Each seagull featured in the crew of the Silent Mary is a reanimated, undead seagull corpse (described as a skeletal seagull according to the English Audio Description). These seagulls are said to fly around the Silent Mary under Salazar's command as the undead crew continued their mission to hunt down Captain Jack Sparrow as they continued to squawk very loudly showing support to Salazar. After the Trident was broken, it is likely that the seagulls have reverted back to their normal forms afterwards.
After Jack Sparrow gives his compass to the innkeeper, a flock of ghost seagulls flies across the Silent Mary sailing past the Devil's Triangle just as Captain Salazar and his undead crew celebrate in escaping the Triangle in a plan to hunt down Jack Sparrow who betrayed his compass.
One seagull lands on the Red Dragon as one of the ship's crewmen notices it only to find out the bird disappeared. The same bird reappears again and frightens the pirate onboard as it is shown that the seagulls are linked to the Silent Mary just as the undead seagulls fly around the Mary sailing past the Dragon which was ultimately rammed and destroyed by Salazar's ship.
Arriving near Hangman's Bay, Lesaro told his captain to send in undead sharks to go after Jack Sparrow as a flock of undead seagulls continued flying around the Silent Mary squawking loudly. During the search for the Trident of Poseidon, a flock of ghost seagulls surrounded both the Black Pearl and the Silent Mary just as Salazar tells Henry Turner to go after Jack Sparrow who is after the Trident just as Salazar and his undead crew travelled to Poseidon's Tomb to go after the pirate.
As the Trident of Poseidon has been destroyed, all curses are broken, so it is likely that the seagulls have been reverted back to their normal forms as it is shown that the crew of the Silent Mary returned from their undead forms to their mortal forms.
In the discontinued video game, Disney Crossy Road as part of the Dead Men Tell No Tales add-on update to the Pirates of the Caribbean world, a singular undead seagull can be found perching on one of the barrel obstacles when playing as one of the characters who originated in Dead Men Tell No Tales. When approached on random occasions, the seagull makes an ominous squawking noise, indicating that the player found the seagull, which unlocks the Ghost Seagull character after finding it.
A ghost seagull next to a pirate onboard the Red Dragon