From The Royal Society For the Protection of Birds
http://www.rspb.org.uk
All types of bread are acceptable to birds, but ideally only if it is just one component in a varied diet. Bread does not contain the necessary protein and fat birds need from their diet, and so it can act as an empty filler. A bird that is on a diet of predominantly or only bread can suffer from serious vitamin deficiencies or starve.
Only put out an amount that birds will eat in a day. Bread left on the ground overnight can attract rats. Soaked bread is more easily ingested than stale dry bread, and brown bread is better than white. During the breeding season, crumble the bread into tiny pieces so that it is only eaten by the adult birds, and never put out an amount that will last more than one hour. Dry chunks of bread will choke baby birds, and a chick on a diet of bread may not develop into a healthy fledgling.
Crumbled bread is suitable in small quantities, but moisten if very dry. Brown is better than white. Although bread is not harmful, it should not be offered in large quantities, since its nutritional value is relatively low.