To minimize the stress on trees (no screws, nails, or sharp wires), this is how I attach birdhouses to trees. Simple, inexpensive, and harmless. Let's get started.
Step 1: Materials
3/4" X 10' galvanized hanger strap ($1.98) - I'm only using about 6 inches of this 10' sheet of metal
8' lashing strap, 2 pack ($10.48)
Step 2: Make a Birdhouse
Of all the birdhouses I've ever made, this is my favorite. I got this wood from a guy who tears down old barns in Kansas. It had this magnificent natural hole, too small for squirrels, I hope. It's dreamy. The birds will be fighting over this one. Even the wood from the back of the birdhouse looks like it came from an old pirate ship, washed in from the ocean. Wait, no, we're in Kansas. Never mind.
Step 3: Attach Hanger Strap, Then Lashing Strap
The hanger strap comes with different sized holes, so just screw that baby in vertically on the back of the birdhouse.
Then, run the lashing strap through the hanger strap.
Step 4: Happy Tree, Happy Birds
Climb a ladder, and strap your birdhouse to the tree. The tree is unharmed and the birdhouse is secure. You may want to check your strap periodically to make sure it is good and tight.