A deficiency means that the bird does not get enough of a particular nutrient. All animals can suffer from deficiency problems but in birds a deficiency very quickly shows signs of:
If the feed contains too little protein (animal or plant) the birds will become weak and may develop infections. They do not grow well and meat production is badly affected. Egg laying decreases or stops.
Lack of calcium in the feed can cause:
If birds lay eggs that have either no shell or a soft shell these eggs will be broken and can be eaten by other birds. If this happens the chicken can develop a habit of eating eggs which then becomes a problem.
To prevent these problems birds must be given plenty of minerals as powdered shell or bone. If birds are allowed to scratch for grain they will take in minerals they need from the soil in the run. A good source of minerals is to feed crushed egg shells to the birds.
If vitamins are not present in the feed then:
These problems can be prevented by adding commercially bought vitamins to the feed or providing the birds with green vegetables in addition to the feed. Vitamin deficiencies can cause birds to start feather picking which becomes a problem.
Source :Pashu Sakhi Handbook