Juneau's dippers appear to be thriving along healthy streams

Posted: Sunday, September 05, 2004

You'll see them along streams - dark gray birds smaller than a robin. They'll fly low, following the course of the stream, and wade or dive into the water to capture aquatic insects or small fish. At rest, they'll perch on a rock or a log and bob up and down.

They're American dippers, Southeast Alaska's only songbird that regularly swims.

Juneau ecologist Mary Willson has been studying dippers for two years. Her goal is to document where dippers live around Juneau, what they eat, how successful they are at nesting, and how their territories change from summer to winter.

Studying dippers is very hard work. When I interviewed Willson at her home in the Mendenhall Valley last week I asked her why she was doing it.

"Dippers are sensitive to changes in stream quality," she said. "Increased sediment, or pollution like acidification or junk in the streams, diminishes their food supply, nesting success goes down, and eventually if the stream deterioriates too much, the birds just go away."

Dippers are also not censused adquately in standard bird censuses like the Breeding Bird Survey in spring or the Christmas Bird Count, she said. "Dippers live along streams, not in forests or open fields or along roads, where most counts take place. If you want to know how many there are, you have to focus on streams."

"We need to have a baseline," Willson said. "If we know how many birds there are and where they are, we have a standard of comparison if something changes or goes wrong. We should get that information as soon as possible and not wait until something happens."

Willson is conducting her dipper surveys with help from biologists Kim Obermeyer and Kathy Hocker, and nearly a dozen volunteers. "This project requires lots of legs and eyes," Willson said, "and it's exceedingly time-consuming. You have to walk up streams, find birds, then try to follow them. Sometimes it can take all day to find a nest, or even two days."

What Willson described sounded to me a little like a fly fisherman's nightmare, or maybe a scene from an Alaskan episode of Survivor. You walk upstream wearing chest waders and felt-soled boots. You work your way over boulders and fallen trees. When you reach a waterfall, you scramble up the bank and bushwhack around the obstruction (nothing to it in your clumsy waders and felt boots, right?), then you scramble back down into the stream again and keep lookin'.

Dippers are easy to spot when they're building a nest or feeding chicks, Willson said, because they'll be going into and out of the nest carrying nesting material or food. But if you arrive when they're incubating eggs, they'll be staying still for long periods of time, so it's hard to see them.

"Sometimes it's profitable to work in pairs," Willson said, using walkie talkies to stay in contact to cover a bigger stretchof stream. "Usually if you look long enough and the birds are there, you'll find them," she said cheerfully.

And your reward? If you do find a nest, that means you or some other lucky member of the team will get to make the same hike (which might take three hours round-trip) every few days during the next few weeks to see whether the dipper youngsters survive and ultimately fly out of the nest.

"Once a nest is located, we plot it on GPS so it can eventually be mapped," Willson said. "We describe the nest site, and measure the habitat - stream flow, width, pH (acidity), substrate (bedrock, gravel, etc.), and vegetation on the banks. Sometimes we sample insect populations in the stream (mayfly, caddisfly, or stonefly larvae, for example), and in one pilot study we surveyed small fish (which dippers also eat).

"Then we band as many adults as we can so we can track them in winter and during the next nesting season.

"Banding takes two people," Willson said. "We stretch a nylon mist net across the stream. (It has a very fine mesh like a hair net. The dippers can't see it, and it is so soft and fine it doesn't hurt them.) Once we capture a bird, it takes only about five minutes to extract it from the net, weigh it, put two plastic or aluminum bands around each leg, and let it go. The bands appear not to bother the birds at all, even in the water.

"Sometimes we can capture both adults within 30 minutes; sometimes it takes several hours or a second visit."

Willson and her teams have surveyed about 34 streams on the mainland around Juneau and on Douglas Island. They've found that dippers nest only on streams of a certain minimum size, and where streams have available nest sites, such as cliffs, root wads, or boulder heaps. They can also nest under bridges if there are cross-beams or I-beams to support a nest.

A dipper nest is a ball of moss about the size of a volleyball, with an entrance hole on the side. The inside may have a little pad of grass in the bottom. "It's a very protected nest," Willson said, "and the chick survival rate is right up with that of birds that nest in tree cavities, like woodpeckers or chickadees."

Willson and her team have documented 90 percent survival of chicks in the nests they've studied - much higher than that of robins, warblers, or sparrows that use open, cup-shaped nests. Five dipper pairs successfully raised two broods this season, and one raised three, which appears to be a record for Southeast Alaska.

Come winter, dippers often move downstream, as ice covers a stream's upper reaches. They may also move to different streams, as birds become less protective of their territories during winter. Willson says you can often see them in the intertidal zone, like below the bridge at Sheep Creek, or on Lemon Creek, Salmon Creek, Wadleigh Creek, or near the mouth of Auke Creek.

"We banded 30 adults this year and a dozen or so last year," Willson said, "so we can identify specific individuals. We know where they nested, and if I can find them, we'll know where they spend the winter."

• Marge Hermans and Mary Willson are members of Juneau Audubon Society. Check the Web page www.juneau-audubon-society.org for lecture and meeting schedules.



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