Birds
Nesting shelves: http://www.wild-bird-watching.com/wbw-nesting-shelf.html Birds that can be attracted to nesting shelves are: American Robin, Barn Swallow, Eastern Phoebe, and Mourning Doves.
Except for Purple Martins, a bird house should be for a single nest. Unless your decision is just to decorate your yard or garden, stay away from nesting boxes built for multiple birds, (duplexes, triplexes).
http://www.wild-bird-watching.com/Bird_Houses.html
Proportions for the birds you want: http://www.wild-bird-watching.com/Building_Bird_Houses.html
Except for Purple Martins, a bird house should be for a single nest. Unless your decision is just to decorate your yard or garden, stay away from nesting boxes built for multiple birds, (duplexes, triplexes).
http://www.wild-bird-watching.com/Bird_Houses.html
Proportions for the birds you want: http://www.wild-bird-watching.com/Building_Bird_Houses.html