Birdhouses make lovely additions to a yard or garden. They can be aesthetically pleasing and, depending on the type and placement of the birdhouse, can attract a variety of different birds. While the primary consideration when putting up a birdhouse is the birds, there are a few other organisms you should consider, too.
The first is the tree itself. It's important to consider just how you're mounting or hanging the birdhouse and the potential harm that certain methods could cause to the tree.
The second organisms, well, those are actually multiple organisms: Predatory animals like cats, raccoons, snakes and squirrels that would love nothing more than to sneak into the birdhouse and grab a quick bite or turn the home into their own.
Consider the bird and the tree
The usual inclination when attaching a birdhouse to a tree is a nail or a screw. That's how we attach most things to wood surfaces, after all. Not every problem needs a hammer, however, or a nail for that matter. In fact, that inclination can potentially do real harm to the tree.
As Mickey Merritt of the Texas Forest Service explained to the Houston Chronicle in 2007, nails and screws that penetrate the outer bark can damage the cambium, the area just underneath the bark. This space is were cells rapidly divide and help the tree grow. Other parts of the tree — including the pholem, the tissue of the tree the transports the sugars produced photosynthesis, and the xylem, the tissue system responsible for transporting water from the roots to other areas of the tree — can also be harmed by nails or screws. In addition to the physical harm they can do, nails and screws also create openings for insects and diseases to sneak in.
Some trees are able to recover from these puncture wounds. A chemical reaction goes into motion when a tree is penetrated that essentially seals off the rest of the tree from the wounded area, preventing any disease and decay from spreading. New wounds keep triggering this process, however, and according to Merritt, it may only take 10 holes, depending on their location, to kill a tree.
So now that nails are a no-go, affixing a birdhouse to a tree requires a little more work than getting a nail at just the right height. Arborist Now recommends any kind of flexible, flat nylon webbing. A fabric fastener, like Velcro, glued to the sides of the birdhouse and to the outward facing straps will allow you to stick the birdhouse to the tree without harming it. You'll need to check the tree's growth periodically to make sure you're not girdling the tree. Nylon straps that also have fasteners and buckles can help with this task because they're easily adjustable.
SFGate has a more detailed way of hanging a birdhouse on a tree, one that sounds a bit more secure than a fabric fastener and glue. You'll need eye screws or hooks, rubber hosing for wires and bungee cords, along with some some precise measuring to make sure everything is exactly how you want it.
Remember that in all cases you need to think about the type of bird you want to attract. Different birds have different height requirements, and some birds are very territorial, so setting up too many bird houses may cause fights. Some will want the house to swing, while others might find this instability a deal breaker when choosing a home. Installing birdhouses in a way that doesn't injure the tree will also allow you to easily move birdhouses to different locations and heights without needing to make more holes.
Consider the predators
Birds, given their many years of evolution, are pretty good at building their nests away from predators. A birdhouse made and mounted by a human, however, may not be given the same kind of thought.
Now, if you're really serious about not hurting a tree with a birdhouse but you still want a birdhouse, simply don't put it on the tree. Trees afford predators plenty of opportunities to get to the birdhouse. Keeping a tree-mounted birdhouse safe requires pruning branches away from the birdhouse, according to The Spruce. Planting prickly bushes at the base of the tree to deter anything from climbing up the trunk will also help keep predators at bay.
If you want a birdhouse that's safe from predators, consider placing your birdhouse in other locations. Cranmer Earth Design offers a few suggestions:
1. Metal pole. It doesn't get much more difficult than a metal pole when it comes to climbing. You add a baffle, and climb-happy predators should be thwarted, especially if they have nothing nearby from which to jump atop the baffle.
2. Slippery building facade. OK, so maybe it does get more difficult than a metal pole. However, while you can often easily mount a birdhouse on a pole, mounting on a slippery facade will be difficult. Plus, you'll need to consider the color of the building, along with which direction the building is facing so as to avoid absorbing or facing too much heat from the sun.
3. Brick buildings. Brick isn't that easy to climb, and unlike trees, drilling into brick isn't going to hurt anything. As with the building facade, avoid sides of the building that get a lot of sunlight. Bricks, after all, soak up heat, and birds want a birdhouse, not a hot house.
4. Wood siding. If you want that tree feeling without the tree, wood siding is another way to go. It's not easy to scale up, and unlike a number of other surfaces, it doesn't get any hotter than a tree would, making it a good choice. Of course, wood siding means a house more often than not, and you may not be crazy about a birdhouse that close to your own house, especially if you want to observe the birds.