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Tiny trackers plot sparrows' flight between Alaska and California

Doug O'Harra
Alejandro Erickson / Flickr photo

They flit among stunted trees and brush as they sing a distinctive three-note jingle. Alert bird watchers can sometimes catch fleeting glimpses of their yellowish head patch rimmed in black as they seek mates and snatch meals from Alaska’s summer insect horde.

But come fall, the golden crowned sparrows of Alaska have always taken flight -- along with uncounted thousands of other migratory passerines that fill the Far North’s cool summer air with song -- and disappear over the horizon for parts unknown.

At Point Reyes National Seashore in central California, there was different mystery.  Their flocks of golden-crowned sparrows spent winters foraging amid the scrub and evergreens near the coast. But in the spring, the one-ounce, seven-inch sparrows flew away. Where to? Except for one odd clue from 1990s, all bird lovers knew was that they winged north and were gone for half the year.

Now a team of California researchers has discovered one of the mysterious and previously unknown summer destinations of their golden-crowned sparrows. And was that place some fabulous enclave at the edge of Earth?

Not exactly. It’s here -- the coastal rim of southern Alaska. These particular Southcentral sparrows were California snowbirds all along.

Carrying miniscule devices that monitor day length as a way to calculate geographic location, four sparrows were successfully tracked on their 3,200-to-5,000-mile roundtrips between California and four different summering areas from Wrangell St. Elias National Park to the Alaska Peninsula.

One of the birds spent its summer of love roaming the Kenai Peninsula, with jaunts to Prince William Sound, Kodiak and the Katmai coast before winging back to Point Reyes.

“This study is helping to unravel the mystery of bird migration and answer the age-old question of where birds go, which helps protect habitat along their entire migratory journey,” said lead author Nat Seavy, a research director with PRBO Conservation Science, a group that focuses on Point Reyes birds, in this story.

“Until now, all we knew was that these birds bred far to the north and undertook one of the longest migrations of all songbirds that winter in central California,” added co-author Diana Humple, manager of PRBO’s Palomarin Field Station. “We’re very excited to finally pinpoint exactly where some of our golden-crowned sparrows breed.”

Five photos of Humple at work on the study were posted with this story by the Marin Independent Journal.

Sorting out where migratory birds spend their mating and wintering periods has always been one of the most important and difficult tasks for ornithologists.

“Bird migration presents a challenge for conservation and the study of avian ecology. No matter how well studied or protected a population may be during any one period in their annual cycle, most migratory birds disappear to unknown locations for the other periods of their lives,” the four co-authors wrote in the study, published this month.

“Not only does the reproductive success and/or survival during one period directly influence the number of birds in the next, but conditions experienced by an individual in one season and location can influence their reproductive success or survival in the next.”

Bird scientists have often been forced to rely on chance for clues to the migratory itinerary — clip a tiny tag or band to a bird at one end of its seasonal journey, and then hope that someone reports seeing it somewhere else along the route.

Did it work for golden-crowned sparrows? Not so much.

“Of the 5,251 Golden-crowned Sparrows banded at Palomarin between 1966 and 2010, only six were encountered away from their original banding site: none within the breeding range and only two outside of California,” the authors wrote in the study. “Of the 6,452 golden-crowned sparrow band encounters reported for North America, only a single record definitively links breeding and wintering locations, a bird banded in central Alaska (near Denali National Park) … on 25 June 1997 and recovered in inland southwestern Oregon on 18 March 1999.”

But recent breakthroughs in the miniaturization of tracking technology gave the Point Reyes scientists another option. 

For the new study, the researchers obtained tiny “geolocators” that record sunrise and sunset times by day. That data can later be downloaded to a computer and used to calculate the avian volunteer’s daily longitude and latitude within 100 miles. The devices, developed by the British Antarctic Survey, weigh about 1.1 grams, only about 4 percent of the bird’s weight.

The researchers captured 33 sparrows in late winter of 2010 and fitted them with the geolocators by using leg-loop harnesses made of Kevlar thread. The scientists also tagged 28 other sparrows and released them without the tracking devices.

Over the next fall and winter, they recaptured 11 sparrows that had been fitted with geolocators and 11 of the untagged control birds. Of the original 33 avian spies for science, seven had lost their geolocators along their journeys. (“We strongly suspect that these individuals were successful at removing their tags simply by picking at the fibrous Kevlar thread harness,” the authors wrote.)

But four returned to Point Reyes with their secrets largely intact.

The one-ounce birds had taken an average of 29 days to travel north, eventually spreading out into four areas along 750 miles of coastal Alaska. Their trips ranged from 1,600 to 2,400 miles in length. When they flew back to California, they traveled somewhat slower, averaging about 53 days in transit.

They were in good shape, too, appearing about the same weight and condition as the other tagged birds.

By using similar techniques, the authors predicted, “knowledge of migratory geography in passerines and other small species is likely to expand exponentially in the coming years.” That information could help conserve birds.

“Today we are facing unprecedented changes in land use and our climate,” Seavy said in this story. “The information in this study will help us understand where our migratory birds may be vulnerable to these changes, and what we can do to help protect them and the ecosystems on which they -- and we -- depend.”