The completed bird box (minus roof, of course!) is based on the dimensions given on the bird box plan page and RSPB guidelines.
Last year this nestbox was fitted with a miniature surveillance camera and was home to 11 blue tit chicks. In spite of a cold, wet spring that saw heavy losses of blue tit chicks in Britain, all 11 chicks survived, thanks to being supplied with a constant source of live mealworms from the mealworm feeder. It became pretty crowded in this box, particularly when mum and dad brought the food.
The two lighter coloured pieces of wood only serve to support the false Perspex ceiling. This was installed to prevent the birds from pecking at the camera electronics and wiring that was installed between the ceiling and the wooden roof.
This year I've installing a small batten part way up the inside of the front wall. It's a long way for the blue tit chicks to scramble from the nest to the opening, and they do like to see what's out in that new world before making that vital first flight!
The Royal Society for the Protection of Birds and the British Trust for Ornithology both recommend all year round feeding. In the spring the parent birds are so busy finding live food for their chicks that they need good quality food and drink that you can easily provide via a selection of bird feeders. See winter feeding and spring and summer feeding
Installing a small camera gives you a truly fascinating insight into the life of such small birds. If you have the time to observe them, it's a project that's definitely worth the effort. Try to get the nest box in place as early as possible in the New Year. If you leave it too late it may not be occupied the first year. House sparrows are in rapid decline in many parts of the country, and most new houses have no suitable nesting cavities. Sparrows are gregarious creatures and often prefer communal nest boxes. See the sparrow terrace project!