often more) and a taller inside, two-holed bluebird nest boxes can support a blink camera on its native mount without stopping a Van Ert trap from snapping shut, or creating an obstacles when bluebirds try to enter.
2- Holes, 2- Problems: Disadvantages can challenges to two-holed Bluebird nest boxes
Something to keep in mind is that while two-holed bluebird nest boxes have been around for a while, they haven’t been around as long as other nest box variations, and as a result, they aren’t well studied. Many of the advantages and disadvantages stated are considered hypothetical, scientifically speaking. That is to say, these are educated guesses with some anecdotal evidence, but there hasn’t been repeated experimentation across a larger sample size to move speculation into theory. Remember the good old scientific method ;).
2-holed nest boxes are harder to find
Two-holed nest boxes are not commercially available just yet. Though, with their growing popularity, it’s a matter of time (though we will have to see if manufacturers will make the large versions that bluebirders really love).
You can find them on here*. Or you can find someone to make them for you – or make them yourself. That’s about it right now.
So accessibility is challenging at the moment.
Two-hole mansions cost a lot more
The cost for a two-hole mansion is a lot more compared to a traditional bluebird nest box. For a well- made cedar box, you’re looking at spending $100 or more.
Pine boxes will be less expensive, but not by much. They’ll still hold up pretty well. But after a good rain, the wood may expand quite a bit, making it harder to open the access panel for monitoring.
The larger size makes trapping house sparrows more challenging.
Any nest box that has a larger interior floor size (4.75×4.75 inches or more) is going to make trapping house sparrows using a Van Ert harder because a house sparrow (or Eurasian tree sparrow) can simply jump over the trap’s trigger.
A way around this is to make a false back. This can be made from cardboard or foam core or whatever you can use to improvise, and then wedged into the nest box so that it makes the distance from the front to back shorter, forcing a house sparrow to trigger the trap.
Another route to take is to modify the Van Ert trap by adding a little card board to the trap’s trigger, thereby extending it, causing the house sparrow to trip the trap.
You’ll have to plug the extra hole when….
While we’re going to discuss two times it’s important to cover the second hole, there may be other reasons that surface as people get more and more used to using these larger two-holed bluebird nest boxes.
- Winterizing: A lot of bluebirders leave their nest boxes up during the winter to help bluebirds or other cavity birds find shelter and roost. An important step is to plug up some of the vent holes and put some insulating material on the ground like wood chips or pine needles. With the two-holed nest boxes, you’ll want to find a way to plug up that extra hole. This isn’t very hard to do, but it is a necessary consideration.
- Catching house sparrows: Another time you’ll want to plug up the second hole is when trying to catch a house sparrow. It kind of goes without saying, but some people ask how to do it with these types of nest boxes since the extra hole would mean a house sparrow could escape. Another option is to modify the trap with cardboard on the door-covering flap that extends the flap so that when the trap triggers, the flap covers both holes. The only reason we don’t like this second option is because a house sparrow might be able to pick and pull at it in time before you can get out to dispose of it.
Two-holed bluebird nest boxes might make it easier for predators to gain access.
A raccoon can do a lot of damage sticking one hand in the nest box. He can also do a lot of damage sticking both hands in the nest box.
Just because there is a potential escape route by having a second hole, doesn’t mean that route is always open.
Now, if the second hole were on a different panel, this may offer a bird more time to get out. But the classic design of the two-holed mansion for bluebirds has both holes on the same front panel.
As of yet, we don’t have evidence one way or another of increased (or decreased) predation events in two-holed boxes. And it’s possible a mother bluebird incubating her eggs may sense danger, and be able to narrowly escape a predator by using the second hole. The upside is only the eggs were lost. But from an ecological standpoint, a breeding female bluebird still lives, and that’s very important.
So this disadvantage comes with mixed opinions until more testing is done.
2-hole bluebird nest boxes may expose nestlings to the elements more than traditional 1-hole boxes.
In 2018, a study was conducted, with the help of citizen scientists, to assess the outcomes of slot nest boxes vs. traditional round-holed nest boxes.
According to Cornell’s NestWatch summary of this study, overall outcomes in round-holed boxes were better than in slot nest boxes. In fact, full nest failures happened more in slot boxes than round-holed boxes.
I know, you’re wondering what this has to do with 2-holed bluebird nest boxes. Don’t worry. We’re getting there. Stick with us. We have to discuss the slot boxes because it may have relevance to the two-holed bluebird nest boxes.
Researchers hypothesize the reason for more nest failures happening in slot boxes was due to more exposure to the elements – vulnerability to storms, rains, wind and extreme weather.
How this applies to two-holed nest boxes? Well, remember 2-holes, 2 problems. more exposure is inherent to the design. Just like with a slot box, that extra opening may allow more exposure to the elements, which could have a negative affect on nestlings.
Remember though, this is all hypotheses at the moment – informed speculation based on what we currently know or what some bluebirders have reported in. Nothing has been field tested yet.
Verdict – Are 2-holed bluebird nest boxes worth it?
Two-holed bluebird nest boxes have a lot to offer, but come with more responsibility. But they have not been extensively field tested. And, the sparrow escape advantages is debatable. While they cost more, they offer an advantage for feeding.
So it boils down to personal preference and your own priorities. In our experience, if you’re managing the house sparrow threat actively, two holes may be too many.
Then again, if you’ve had a really bad mishap with a house sparrow, you might want to offer your bluebirds every advantage to escape, even if it’s a narrow advantage. In that case, this box might be just right for you.
It might also be useful when running a bluebird trail, where you can only do nest checks every few days. That potential for escape (no matter how small) may be critical for your trail.
So our verdict is this:
Every nest box style was designed with purposes and special advantages. But each nest box design also has limitations. There is no perfect nest box design out there. The two-holed bluebird nest box has some important features worth deep consideration, just like many nest box styles have. If the features fit in your list of priorities, then this is the right box for you.
Nest box styles we absolutely don’t recommend:
Nest Hollow emphasizes finding a nest box type that suites the needs of the space you’re placing this box. Peterson nest boxes have their place, just as Gilbertson nest boxes. But there are two nest box styles (not counting any decorative nest box) that we do not recommend:
Slot boxes – rather than a round-hole opening, slot boxes look like a mail slot. They are advertised to be house sparrow resistant, but luck on that is not significant enough. These boxes are shallow, difficult to add Noel guards to, and require you to modify Van Ert traps in order to catch house sparrows.
Plastic boxes (with the exception of the Gilbertson style nest box – bluebird nest boxes made of recycled plastic are advertised by major bird shops as well as on Amazon. But we don’t recommend these because bluebirds typically don’t prefer these, and because they cause nest boxes to overheat quickly, killing the babies inside.
References
Bailey, R. (2024a, October 8). Round versus slot entrances: New study says “go round.” NestWatch. https://nestwatch.org/blog/round-versus-slot-entrances-new-study-says-go-round/
Leroux, S. L., A. E. McKellar, N. J. Flood, M. J. Paetkau, J. M. Bailey, and M. W. Reudink. 2018. The influence of weather and parental provisioning on fledging success depends on nest box type in a cavity-nesting passerine, the Mountain Bluebird (Sialia currucoides). Wilson Journal of Ornithology 130(3):708-715. https://doi.org/10.1676/17-084.1
