I saw it feed on something, black coloured. Not sure of what.
EDIT (15 Jun 2019)
This is not food, but regurgitation of pellets.
The passing of pellets allows a bird to remove indigestible material from its proventriculus, or glandular stomach. In birds of prey, the regurgitation of pellets serves the bird’s health in another way, by “scouring” parts of the digestive tract, including the gullet. Pellets are formed within six to ten hours of a meal in the bird’s gizzard (muscular stomach). – From Wikipedia