The best choice for a birdhouse in your garden would be one that is custom-built for a type of bird that naturally occurs in your area.
Birds that prefer birdhouses for nesting are those that naturally nest in cavities. For the most part, these birds will typically nest in holes naturally occurring in trees.
Birds will move into an optimally built house that protects them from rain, wind, and cold, therefore water- and weatherproofing is of the utmost importance.
Natural predators will also find it more difficult to get to the nest if it is in a well-constructed birdhouse. Such predators include ravens, crows, red squirrels, and weasels, depending on the geographical location.
Why Choose Wood
Wood is a natural material that decays easily and fairly quickly if left exposed to the elements. Therefore, it may seem like a better idea to use a different man-made material for your birdhouse to ensure longevity. Unfortunately, this is not the case.
Wood is the best choice for a birdhouse such as Cedar or Cypress, (mentioned earlier), as it provides optimal insulation for eggs and young and is familiar to the birds that will come looking for a home.
You can learn more about what type of wood is best for a birdhouse here.
Due to the nature of wood, weatherproofing is critical to ensure that your little shelter does not come tumbling down with the first rain.
Roughly hewn wooden slabs, plywood, pine, or tree sections are good choices but avoid creosote-treated wood and pressure-treated wood, as the treatment could harm the eggs or young.
Wood with a green-colored stain has been treated with a copper-based preservative will produce poisonous vapors when it is exposed to water, so you should avoid it at all costs.
To extend the life of your birdhouse, you should paint the exterior and seal it against potential water damage. It is always best to paint the exterior in dull, natural hues that blend in with the environment as this is least likely to attract predators (source).
Waterproofing Your Birdhouse
The best way to waterproof your wooden birdhouse is by making sure you treat all exposed wood, the roof, and limit water entry to the interior.
When constructing your birdhouse, it is a good idea to first use glue to glue the panels of your structure together before nailing or screwing them together. This will assist in keeping the finished product waterproof.
A slanted roof or pitched roof will allow water run-off, will last longer, and it will be easier to protect than a flat roof. The roof should also extend at least one to two inches past the sides of the birdhouse.
Once you have finished building your birdhouse, the final waterproofing can begin!
Equipment Needed:
Wood filler and sandpaper
A strip of metal or waterproofing tape
Raw linseed oil or a clear lacquer sealant
Optional: Lead-free and non-toxic paint
If you're building a birdhouse from scratch then you might want to check out this guide on the essential materials to make a birdhouse.
Steps To Follow:
Use wood filler to fill any small holes or depressions in the birdhouse.
Sand the wood filler down to make the exterior smooth and flat.
Paint only the exterior with raw linseed oil or clear lacquer sealant and allow it to dry.
Apply a strip of metal or waterproofing tape to the roof seam if it is pitched.
Optional: Paint your birdhouse in a dull, natural hue to blend in with the environment.
Affix your birdhouse to a sheltered spot, such as under the eaves of your roof.
You can use Linseed oil to seal your birdhouse as it is natural and non-toxic should birds peck at the wood and ingest it. However, it does take a long time to dry - up to a week in some areas!
So, if you're looking for a faster solution, you can also use a clear and non-toxic lacquer sealant of your choice.
It is important to ensure that you paint the exterior thoroughly and apply at least two to three coats to the roof itself as it will take the most punishment from the weather.
Once done, you should place your birdhouse in a space that isn't directly exposed to the sun and will be partially protected from rain, snow, drips, and wind as well - the eaves under your home roof will make a cozy spot (source).