When working with wildlife to help support animals and birds in a habitat which is perhaps recovering, if supplementary feeds are used then they can 'skew' the balance of nutrients in the diet and cause a problem. However important the additional feed is, it is important to give thought to the total diet and how it may change.
In the case of red squirrels some of the projects where supplementary feeding was necessary noted an increase in soft bones (nutritional osteodystrophy) due to low calcium levels. Use of Nutrobal in squirrel supplementary feeds was important to help prevent this.
More recently on the recommendation of Jersey Zoo we helped the project on Denis Island in the Seychelles (unfortunately at long range!) in their work which involves using supplementary feeding of the Seycelles Magpie Robins.