The Coast Guard was contacted by birdwatchers on Gotland, which lies about 90km to the east of Sweden, about the injured birds, one of which was found dead. So far the agency's satellite images and aerial reconnaissance haven't turned up any signs of a spill.
"We always take such information seriously because it may indicate that there is an oil spill somewhere out in the sea," Mattias Lindholm, communications officer at the Coast Guard, told Swedish Radio's local channel on Gotland. He said the agency will continue its search for the next few days.
Two weeks ago, similar discoveries of oil-covered birds were reported from Öland, an island just off the Swedish mainland. Lindholm said the reports were sadly a reoccurring event in the Baltic.
"This happens a few times a year and it is always sad when animals are harmed in the environment in this way," Lindholm said.