Despite what some so-called experts might tell you, this is actually very straightforward. Does the gull in question look like this?
If so, then you’re in luck – that’s a Slaty-backed Gull alright. You can add it to your list and everyone will look up to you.
If, however, it looks like this:
Then someone's fucked up big time. This is the closely related, but sadly much commoner Salty-backed Gull, often found hanging around outside seaside fish and chip shops (OK, it doesn't look much like salt, but what do you expect for 5 minutes pissing around in Photoshop?). And if you still need that, then you’re a fucking tart.