CASPIAN TERNS – FABULOUS BIRDS BUT NOT ALWAYS APPRECIATED

The Caspian Tern (Hydroprogne caspia) is the largest of the world’s terns and one of the most widely distributed. The size of a medium-sized gull, it is a most impressive bird, with its striking silvery-white plumage, black crown, and big red bill. Its vocalizations are no less impressive, a loud rasping call that one could imagine pterodactyls made on their breeding grounds.

Caspian Terns breed on islands in lakes or bays, usually in colonies of hundreds to thousands of birds. The largest Pacific Northwest colony is on East Sand Island, at the mouth of the Columbia River, with about 10,000 pairs. This may be the largest colony in the world. The first eggs are laid in mid April, the first hatching about a month later and the first fledging about a month after that. Thousands of birds are still present in late July, but the colony empties shortly after that.

Most pairs lay three eggs, and when they hatch both adults are kept busy bringing in fish from surrounding waters. They fly well above the water, up to 10 meters or more, and dive when they see a fish near the surface, submerging their body completely. If they succeed, they head for the colony. The young have a high-pitched call, like most young birds, and in and around a colony the constant calling back and forth between parents and offspring brings to light the meaning of the word “cacophony.”

Terns are all fish-eaters, and big colonies of Caspians can only thrive where there are a lot of fish in the nearby waters. The Lower Columbia River is such a place, rich in many kinds of fish at the transition from fresh to salt water. Among them are salmon of several species. After their early growth in fresh water, salmonid smolts descend the river to spend some years in salt water before returning to spawn. It has been estimated that 100 million smolts come downriver each year, most of them released from hatcheries.

The terns, and other fish-eating birds nesting in the same area, are there to receive them. It has also been estimated that the tern colony on East Sand Island accounts for predation on about 5 million of these smolts. Although this is only 5% of the estimated smolt run, it is enough to greatly concern wildlife management personnel in both Washington and Oregon.

In fact, concern seems sufficiently great that the terns have been systematically persecuted. Over the past few decades, Caspian Tern colonies have formed at numerous sites along the Washington coast, and at each of them the birds have been hazed until the colony disbanded. Because they have been chased away from islands, they have tried to nest on rooftops and other humanmade structures, but each time they are discovered, they are soon displaced.

Because a few of the colonies on Columbia River islands are thriving, there are enough excess terns that they are still to be seen and heard flying over all of our coastal waters, but many of them fail to breed because they are not allowed to. Salmonid fishes are of great importance to the Pacific Northwest economy, and Caspian Terns are not; the equation does not favor the terns.

POSTSCRIPT (added 23 June 2011)

The largest PNW tern colony, on Sand Island at the mouth of the Columbia, has been severely disrupted by predators, especially Bald Eagles, and will probably fail this year. Fortunately, the terns are long-lived and will presumably breed next year, but the eagles are proving to be real villains in this case, disrupting colonies of terns and other seabirds severely enough that they pose a long-term threat to the populations of these birds. Anthropogenic changes may have made the situation worse, with a decline in other eagle prey and a reduction of the seabird colonies to few sites.

Dennis Paulson

IT’S SHOREBIRD TIME AGAIN

Every year in April and May the Pacific Northwest experiences a mad rush of thousands and thousands of shorebirds—sandpipers and plovers—on their way north to their Arctic and Subarctic nesting grounds.

This is nowhere more obvious than around two big coastal estuaries, Grays Harbor and Willapa Bay. These estuaries are extremely productive of the invertebrates that the shorebirds eat. Prey animals are present at very high densities in the mud and sand of these estuaries, for example about 10,000/square meter at Bottle Beach, on the south side of Grays Harbor, one of the most productive areas.

This seems astronomical but pales in comparison with 100,000 individuals/square meter of one species of amphipod in the Bay of Fundy. Nevertheless, it is sufficient density to support the tens of thousands of shorebirds that visit the area for up to a month each spring. Individual birds stay only a few days, fattening up for a flight that may be only 200 kilometers to Roberts Bank in British Columbia or as much as 900 kilometers to the Copper River delta in Alaska, then on to arctic breeding grounds.

The invertebrates that fuel these birds are primarily amphipods and polychaete worms. The amphipods are about 5-6 mm long, and some of the polychaetes are in the same size range. Other polychaetes are called “thin worms,” up to 30 cm long but only 1 mm in diameter! They can be in such high densities as to almost bind the mud together.

The three most abundant species in spring migration are Western Sandpiper, Dunlin, and Short-billed Dowitcher, in descending order. Most of the big flocks you see will include these species. There are many other species present as well, and searching them out gives the observer variety as well as spectacle. Other common species include Black-bellied and Semipalmated Plovers, Greater Yellowlegs, Whimbrel, Marbled Godwit, Ruddy Turnstone, Red Knot, Sanderling, and Least Sandpiper, with numerous others even less common but out there somewhere.

No migration is evident at the beginning of April, but by the middle of the month, Greater Yellowlegs and Short-billed Dowitchers have arrived in numbers. During the last two weeks of the month, all the species move in, most of them in full breeding plumage, and peak right around the first of May. Numbers fall off after that but are still impressive through mid May, and some of the later migrants are present until the end of that month. Species that peak late include Red Knots and Long-billed Dowitchers.

Herb Wilson of Colby College, Maine, found that these hordes of shorebirds had little effect on the invertebrates on which they fed. He erected “exclusion cages” at Bottle Beach, meter-square cages that kept shorebirds out, and after the migration compared the numbers of invertebrates under each cage with the original numbers there and the numbers just outside. He found no fewer invertebrates outside than inside; thus the numbers had not been reduced by the birds.

The spectacle can be seen in feeding areas at lower tides and at roosts at higher tides. The ocean beach itself has throngs of shorebirds on peak migration days, but it is discouraging to see the high levels of human activities (joggers, horseback riders, dog runners, mopeds, pickup trucks) that disturb each flock again and again on a busy weekend day. The birds get back to feeding immediately after each disturbance, and we can only hope that they are able to take in enough nourishment to make it to their next destination on time.

Dennis Paulson