A container, usually made of wood or sometimes plastic, placed high on trees to host a bird nest inside.
Usually among the first things that people who are starting to practice the humble art of woodworking plan of doing, because the construction doesn't require much skill, and occupants usually don't complain. Much.
Also, in Finland, the oldest-generation Apple Macintoshes were called bird nesting boxes, because of the great resemblance. (See MacQuarium.)