Always use corrosion resistant screws and hardware. Drill countersunk pilot holes in primary work pieces (first piece the screw is inserted into). Drill regular pilot holes in secondary work pieces.
You shouldn’t have to pay for any wood if you keep your eyes peeled. Watch for wood scrap piles at fence and home construction sites. These are good sources for the small sizes usually needed for bird houses.
For increased ventilation in warmer climates, floor and side panel corner gaps can be larger than plans specify.
Some woodworkers may prefer to drill strategically located holes for ventilation and leave floor panels whole.
You may prefer Side Opening Doors instead of hinged roof doors, especially for the larger nest boxes.