Common Terns are by far the most numerous tern species in New England. In Maine, the majority of Common Terns nest in colonies on off-shore islands, where mammal predation and human disturbance is at a minimum and small fish are plentiful.
In summer, adults have black crowns, orange legs, and orange bills tipped with a variable amount of black. On their nesting grounds, and in flight, they often give loud Keeeeeerrr calls. (Listen to the following embedded sample.)
Immature birds have brown heads, and back and wing feathers with light brown edging. By fall, the brown will have worn away, and the young-of-the-year will look much like non-breeding adults. (All About Birds has photos of breeding and non-breeding adults as well as immature Common Terns for comparison.)