Not specific to hawks, but gives information on fledgling body mass.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1634777
The main result presented here is that nestling swifts do have the ability to make individual facultative adjustments in their pre-fledging mass loss, and are not tied to an inflexible developmental schedule. This apparently adaptive flexible response is seemed to be achieved by nestlings simply limiting their food intake during the last two weeks before fledging. This occurred through an obvious reduction in individual nestling begging effort and the subsequent reduction in the rate of food delivery by their parents (S. Markman & S. M. Denney 2000–2002, personal observation; and see Martins 1997). It is possible that our weighted nestlings lost more mass prior to fledging because they experienced greater energetic demands in the nest as a result of carrying around the experimental mass. However, given the range of movement by these birds in the nest (see below) this seems unlikely to have been a sizeable effect. The similar level of mass loss by individuals in the trimmed experiment group also clearly argues against this as an explanation for our results here. Therefore, during the last two weeks in the nest, nestling swifts appear to voluntarily lower their food intake and individually reduce their body mass (but interestingly not increasing their wing growth) in order to achieve the appropriate wing loading for flight.
The human spirit needs places where nature has not been rearranged by the hand of man. ~Author Unknown