You will find that a Harris's will fly to you at a high weight. You will have to bring the weight down to make a young hawk take the plunge and tackle quarry.
Once they have had a few kills you can try the weight up a bit until it goes off the boil. As muscle and confidence grow its weight will increase, its all trial and error. There are a number of factors to take into account, the weather, they need more fuel when its cold. The quality of meat, ie red/white meat all have differing calorific values. Also the amount of exercise its had will determine the amount of food needed to maintai