The red-throated diver was photographed by a visitor to a lochan in a remote part of North Uist.
The birds are on the RSPB's amber list after a recent decline in numbers.
Jamie Boyle, site manager of the RSPB's Uist reserves, said plastic bags can kill birds if they eat, or become entangled, in one.
He said: "We urge people to take great care in the way they dispose of plastic bags or, indeed, any other rubbish, particularly balloons and Chinese lanterns.
"They pose a direct threat to our wildlife and it is depressing to think that plastic bags are even reaching remote lochans in a place like North Uist."
Mr Boyle added: "Marine birds such as red-throated divers are particularly at risk both at sea and on their breeding grounds where they can mistake the bags for fish or mistakenly use it for nesting material."
RSPB Scotland has urged people to discard unwanted plastic bags carefully after a rare bird was photographed with one caught in its beak.