Northwest passages

Natural History, May, 1998 by Nils Warnock, Mary Anne Bishop, Horacio de la Cueva

Bordering Prince William Sound, the eighty-mile-wide Copper River Delta in south-central Alaska hosts millions of shorebirds on the final leg of their spring migration. This site and other shorebird magnets--such as San Francisco Bay, Grays Harbor in Washington, and the Fraser River Delta in British Columbia--are stops along the Pacific flyway, a migratory route that winds up the western coast of North America toward the sub-Arctic and Arctic tundra, where the birds will nest.

The sheer numbers of birds point to the magnitude of the migration, and research on this natural phenomenon has led to a recognition of how vital the stopover sites are to migrant species. But relatively little is known about how individual birds make the journey. How do they use the sites during migration? Do they make relatively short hops from one stopover site to the next or do they undertake long-distance jumps to far off sites, leaping their way up the coast?

Along with other members of a collaborative research team from the United States and Canada, we theorized that by radio-marking and tracking individual sandpipers, we could learn more about the mysteries of migration. We decided to focus on the western sandpiper. With a population of between two and four million, this is the most abundant shorebird species along the Pacific coast. Weighing on average less than an ounce, western sandpipers are not quite as big as northern cardinals. They nest principally in the tundra of western Alaska and winter mainly in coastal lagoons and estuaries from California to Peru.

The advent of radio transmitters with signals detectable by satellite has revolutionized the study of the seasonal movements of large birds, but the weight of these devices--about three quarters of an ounce--precludes their use on small, sandpiper-sized migrants. We pinned our hopes on transmitters weighing just hundredths of an ounce and about the size of a pinkie fingernail. Each transmitter would have its own frequency and could be heard a few miles away in a small plane or a specially equipped truck. To radio-mark the birds, we first set up mist nets at saltwater and freshwater ponds where the sandpipers fed and rested. Those caught in the fine mesh were removed, weighed, and measured. We then attached the miniature transmitters to their lower backs with a bit of glue. This method of attachment appealed to us because once the birds reach their breeding grounds, the transmitters fall off when the birds molt their old feathers.

After a 1992 pilot study, we expanded our efforts in 1995 and 1996. We captured and radio-marked migrating birds at busy San Francisco Bay and at the less urbanized coastal estuary at Grays Harbor. Because some western sandpipers migrate not along the coast but through the and interior of western North America, we also marked birds at Honey Like, California, a freshwater wetland in the western Great Basin.

Monitoring coastal wetlands known to attract large concentrations of western sandpipers migrating from San Francisco to Alaska proved to be an enormous challenge. When a bird's signal was no longer heard at its capture site, indicating that it had moved on, we contacted other trackers to the north and asked them to begin listening for the bird's radio frequency. Along with our many collaborators, we searched fourteen coastal arid four inland sites by land arid sea--from Humboldt Bay, California (about 250 miles north of San Francisco Bay), all the way to the delta of the Yukon and Kuskokwim Rivers in Alaska, some 2,500 miles from San Francisco Bay.

The path of one particular western sandpiper, known as Number 4.521, proved to be extremely informative A fat made weighing more than an ounce, tins individual was captured on April 17, 1996, in an area encroached upon by urban sprawl, a salt pond at the south end of San Francisco Bay. He spent the next three days shuttling between the bay's low-tide mudflats--where lie fed on small clams, worms, and other invertebrates--and adjacent salt ponds, where lie roosted and occasionally foraged during high tides. After April 21, his signal was not detected for four days, but lie probably made some stops along the California or Oregon coast before we heard his signal again on April 26, during in early morning aerial survey at Grays Harbor. Number 4.521 was oil the move, arid just a few hours later, he had crossed the border into Canada arid was relocated feeding with a flock of birds on mudflats near Vancouver, British Columbia. He rested at that province's Fraser River Delta for a day and a half before resuming his migration along the rugged, remote northern Pacific coast. Three days went by, with none of the monitoring sites picking up his signal. Finally, on May 1, lie was detected when he stopped some 1,000 miles to the north at an estuary by Yakutat, a fishing village on the Gulf of Alaska.

Less than twenty-four hours later, our bird was 185 miles northwest, at Alaska's Copper River Delta, where he paused for five days to refuel on the invertebrate-rich mudflats. He moved at a leisurely pace across this immense delta, first spending a few days at the far east end, then one day near the mouth of the Copper River, and another few days around the barrier islands at the west end. He then shifted direction and flew west some 460 miles to Bristol Bay, where, two days later, we heard his signal for the last time. He may have mated and nested on the low, wet tundra near Bristol Bay or continued north toward the Yukon-Kuskokwim Delta. In all, we followed this tiny sandpiper for twenty-two days, from California to western Alaska, on an odyssey of 2,600 miles.