While all of you were sending emails about this behavior, I was watching it.  A herring gull took a large clam, probably Mercenaria, and dropped it on the road at Point Lookout, MD, giving my vertebrate class field trip a fine view.  No other gulls were close, so it was able to retrieve its meal.

Birds can be significant predators of land and shallow water mollusks, which is part of why clams are purportedly happy at high tide.

It might be possible to experiment on the gulls by providing fake clams of known size.  They should take them, being gull-ible.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!