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Relocating the Caspian Tern


The largest colony of Caspian Terns in the world, approximately 9,000 breeding pairs, is found along the muddy banks and the sandy islands at the mouth of the Columbia River in Oregon. "The Caspian Tern is declining globally, but we have seen an increase in population on the west coast of the United States," says Dr. David Craig of Williamette University, "There is something good happening here for the Caspian Tern."

Dr. Craig is trying to find out what conditions at the Columbia River attract Caspian Terns during breeding season. Using satellite data, he can identify where the terns are and what they are eating. The Columbia River is dredged regularly so that large ships can navigate the naturally shallow channel. Dredging means that mud and silt from the bottom of the channel is removed and deposited in the river stream, creating sandy islands. Dr. Craig believes that these islands are perfect habitat for Caspian Terns. On these islands, the birds are free of predators and have access to the many different species of fish that they use as a food source. In other words, these sandy islands are the perfect place for the terns to make their homes during breeding season.

Click here to see where they are! Salmon seem to be a favorite food for the Caspian Terns, and the Columbia River is a perfect hunting location. However, some species of salmon in the river are endangered. Scientists are trying to protect the fish populations from reaching dangerously low numbers. Caspian Terns will eat other species of fish and sometimes insects or crayfish. Therefore, scientists have tried to move the Caspian Tern colony to areas of the river basin where they might eat fish other than salmon.

The Caspian Tern is a very social bird. Dr. Craig and his colleagues used this characteristic to relocate the birds away from the endangered salmon over a period of three years, from 1998 to 2000. Decoys, often fake birds, and solar powered CD players that repeat the call of the Caspian Tern successfully drew the colony away from the shrinking salmon population. Caspian Terns do not need to return to the same breeding grounds year after year, therefore birds will still breed if the colony is moved. In the future, Dr. Craig is interested in finding other suitable breeding habitats for Caspian Terns. However, he questions how common small, sandy, predator-free islands are locally and globally.

Dr. Craig asks Signals of Spring students to consider the migration path of Caspian Terns; do they zig-zag or fly along the coast? He is also interested in what they eat along their journey, How might the type of fish, salmon versus other species, that Caspian Terns eat during their migration impact an evolutionary change in the population? Animal populations can change and adapt to available food sources in their environment. It is possible that the type of fish that a tern colony feeds on during their migration may cause an evolutionary change in the population. If a specific characteristic, such as a long sharp beak, makes it easier for the tern to catch the new type of fish, birds with this feature will survive better in that environment and pass this trait on to their offspring. Then, over thousands of years, long sharp beaks might become more common within the colony. This process of adaptation has been observed in almost every species on Earth.

The Caspian Tern is the first tern species to be tracked and studied in such detail. Dr. Craig's research may provide insight into the migration and behavior of other tern populations, such as the Artic Tern, which has the longest migration of any bird.

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