"Gulls are an indicator of our coastal marine ecosystems," said Louise Bright, the study's lead author. "We need to be restoring ecosystems along the coast, and that includes restoring fish populations."
Gull numbers started shrinking after a peak in the mid-'80s, reversing an increase that started in the early 1900s.
Gulls historically had an almost completely marine diet of small fish and shellfish, but gradually began eating more land-based food, including garbage and earthworms.
"These birds are the ultimate generalist — they can eat whatever’s around," Bright said in a news release.
"If they are experiencing a population decline, the gulls may be telling us that there have been some fairly profound changes to local marine ecosystems. They’re presumably turning to land-based prey sources because the things they prefer to eat are less available."
Industrial fishing means there aren't enough fish, and fewer varieties are available.
Reduced marine food and quality also explain why the populations of two other bird species — Marbled Murrelets and Western Grebes — declined by 90 per cent, study co-author Peter Arcese said.
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