Seagulls are similar to the main game's pigeons in which they are "collected" by shooting them. They are much easier to locate because of their louder sounds, and all of them can be found in easier-to-reach places. There are fifty seagulls total in both games.
Seagulls are one of the many birds found in Grand Theft Auto V. They can be seen flying in the air at coastline areas such as Vespucci Beach, however, they are not required to be found or killed. They may occasionally land on the ground or on structures, and they will always fly away from the player if he approaches.
In the enhanced versions, the player can consume a Peyote Plant and begin hallucinating that he is a random bird. One of these birds is a Seagull, as seen here.
GTA The Ballad of Gay Tony - Seagulls Guide (1080p)
GTA: The Ballad of Gay Tony - Seagulls Walkthrough
All fifty in The Lost and Damned were found by the gaming community within five days of release, before any official information was released.
Although seagulls do not appear in Grand Theft Auto IV, they can still be heard at places near water or beach (like Firefly Island). Not to confuse with the sound from the seagulls and from the shore area, they are different.