Terns
Common Tern flying and watching for fish
Terns are the more elegant cousins of gulls, and most species eat only small fish.
Common Tern flying and watching for fish
Terns are the more elegant cousins of gulls, and most species eat only small fish.
■ Why do some birds nest in colonies? Colonial nesting habits develop when suitable nest sites are limited and food is patchy and unpredictable across a wide area. Terns nest on small islands with no ground predators, and the schooling fish they need for food are extremely unpredictable. The disadvantages of living in crowded conditions include increased exposure to disease and parasites, and increased competition (for food, nest sites, nest materials, and mates). Advantages of colonial nesting include improved predator defense and better information about food sources. A larger and denser colony can mount a much stronger attack against a predator than a sparse colony or an isolated pair. Even when parents are absent from their own nest for long periods in search of food, the colony as a whole will still be defended by other colony members. A colony also functions as an information exchange, allowing birds to take advantage of food sources discovered by their neighbors. The chance of discovering small schools of fish is greatly increased with more birds searching, and once a school is discovered other terns can quickly join the feeding frenzy.
A colony of Common Terns: each pair defends a small space around their nest, but everything else is shared.
■ Finding fish in the open ocean is a challenge, and terns require small fish that swim within a few inches of the surface. The tern’s strategy is patrolling at a low height, scanning for fish. When they find fish at the surface they stop and hover, about ten feet over the water, selecting their target and waiting for the right moment. Then they turn and drop straight into the water headfirst, hoping that they can close their bill around a fish. They never rest on the water; instead, they take off immediately, and if they’ve captured a fish they either swallow it while flying or carry it back to their nest. Along the way, they hope to avoid marauding gulls and other birds that want to steal their fish (see kleptoparasitism, this page).
A Common Tern hovering and diving for fish
■ Terns are exquisitely adapted for flight, and none more so than the Arctic Tern. This species nests in the Arctic, then migrates to the Antarctic and back every year. It lives most of the year in sunlight, traveling from Arctic summer to Antarctic summer, and spends most of the year near icebergs. Terns are not well suited for swimming, so they’re on the wing during long stretches of migration. Their migration route is not a straight line, but takes wide loops around the ocean, and an individual bird might travel 60,000 miles in a year. (The bird migration distance record is currently held by the Wandering Albatross, which has been tracked moving an average of 114,000 miles a year, cruising around the southern oceans.)
Beginning at Arctic nesting sites, Arctic Terns migrate south (orange lines) to their Antarctic winter range (blue), then return north on a different route (green).