Gulls are a highly opportunistic species which have adapted well to human habitations. When fish get scarce in winter, you are just as likely to see them in the parking lot of a local mall scavenging for food. Often, it seems like they are more willing to expend energy chasing another bird with a bit of food rather than finding food on their own.
That is what you see on the Bay too. They will shadow flocks of ducks trying to steal the food the ducks have foraged from their dives. If you look carefully, you can see the reason for the flock taking off in the picture above. There is a gull a little left of center in the image intent on stealing.
Here is a Herring Gull or, perhaps, a Ring-billed Gull cruising over the flock looking for a chance to steal some food from a flock of Greater Scaups.
The size of the Great Black-backed Gulls can be intimidating. There is nothing to compare them to in this photo, but these gulls have a wing-span of about 5-1/2 feet, much larger than the ducks from which they steal food.
The duck in this picture is just diving below the surface (the most common defensive maneuver they use) to escape from what looks like it may be a Yellow-legged Gull. I have a lot of trouble identifying gulls, by the way, because they have so many variations. Almost all species of gulls have about four different looks depending on age and season. It can be very confusing. Notice the duck with a shell in it's mouth.
You would think when a duck drops a mollusk, it would sink like a rock and a gull would have no chance of recovering it, but it surprising how good that are at their thieving. Some of the bubbles around this gull were created by the duck that dropped the mollusk and dove under to avoid the gull.
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