A raven is much larger and glossier than a crow, and has a blue sheen. Its beak is larger in proportion to its head than a crow's and it has a graduated tail.

 The blue gloss can be seen on the bird's feathers. The raven tore pieces off the meat carcass and hid them in various places, to return to later.

This fellow was in a sorry state when he was rescued in NW London. His large size, blue gloss, disproportionately large beak and graduated tail indicated that he was a young raven, although unusual for the area, rather than a crow. Also, when released a few days later, he flew far higher than a crow's normal flight path.

(The only known colony of ravens left in London were those in the Tower of London, but they have currently been sent away to an aviary because of worries of avian flu (2006). -- Will the country collapse, as legend has it? . . .)