Cormorants eat a lot of fish. This Double-breasted Cormorant was spotted on Belle Isle. If you drive through Canada over to Buffalo, there is a bridge on Rte 403 where you can see thousands of them congregating near the harbor in Hamilton, Ontario. They commonly nest on the ground and large pebbles are often found in their nests. The birds treat the pebbles like eggs. S. notes that some of their calls sound like seals and that they migrate throughout most of the United States, but live year-round along the coast of Alaska. In China, Japan and other places, fishermen trained the birds to help with the catch. A small snare would be tied around the bird's neck, preventing it from swallowing large fish. The birds would be released and after catching a large fish would then return to the boat where the fishermen would then remove the fish from the bird's bill/throat. For more information on these birds, see here.
UPDATE: Saw these again in April 2010, at almost the same location on Belle Isle.