This one may have figured it out, not all gulls are equally brilliant, but I've seen them in the Pacific Northwest (Puget Sound) pick up clams at low tide and fly over rocky areas to break them. They drop the clam and they circle back around to retrieve it and pick the meat out of the broken shell.
I worked for the National Park Service on San Juan Island one year, and up at British Camp there is a muddy bay at low tide. The gulls eat the clams that way, and the sharp-witted crows mimic them, but without as much success. On foggy days it was always kind of amusing to hear the birds at work down at the shore--the sharp "crack!" of the gull-dropped clams hitting rocks, and the occasional "splat!" when the crows dropped them and missed, instead hitting the mud.
I've seen crows mug gulls to get their food, and it usually is a group of crows around the gull, so when the gull charges one crow to drive it off (leaving it's food on the ground) another crow races in and steals it.
_________________ Northwesterner
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