Another physics issue of importance is energy vs. muscle. A bird needs some excess fat in order to create muscle. A completely fatless bird is burning its own muscle in order to fly. (See LG4).

Something the apprentice may not know is that each year you have the bird, its flying weight will probably increase slightly. This is partly to do with muscle increase (more likely in an eyas than a passage bird.)

But it also has partly to do with the relationship -- the hawk trusts you, knows what you want to do. And hawks do get habit-bound -- you may have observed a wild hawk take the same perch at the same time in the morning, leave for another at about an hour and 15 minutes later, etc.

You may observe your hawk flying at a physically higher level when she's got some extra padding. When they're high in weight they play on the wind more, chase sparrows they don't stand a chance of catching, and generally enjoy themselves. Within limits, this is a good thing for the hawk's flying skill and its relationship with you.