The very first line of the Wikipedia entry on Gull says: "Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. ". Colloquially speaking all gulls are seagulls.
The entry lists 54 species of Gull, and indeed from a pedantic perspective, none of their common names are "seagull". Nor are any of their binomial names Latin for "seagull". But there is Larus pacificus, either very calm or associated with the ocean of the same name. Also there is Larus atlanticus, and Larus Marinus (pretty dang close).
I'd just like to interject for a moment. What you're refering to as a Gull, is in fact, Sea/Gull, or as I've recently taken to calling it, Sea plus Gull. Gull is not an categorization unto itself, but rather another component of a full identity made useful by the kingdom, phylum, class, order, family, genus, and species components comprising a full identification as defined by its scientific classification.
It counts for me! But we need something objective, this is SCIENCE after all. A question, when the Swedish Larus marinus, a.k.a. Havstrut walks, what does it look like? I think you see where I'm going with this, is there a bit of a swagger? Does L. marinus have strut?
I really don't understand what the point of the distinction is. It's not like there's something else which is a seagull but not a gull. Seagull is just another word for the same bird... Or am I missing something?
My whole point of posting was to point out how inane, and pedantic the distinction between Gull and Seagull actually is, which is the distinction that OP made. And of course on the Fediverse that generated a whole lot of conversation, including this sentence.
There are weirdly rigid common names around birds. There is a whole thing about renaming them right now. They are essentially regulated terms that low level pedants respect. They are the same types of people who would correct you for calling Frankenstein's monster 'Frankenstein'.
The plant community is better. You could call a "sunflower" a "tall flower" and nobody would care. You might get a "oh, I've never heard that one" but never "there's no such thing as a 'tall flower.'" They just fall back to the scientific names when clarity is important.
IMO common names should just be useful. I will call any gull a seagull when talking to non-bird people because that is a term that is commonly understood and how effective communication works.
I understand the need for having one particular defined name for a species, honestly. That makes some sense to me. But just because taxonomically a bird is not called a seagull doesn't mean that it is not a seagull. Otherwise what is a seagull? There is no bird that has the 'official' name "seagull". So what, seagulls don't exist? It's a semantic distinction that is meaningless outside of its narrow context.
I absolutely agree that there should be a official name. My problem with birds is that there are 2 official names. The American Ornithological Society approves both of them (kind of). One is Latin/Greek/whatever in Genus species format - that is the one for science literature and taxonomy. The other is in English and silly in my opinion because that's where people will use it to say nonsense like there is no such thing as a seagull.