Tuesday, June 24, 2008

Relocating a Caspian Tern Colony

The mouth of the Columbia River is home to the largest colony of Caspian terns in the world. At least 9,900 pairs of terns nested on a sandy island there last year. Unfortunately, the Columbia River is also home to endangered salmon and steelhead. Terns eat millions of the juvenile fish each year as the fish migrate towards the Pacific Ocean. So wildlife managers are trying to relocate the tern colony to other sites.

One reason for the super-colony at the mouth of the Columbia River, Roby said, is that the birds' historical nesting sites in the western United States have been destroyed by human activities. The draining of marshland habitat in some locations, and the flooding of historical nesting sites in others, has decimated their favored nesting habitat -- bare sand islands.

Now, working with a plan developed by the U.S. Fish and Wildlife Service, the Corps and NOAA Fisheries, the OSU-led team is starting to restore alternative nesting colonies for Caspian terns. In addition to the Crump Lake site, the Corps created an artificial island in Fern Ridge Reservoir in the Willamette Valley, and plans to build three half-acre islands in the Summer Lake Wildlife Area in southern Oregon this summer and next. The plans at Summer Lake include the construction of a half-acre floating island made of recycled plastic with a coarse sand and gravel surface.

Crump Lake and Summer Lake are historical nesting sites for terns, Roby said, but Fern Ridge is not. And thus far, the terns have been slow to embrace Fern Ridge as a nesting site....

The Caspian tern management project also calls for establishing and/or restoring three alternative nesting sites in the San Francisco Bay area, where the OSU-led team also has a research crew.
If the project succeeds, the colony at the mouth of the Columbia River will still exist, but will be about one-third of its current size. So far the project has moved 135 of the 9,900 pairs to the new nesting site at Crump Lake, enough for the linked article to describe it as a success. It is too early, though, to judge how well the project will work. One problem is that we do not yet know how well terns will adapt to the new nesting locations. Terns bred there in the past, but local conditions such as food availability and nest predation may have changed since then. The linked article mentions gulls as a potential nest predator at the Crump Lake site, where they are more numerous than they are at the Columbia River colony.

A second potential problem is suggested by the first quoted paragraph above. The current situation at the mouth of the Columbia, with a large colony of terns preying on two endangered populations, is largely man made. Habitat changes elsewhere caused nesting terns to relocate to the Columbia River. The fish populations' decline is human-caused, as well; dams and other obstacles block many waterways that formerly served for spawning. Relocating terns could have unpredictable effects on prey populations at the new sites. If a local prey population crashes, another relocation project will be required.