"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.
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.