Ron et al,
This is a very interesting observation. I would guess that they were Ring-bills, but that olive consumption is not necessarily unique to that species. Gulls seem to be fairly omnivorous. My buddy sent me pics last week of Herring Gulls getting rats in Salem, Massachusetts harbor as the water rose with Storm Sandy and the rats were seeking new digs. All gulls love dumps and who knows what all they find there? Russian olives (the pulp, that is) reminds me of mild watermelon. Many, many birds eat the fruit, some for the pulp, some for the pit, from Yellow-rumped Warblers to sapsuckers to, now, gulls. Thanks for your post.
Dave Leatherman
Fort Collins