Very interesting sequence, Harris, the gulls (Larus ridibundus) feed on the ground due to the agricultural treatment. When you approach the flock and come close to ground, the gulls flew up. First I thought it would be caused by your drone, but they come against you. This it because they flee the raptor approaching from the other side of the field, right from behind the hill top. It can be seen in the very last second of this take. Very impressive. Unfortunately I can not determine what it is. You have some more seconds to watch? Where is this footage from?