We have generic tests for the intelligence of birds.

Most famously used on Crows/Parrots, Seagulls are smarter than average and can solve some advanced puzzles, but they're mostly below Corvids or Parrots. We've also tested their memory, they only care about food and can remember places they've gotten food and even the faces of people who they've got food from year to year, but we don't think over too much longer periods.

If a seagull lives to it's thirties I imagine it's flying around with nothing but Smashmouth banging around it's brain all day.

I can't relate to being disturbed by the idea that other creatures remember their own lives.

There's a whole Flock of Seagulls that's been around since the 80s!

Dumb? The sea seagulls maybe, but the ones moving inland are definitely not. Here in The Netherlands there's an evolutionary difference in growth and cognitive behavior noticeable, as the inland seagulls are able to get much more food. The food is also more diverse and contains more fats. Because the food they eat is our trash, they are also exposed to many fungi and bacteria improving their immune system. To be able to get to our trash they need to open containers and reach into difficult places, improving their cognitive skills. There is a big difference noticeable with seagulls who stay at sea, as they rely on just fish which is harder to get as thanks to us the fish population is greatly reduced. Sea seagulls are often under fed and get ill more often and die due to poisoning (like lead from ships which ends up in the fish they eat). They are doing bad these days, which is what forced groups to find food elsewhere, like in the cities.

And now they are plotting to take control over humans after ages of control by cats. I've seen them and my cats hoarding machine guns, grenades and rocket launchers.

I tried to find the article I read a while ago (was in Dutch) but can't seem to find it. Here's a similar article though: https://phys.org/news/2016-07-biologist-evolutionary-brought-seagulls-ocean.pdf