Guts. They're the key to a raptor’s ability to soar. Guts and wing area. A shorter gut means less weight, a lower wing-loading and “up she goes!” That's why a sparrowhawk soars and a similarly dimensioned wood pigeon flaps like crazy. The pigeon is full of guts for processing seeds. The hawk with its high-protein diet doesn't need so many.
This came out of a fascinating talk given by Keith Offord to the Kidderminster branch of the WMBC last night. Here's more: raptor is a taxonomic term. Yup, the birds have hooked bills, binocular vision and long, pointy claws; but so do owls, which technically are not raptors. Nor are New World vultures, more closely related to our storks. They evolved convergently to the same body pattern. (Actually all vultures have somewhat subdued claws but I’ll skate over that.)
And finally, why are some species’ females bigger than the males but not others? Nobody really knows but there's a correlation between this sexual dimorphism and the speed of the birds’ prey. Dead stuff doesn't move fast at all and the sexes of the carrion eaters, like vultures and kites, are the same size. At the other end of the scale birds are the zippiest things on the planet and the female of our sparrowhawk can be twice the size of the male.
So there's another thing for him to worry about, along with his lack of guts. (Just kidding: I wouldn’t tangle with a male sparrowhawk.)