The Caspian tern is the largest of the terns, and is a colonial nesting species that can be found on every continent with the exception of Antarctica. They are about the size of a medium-sized gull, but are easily identified by their black cap and large, red bill with a dark tip. These terns are plunge divers that mainly eat fish, though they have been known to capture and eat crustaceans and even small leopard sharks. Prey items that are not consumed immediately, are brought back to the colony in the bill to be eaten, fed to mates or young, or displayed to court potential mates. Breeding colonies range from few birds to thousands of pairs, and in the Pacific Flyway, colonies can be found in northern Mexico, the Salton Sea in southern California, and all the way to the Copper River Delta in Alaska. For the last two decades, the largest Caspian tern colony in the North America has been located in the Columbia River estuary on East Sand Island, near Astoria, Oregon. During that time, that colony has been home to about 12,000 – 20,000 Caspian terns, which represents 50 – 65% of the Pacific Flyway breeding population. The Caspian terns nesting at this densely packed colony location has generated some controversy in the Pacific Northwest over the number of out-migrating salmonid smolts that they consume during the breeding season.
The breeding season lasts from mid-April to the end of July at most Caspian tern colonies in the Pacific Flyway Region. Caspian terns nest on the ground, and lay their eggs in a small nest scrape. As such, they typically form nesting colonies on flat, sandy or gravelly areas. They also like to have a clear view of the horizon to help them detect predators, so will avoid nesting or roosting in areas with dense, enclosed vegetation. Caspian terns don’t usually breed for the first time until they are around 5 or 6 years of age, but can live for over 20 years. Clutch sizes range from 1-3 eggs. The egg incubation period lasts about 27 days, and after they hatch, it takes the young about 37 more days for the chicks to reach fledging age. However, plunge diving for fish isn’t easy, and the adults provide extended post-fledging care to their young to teach them how and where to forage. Newly fledged young will spend the fall and winter with one of their parents to learn the skills they need to survive. In the Pacific Flyway, wintering areas stretch from just north of the US/Mexico border down through Central America. Satellite telemetry data from our study has highlighted four major southern migration routes from breeding colonies in Oregon and Washington to a stop over area at the Salton Sea, but just a single route to wintering sites as far south as the Gulf of Fonseca, Nicaragua after crossing the US/Mexico border.
In North America, bald eagles and peregrine falcons are the main avian predators of adult Caspian terns. The mere presence of these raptors high above can cause an entire colony to flush and take to the air, but mammalian predators such as raccoons, opossums, and coyotes are also a threat to colony productivity. As a protection strategy, Caspian terns tend to be attracted to nesting sites with gulls because their presence may indicate that the site is relatively free of these mammalian predators. It is a pretty good strategy, but sharing a colony location with gulls carries some risk too. Larger gulls will, given the chance, readily eat tern eggs and chicks, and some individual gulls have been observed to behaviorally adapt; specializing on eating tern eggs and chicks to raise their own young. The combination of disturbance by bald eagles and nest predation by gulls can greatly limit Caspian tern nesting success.