Apparently you shouldn't call them seagulls, they are just gulls.
These gulls (maybe all gulls) spend the day picking up shellfish, cockles etc off the beach, flying to a great height and then dropping them on the pebbles to smash them open! This system appears to work well and I'm always amazed that they can find the cockle amongst the pebbles again.
I caught this gull just as he/she was dropping it to the ground.
- Panasonic DMC-GX8