There is a hobby called Birdwatching. Some just observe birds with naked eyes, some use binoculars, others make photos using telephoto lenses. But precise identification of the species observed is a must. And with large gulls such as this one, it's a tough challenge.
Mostly, large gulls begin their life covered with white-brown plumage for the first 3-4 years, and only when they become adults, they turn into those grey and white birds. And different species can look very similar and can be extremely hard to identify reliably.
The bird in the picture is a Yellow-legged gull. And it is almost indistinguishable from European herring gull, who also sometimes have yellow legs. Only by noticing very subtle details such as bright yellow legs, slightly darker shade of gray and slightly longer wings can we identify the YLG. Lesser black backed gulls also look very similar, adding to the confusion.
The bird in the picture has its autumn plumage up - its head is covered with brown feathers.