Seagulls are birds...
Yes there is such a thing, though more properly just called gulls. It's a family of birds, with dozens of species world-wide, several in UK.
Sadly, kittiwake is not the commonest, but is restricted to certain cliff-nesting sites, spending the rest of the time at sea.
The commonest is probably the black-headed gull, with brown hood and white flash on the wing-edges. Next the herring gull, with pale grey wings and yellow and orange beak. Both often found far from the sea, & often eat human rubbish from tips etc. Several others, some rare -- great & lesser black-back, little, Mediterranean etc.
Otherwise food is fish, carrion and even live prey such as birds and rabbits.