You seem to be conflating evolution with learning. Seagulls didn't evolve to steal from trashcans, they learned to.

As for the main point of your question: the clams are replaced every so often, so they won't end up getting used to the city water and stop serving as a marker.

Got it, learning is something that can be acquired relatively quickly, while evolving needs generations. Thanks for the explanation.

Valid question though, and the answer to what I think you wanted to know is >3 months:

> the clams are “paid back after three months of work by releasing them to a place from which they will never be caught again”. [---] this is done because they eventually become resistant to contamination in the water.

https://www.zmescience.com/ecology/poznan-mussel-water-plant...

When I lived in Aberystwyth on the Welsh coast you could always tell a newcomer to the town because they'd eat outside without aerial cover. The herring gulls there are pterodactyl-like, massive and always watching for the slightest opportunity to fill their beaks. They're clever birds too, I've seem them work cooperatively to deprive some poor fresher of their lunch and hawks had little success in driving them away. I've actually come to miss them a bit having moved inland, I'm moving back to a city by the sea this year and I wonder how long that will last!

Depending on where you are, you may be thinking of the great black-backed gull.

I've just googled, and herring gulls are indeed the birds I was referring to here in Norway, good catch. We have the same, actually, when tourists/guests of the city are easier targets. A few times I had to "defend" (mostly, by hiding) my lunch sandwich or bun as well, if I wanted to eat somewhere with a view over the sea. However, even a few kilometers from the sea, they can still be on a "hunt". Very interesting creatures!

"seagull" isn't one species. Pedantic ornithologists will say there's no such thing as a "seagull" since the group is more correctly just called "gulls" and many of them are found very far from any sea, but everyone understands the term anyway.

There are many different species of gull, they vary a lot in size (and a decent amount in plumage). Great Black-backed Gulls (Larus marinus) have about a 165cm wingspan, Common Gulls (Larus canus) have about a 120cm wingspan, and Little Gulls (Hydrocoloeus minutus) only about 61cm wingspan! And that's just for a few of the 20+ gull species found in North America, there's even more variation worldwide.

So the other slightly larger gulls are probably just other gulls, of a different species. The genus Larus in particular has several species that are quite similar in plumage, e.g. Ring-billed gulls (≈122cm wingspan) and Herring Gulls (≈147cm wingspan) look quite similar in flight or from a distance.

Gulls are scavengers. They will hunt for themselves, but will also scavenge food readily. They're well known for stealing fish from other birds such as Ospreys, eating carrion, etc. They learn what they can eat from observing other gulls (their parents and flockmates), eating trash isn't an evolved trait separate from scavenging but instead a learned behavior of what to scavenge.