Common terns fly as buoyantly as butterflies. Their bodies rise noticeably with each beat of their wings. Hundreds of them have been passing along the lakefront over the past couple of weeks, heading south to Colombia, Brazil, and Argentina. Some will reach the Strait of Magellan.
The species you are most likely to see are common and Forster’s. Unfortunately for the novice bird-watcher, these two are practically identical. It takes a very good look under very good conditions to make a positive identification. Birds do have a strong tendency to look alike, a trait that adds both charm and painful confusion to the sport of watching them.
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Both of these terns are black-capped in summer plumage. The average wingspan of the common tern is 30 inches. The average wingspan of the Forster’s tern is 31 inches. This is not a difference that is easy to see in the field. The National Geographic guide Birds of North America says that Forster’s terns have longer bills than common terns, but I can’t even detect a difference in the paintings in the book. Forster’s terns’ beaks are slightly paler, a little less fire-engine red, but the knowledge of this, like many birding field marks, is something you can use only after you have looked at several hundred terns of both species.
Forster’s terns are mainly insect eaters who catch their meals on the fly like swallows. Patrolling the air above their home marshes, they chase down flying insects or pluck small creatures from the water’s surface without wetting a feather.
As its name suggests, the Caspian is a cosmopolitan bird. The type specimen came from the Caspian Sea, but it also breeds in Green Bay and New Zealand. It dives for fish like other terns, but it also soars like a gull and sometimes flies with its beak pointed forward.
Black terns live very much as Forster’s terns do. They nest in marshes, in loose colonies, often building their nests on the roofs of muskrat houses. And they make a living by hawking insects from the air. They are fast and agile enough to chase down dragonflies.
