Black Terns breed in marshes in the Old World and in America, including much of Ontario.
They are very aggressive in defending their cattail marshes: hard hats are recommended – or better still, just stay away.
Since they often nest on floating debris, it is important not to make waves near their nesting colonies.
Black terns are much smaller than Common Terns. Adults have black bodies, the only white being under the tail, when in breeding plumage.
Flying young and winter adults are mainly white below and about the head, except for black on the nape and behind the eyes but are still much darker above than other terns.
In this plumage they have sometimes been misidentified as Least Terns, but these southern terns are accidental in Ontario, and show much white at the base of the tail, and on the trailing edge of the wing: adult Least Terns with their bright yellow beak and feet are distinctive.
Dr. J. Murray Speirs