On the trails: Searching for food, winter dippers survive

Posted: Sunday, February 17, 2008

Not long ago, after a wonderful snowfall that left more than a foot of gorgeous, soft snow in all the open areas, I snowshoed along Switzer Creek, rediscovering just how old my legs really are. Fortunately, there was no hurry - indeed, there was good reason to plod slowly along with frequent stops. The lower part of the creek was lively with American dippers, which nest along mountain streams but can be found near many kinds of aquatic habitats in winter.

Dippers are feisty, often very aggressive, songbirds, even in winter when they are not defending nesting territories. Switzer Creek is an important winter feeding area for dippers, because it is spring-fed and does not freeze over. Dippers typically feed on underwater prey, including aquatic insect larvae and small fish, so an ice-covered stream is a poor foraging prospect. Therefore, when other streams are covered with ice, little Switzer Creek can be a popular place. On this day, I guessed that there might be as many as 12 dippers here, in less than a mile of stream. It was hard to be certain how many birds were present, in part because they often lurked under overhanging snow ledges on the streamside, in between foraging bouts.

At times there were vigorous chases, two or even three birds doing loop-the-loops over the stream or making wide circles out over the meadow. Because the birds were not banded, it became impossible to keep track of who was who, or how many there really were.

Dippers in winter, as in summer, commonly feed on the aquatic larvae of mayflies, stoneflies, and caddisflies. On this snowy day, I watched one dipper spend about three minutes to extract a caddisfly larva from its tubular case. The birds got a nice, juicy two-inch-long morsel for its efforts.

In addition to aquatic insects, dippers also eat small fish. They sometimes eat yolk-sac fry of various salmon species, pried up from the gravels, or catch small, free-swimming juvenile salmonids and sculpins. Here in Switzer Creek, much to my surprise, I once watched dippers catch tiny starry flounders about half an inch long. It took me a while to figure out that those odd-shaped prey that the dippers were gulping down were actually fish - and then I needed to get a fish biologist to tell me what they were.

It is unusual to see so many dippers in such a short reach of stream. Twelve or even just 10 dippers in 5,000 feet of stream is a real crowd, given the feisty nature of the birds. The other streams around here were almost totally frozen at this time, so there weren't many places for dippers to feed. These usually solitary birds also congregate at salmon runs in the fall, feasting on drifting, improperly buried salmon eggs - a nice glob of fat to be had with little effort.

Wintering dippers can also be found on the deltas of local streams, where the fresh water joins salt water. At low tides, fresh water channels flow over the tide flats. The deltas of Sheep Creek, Gold Creek, and Auke Creek are good examples of sites to observe foraging dippers in winter. Here they capture small fish, along with lots of amphipods (tiny shrimp-like crustaceans). Dippers flip over flat stones and often uncover dozens of amphipods huddled together. A dipper that finds such a trove can chug down over 50 amphipods in two minutes. However, this kind of prey provides relatively few calories and little fat, compared to insects or fish, so it is not very rewarding. Consumption of fish is probably important to dipper survival in winter.

Very cold weather appears to cause high mortality of dippers here. Over the past few winters, we have observed that fewer of our banded individuals come back to their nest sites after severe winters than after milder winters. Although dippers move around a lot in winter, and some may go, say, to Petersburg, they are likely to return to their nesting streams if they survive. If they don't return, they are probably dead. Ice-cover on the streams in cold winters is only part of the story, however, and I'm guessing that cold stress for this high-energy bird may be a serious problem.

Right now, our local population of dippers is depressed, because the past two winters have been relatively severe, with prolonged very cold periods (compared to the two winters before that). As a result, many regular nest sites were unoccupied in the past two springs. It will be interesting to see how long it takes for the population to recover.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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