Here's a link to my favourite wildlife stuff retailer, I'm not suggesting you buy their stuff (they are a bit pricey) but there are little cartoons of where and what kind of box you can put up in a smallish garden.

Living with Birds: Buy Nest Boxes, Garden Packs Wildlife Shop

I have to admit to having various of the woodcrete boxes, including the double house martin boxes, they're pretty good.

__________________
Manda.

"Wouldn't it be nice
For maybe an hour
To not have a care."