On my recent visits to the skimmer colony at Nickerson Beach, near Point Lookout, NY, one or two Great Black-backed Gulls would (individually) kill (though I personally never saw the kill) and devour a young Black Skimmer or two. This always took place at the western end of the colony (though I have know clue as to why....)
Over the past 30 years I have seen the following:
A GBBG grab a yoyung skimmer by the head, shake it, and break its neck in 3 seconds flat at JBWR.
A GBBG knock a Glossy Ibis to the ground as it flew over a marsh, this also at JBWR.
A GBBG grab a Mallard by the wing in an attempt to kill it. The Mallard "flew" round and round in circles until it escaped.
A GBBG try to grab a Snowy Egret that defended itself by stabbing the gull with its beak.
A single GBBG off of the Montauk Lighthouse grab a scoter by the neck and drown it in about 20 minutes. Several other GBBGs joined in to rip the thing apart once the duck had expired. This occurred on a Christmas Bird count back in my birding days (1977-1983).
ps: Feel free to share any of your predatory gull stories.