What I've seen though on a hot summer day, when a exhausted pigeon splashed down in a canal,
a single large gull swooped down fast from a pole it sat on,
the moment the pigeon touched the water.
And tore it to pieces. Took maybe 3 seconds.
But they are mostly well fed here, from all the people who enjoy their masterful glide,
with most minimal twitches of their wings, 'endfeathers' they use like fingers to feel the air,
almost standing still in the air, while picking off some food from in between someones fingers on an outstretched arm,
or a string of 'bait' laid out on a guardrail,
countless cameras and smartphones catching their moves :-)
Neither. Seagulls are natural born opportunists. If they can grab your hamburger or hunt and eat a pigeon – they'll do that. Just the same for landfills or farms.