If you want to add a bird nesting box to your garden, there are many questions about how to create one.
A nest box will need to be big enough for birds to get in and out of, in addition to being big enough for them to lay and incubate eggs. But how do you know if your DIY design will meet these criteria?
Building a bird box is a great project to engage in, and as long as you are willing to try, you should find it fairly easy (even if you are not a great DIY’er).
There are few things you have to think about such as the size of the entry hole and the dimensions of the box. You can mess this up if you aren’t careful and won’t have any birds getting in at all!
What Size Should a Nest Box Be?
The size of the nest box will depend somewhat on the bird species you are trying to attract.
If you want to attract larger birds, like pigeons or corvids, you will be using a larger box.
The following text assumes you will be trying to attract small birds, like blue tits and great tits.
It may be Useful to look up sizes of boxes for a variety of different birds. However, the main body of the nest box can approximately stay the same size for most small birds (including robins, tits, nuthatches, sparrows, etc.).
But you will need to make the hole in the front bigger or smaller depending on the bird species that you want to attract. Later, I will talk more about those specific sizes for you.
The size of a nest box will also depend on the number of chicks they typically have.
From a general perspective boxes should be approximately:
- 225 mm high
- 180 mm deep
- 180 mm wide
This will give even small birds’ reasonable enough space to build a nest in, and rear young, without issues around overcrowding.
If you are building a nest box for larger birds, like barn owls, of course the box needs to be a lot larger to ensure that it can fit comfortably in the box.
Something like 670 mm high by 570 mm wide by 500 mm deep. And the hole should be approximately 400 mm high.
Of course, the larger the bird, the larger the box will need to be, and the chicks will also be larger.
There is no one size fits all bird nest box on the Market, each box will drastically differ depending on the local species.
Don’t just go for a standard size, take the time to study the target species, their natural nest sites, and what they will always require.
You may find the below information useful, although it does not cover all birds:
- Wren: 200 high by 200 wide by 180 deep.
- Barn Owl: 770 high by 220 wide by 410 deep.
- Robin and Spotted Flycatcher: 200 high by 200 wide by 180 deep.
- Tree Sparrow: 210 high by 150 wide by 150 deep.
- Blackbird: 180 high by 300 wide by 150 deep.
- Jackdaw: 410 high by 200 wide by 200 deep.
- Nuthatch: 230 high by 190 wide by 190 deep.
- Kestrel: 340 high by 300 wide by 520 deep.
You want to be using plywood certainly, and it should be at least 15 mm thick, because that stops the box from bending.
Plywood, or solid wood, also will give, some measure of insulation from either heat or cold, thereby providing some safety from extreme weather for the chicks. It should help your bird box to last well for year after year, without cracking or rotting.
The nest box size is not something you can afford to ignore; it is essential!
How Big Should The Hole at The Front of a Nest Box Be?
When it comes to your nest box, one of the most important considerations is the hole.
Different kinds of birds need different hole sizes, and most birds want a hole they just fit through.
This prevents bad things from following them into the box, as well as making them more comfortable for feeling safe using it as a nesting box for their vulnerable chicks.
So what size should you use?
You should consider the generic size for small nesting birds, which would be a 32 mm hole, if you are not sure what kind of bird(s) you are going to try and attract.
You can also use a standard 32 mm drill bit to complete this and it should fit most of your garden birds, depending on the type of nest box they might be looking to utilize.
Although, if you want be sure your nest box is designed for a specific kind of bird to use, consider the following:
- Robins prefer an open-fronted box, which is camouflaged in foliage. An open front will work much better than a hole. You can install a piece of wood that crosses the box front to make it secure, so the eggs remain in place; however, leave the upper half of the front open.
- Blue tits, on the other hand, prefer holes around 25 mm. This is likely the smallest hole used in a nest box. It allows the birds to slip in comfortably, but no predators can fit inside the hole.
- Nuthatches will use a box with a hole around 28 mm. They are still on the smaller side and prefer holes that are not too large; however, they need slightly more space than the blue tit.
- Both the great tit and house sparrow would use a box with a hole around 32 mm, since they are larger bodied birds and need a little more access into the cavity of the box.
If you’re looking to draw house martins in, think about attaching a cup-shaped nest under the eaves of your home rather a simple box.
Finally: If you plan to provide a nest for starlings then you will need a bigger box that has a hole of 45 mm to allow them inside.
How High Up Should The Hole Be?
The height of the entrance hole is also crucial – if you make the height too low you will most likely find that birds will not use the box.
The reason for this is that predators will be able to reach in and grab chicks if the hole to the bottom of the box is not suitable.
The hole needs to be at least 125 mm up from the base of the nest box.
This means that predators like cats will not be able to reach in and insects the bird chicks, and the parent birds will feel secure in the box.
You don’t want the hole to be lower than this or you risk the chicks getting injured.
If you can, you should doubly roughen the wood on the underside of the hole to give the chicks even more grip when they want to climb out of the nest box.
Do not go over 125 mm too much as the little birds could have trouble climbing out.
Some birds such as the blackbird and pied flycatcher do need less depth:
- Blackbirds do well with about 70 mm from the base to the hole
- while the pied flycatcher does better with 90 mm.
Overall however, a deeper gap is just safer for the chick. If it does not seem deep enough, be especially careful to put the box out of the reach of predator cats.
What Else Should I Think About When Building a Nest Box?
It is a good idea to put in some drainage holes in the nest box.
This will allow dampness to run out, and if they are on the bottom, even in the worst of weather, the holes will not make the box too cold as they will not have wind rushing through the holes.
Boxes without holes can quickly become humid with mold, both dangerous for birds.
You should try not to treat the wood, but if it is in a very wet place, consider the leather cover or a mini roof to keep water from coming into contact with the wood.
This will help keep the nestlings snug and dry inside, but it will also help the nest box last longer.
It’s a good idea:
- To have a hinged door that allows you to open the nest box.
- From time to time, there may be dead chicks or old nest debris that is necessary to remove from the nest.
While the parents will try to clean it up, if an older chick does die, they may not have the ability to remove it.
In the Wild: It would be eaten, but in a fenced box, it will lay there until you remove it; this is important to be able to do.
Conclusion
Be sure the bird box is spacious enough to fit the desired bird species and have the entry hole sized for that species.
This will help the birds feel secure and more inclined to use the box year after year!
At the end of each season, always check on your box to make sure it is still secure and safe to use the following year to continue to attract birds.
Replace worn string or broken wood, and take a box down if it’s unsafe, even if you cannot replace it immediately.