Is it legal to shoot a seagull?

Probably not. The Migratory Bird Treaty Act of 1918 forbids the intentional killing or attempted killing, of more than 800 species of migratory birds, including 27 species of Gull.

Belcher's, Larus belcheri

Black-headed, Larus ridibundus

Black-tailed, Larus crassirostris

Bonaparte's, Larus philadelphia

California, Larus californicus

[Common Black-headed (see Black-headed)]

Franklin's, Larus pipixcan

Glaucous, Larus hyperboreus

Glaucous-winged, Larus glaucescens

Gray-hooded, Larus cirrocephalus

Great Black-backed, Larus marinus

Heermann's, Larus heermanni

Herring, Larus argentatus

Iceland, Larus glaucoides

Ivory, Pagophila eburnea

Kelp, Larus dominicanus

Laughing, Larus atricilla

Lesser Black-backed, Larus fuscus

Little, Larus minutus

Mew, Larus canus

Ring-billed, Larus delawarensis

Ross's, Rhodostethia rosea

Sabine's, Xema sabini

Slaty-backed, Larus schistisagus

Thayer's, Larus thayeri

Western, Larus occidentalis

Yellow-footed, Larus livens

Yellow-legged, Larus michahellis

Sources:

http://www.fws.gov/laws/lawsdigest/migtrea.html

http://www.fws.gov/migratorybirds/RegulationsPolicies/mbta/mbtandx.html