Scientists are worried that puffins are getting underweight and dying because they haven't got enough fish to eat in the North Sea.
The Firth of Forth in Scotland is home to one of the UK's largest puffin colonies.
But experts who've been counting the seabirds there say their numbers have fallen by about a third in five years.
They think climate change could be to blame for the birds not having enough to eat.
Experts count puffins, which are also known as the clowns of the sky, every five years to try to keep track of their numbers.
Their teeth point backwards
They're only about 30cms tall
A baby puffin is called a puffling
Their bills can hold lots of fish
They spend their winters at sea, floating, swimming and diving for food, and only come to land to nest.
Experts have labelled some of the birds in the Isle of May in the Firth of Forth with rings, and found that their numbers have fallen from 70,000 pairs in 2003, to 41,000.