Hall Effect Tracing The top tracing shows the gape in millimeters. As the birds head thrusts toward the seed the beaks open a few millimeters wider than the seed. Once the seed is grasped there is a plateau in the record as the head is brought up. The brief openings occur as the seed is moved to the back of the beak. The rightmost part of the curve shows swallowing. The lower record is the velocity of the opening. The multiple velocity peaks during the intial gape is characteristic of most responses of adults.