There is a shortage of natural nesting sites, which is thought to be one of the major factors behind the decline of some of our once most-common birds. By putting up nest boxes, such as this, which replicates a bird's natural nesting preferences, you can encourage them to breed near your home
32mm long
http://www.birdfood.co.uk/ctrl/product:332;/birch_log_32mm_nest_box#.Uvz6X9qPO5c