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Warblers Refuel at City Creek

When the yellowest of all warblers - the aptly named yellow warbler - makes a migratory pit stop at Coyote Creek in the heart of San Jose it usually gains weight. A long-term bird-banding program at the Coyote Creek Riparian Station found that over half of these winged visitors fattened up during their rest and refueling stop on the long fall flight from North America to Mexico.

"It shows that even the small fragments of creekside habitat we have left are really important to neotropical migrants," says the Station's Chris Otahal.

The station placed a series of nets on poles to trap the yellow warblers - 4-5-inch-long members of a brightly colored, constantly flitting bird family that one guide calls the "butterflies of the bird world." The warblers fly into these extremely fine "mist nests" and fall into a pocket. Station volunteers then weigh, measure, band and release them for recapture and comparison later.

The program found that many Dendroica petechia used the site for extended periods during the fall migration (mean of four days, range 1-13 days). Of these resting birds, 57.9% gained mass, 15.8% maintained mass and 26.3% lost mass, indicating that most birds used the area for refueling. Fat load changes ranged from a loss of 1.5 grams to a gain of 5 grams (mean gain was 0.5 grams). Such gains are quite substantial considering the average mass of these birds is only 10.1 grams. Each gram of increased fat allows an individual bird to increase its flight range by over 200 kilometers.

Otahal was surprised by the relatively small amounts of fat put on, compared to eastern migrants.

"Western migrants may have a different strategy, picking up a little mass and moving on to the next site in little hops, as opposed to the larger hops and bigger gains of eastern birds," he says. Otahal speculates that this difference may be due to the habitat differences, with western migrants having to find islands of seasonal food and water in a largely dry landscape while eastern migrants have large, contiguous wooded areas in which to rejuvenate.

Otahal says more warblers have been visiting Coyote Creek lately. "The water district's small creek restoration sitehas brought more birds in," he says. The species is closely associated with riparian areas, depending on the cottonwood and willow trees of creek banks for resting and breeding grounds. Farming and urban development in the warbler's historic creekside breeding grounds in the Central and Santa Clara Valleys has resulted in a "dramatic decline" in California breeding populations since the 1930s, according to Otahal.

But others among the seven neotropical migrant species banded at the Coyote Creek site were worse off. Most of the Pacific Slope flycatchers, for example, had used up all their fat reserves and were starting to burn muscle tissue during their stay at Coyote Creek. "These birds are so stressed, there's no room for error," says Otahal. "If this site disappeared, it would be hard for them to make it any further."

Contact: Chris Otahal (408)262-9204

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