The Scottish Society for the Prevention of Cruelty to Animals asked any witnesses to come forward, The Scotsman reported. The bird was the second gull to be shot with a crossbow this year.
Iain Allan, a chief inspector for the SSPCA, said the bird appeared to have been shot in the chest at close range.
"This was a sickening incident which would have caused this bird a great deal of pain and suffering," he said. "We can't be sure where the attack took place as it's likely the gull was still able to fly after being shot."
The bird was discovered Thursday.
In February, a herring gull was found shot in the neck. Like the one found last week it was so badly hurt it was put down.
Exploding whale video goes viral on Internet
Wisconsin business offering 'therapeutic cuddling' forced to close