Bread can be digested by birds though, most human-eating (with sugar and salt) bread has no nutritional value for birds.
If bird eats too much bread, bird will only feel full, but it will also lead to malnutrition and obesity. Some breads are healthier for humans and equally healthy for birds. Whole-grain and preservative-free bread are healthier for humans and birds.
Whole grain bread contains plenty of whole grains and seeds and is rich in carbohydrates, protein, unsaturated fats (good fats), vitamins, and minerals. Whole-grain bread is like whole-wheat bread with added grains and has a lower glycemic index.
Anyhow, bread should never be feeding regularly to birds.