I tried one last time in mid-December to fill my caribou harvest permit from the Kenai Mountain herd. After post-holing on snowshoes for the better part of two days, it took my buddy and I all of 15 minutes of glassing upper Cannonball Creek before deciding to abandon the hunt. The short winter days, the minus 20 degrees, the 3 feet of snow, and the almost total absence of wildlife other than a handful of ravens, chickadees, redpolls, and gray jays made us think twice about continuing on.
While taking a third day to back our way down the almost frozen Resurrection River, we flushed an American dipper looking very healthy and amazingly ice-free. To see this Robin-size bird bobbing and “dipping” in a small pool of open water surrounded by ice ramparts, even as I flicked a two-inch icicle off my moustache, got me thinking about how this was even physiologically possible in the winter.
The dipper or water ouzel is the only aquatic songbird in North America. The dipper spends a lot of its time walking and “swimming with its wings” underwater in pursuit of aquatic insects, fish, and even salmon eggs. They sometimes dive into the water from above like a kingfisher or float on the surface pecking at prey. Anybody who has spent anytime hiking or fishing along mountain streams on the Kenai Peninsula has run into these birds during the summer.
What I didn’t appreciate prior to my recent encounter is that dippers tough it out through the winter. In Arthur Bent’s life history series on birds, he notes that the dipper “is a hardy mountaineer, indifferent to cold and impervious to it. He lives all winter as far north, or as high up in the mountains, as he can find open water.” In Alaska, dippers have been found at ice-free springs on the North Slope and as far west as Norton Sound at minus 50 degrees!
How is this possible for a bird that weighs 2 ounces when bears hibernate and most other birds fly south? It turns out that dippers are well adapted to cold water environments. They have unusually dense plumage with 40 percent more contour feathers than similar-sized songbirds, presumably to buffer against the wear and tear of foraging in abrasive stream currents. Dippers have heavy down between feather tracts like waterfowl, which means that with less than a 3-fold increase in their metabolism, dippers are able to maintain normal body temperatures when it’s as cold as minus 40 degrees. They keep their feathers well waterproofed by preening frequently from a uropygial or oil gland at the base of their tail that is 10 times larger than most birds their size.
Dippers have relatively more hemoglobin than most birds so their blood is better oxygenated for extended dives that average about 30 seconds. Upon submersion, their heart rate immediately drops, followed by a further gradual decline so they burn less energy. And, although they are songbirds, they have nasal flaps like truly aquatic birds to prevent water from getting up their nostrils.
The dipper’s eyesight is critical to its ability to forage underwater in fast, cold streams. So their iris’ sphincter muscle is enlarged to accommodate underwater vision. A third, transparent eyelid called the nictitating membrane closes like a curtain (rather than up or down) to protect their eyes. And feathers cover their entire eyelid, presumably for added insulation.
These unusual adaptations allow dippers to overwinter almost anywhere there is open water. Rather than migrating to warmer climates, as most birds do, dippers move to wherever there is open water, whether it occurs further downstream, in adjacent watersheds, or the Cook Inlet. It is precisely because dippers have high fidelity throughout most of their life (reportedly up to 10 years) to a single watershed that they are superb bio-indicators of local water quality. In one study conducted in Italy, dippers were present in 93 percent of clean streams but absent from 94 percent of polluted streams.
As a carnivore, dippers accumulate contaminants into their bodies (particularly in fats) through the food chain. A recently published article in Environmental Science and Technology reported that contaminant loads in dipper eggs can be indicative of the mercury, chlorobenzenes, and polychlorinated biphenyls (PCBs) that are carried into stream ecosystems by salmon returning to spawn. These lipophilic contaminants are picked up by salmon as they forage in the open ocean, passed through salmon eggs and fry to foraging dippers, which are in turn passed to their own eggs. As Sir Walter Scott once wrote, “oh what a tangled web we weave.”
If you’re looking for something to read during these first dreary days of 2013, download chapter 13 from John Muir’s book, “The Mountains of California.” This whole chapter is lovingly devoted to Muir’s favorite bird, the water-ouzel, the only songbird that sings year-round. Muir writes “the ouzel never sings in chorus with other birds, nor with his kind, but only with the streams … the deep booming notes of the fall are in it, the trills of rapids, the gurgling of margin eddies, the low whispering of level reaches, and the sweet tinkle of separate drops oozing from the ends of mosses and falling into tranquil pools.”
So even if I didn’t get a caribou, I’ve rediscovered a pretty cool bird.
John Morton is the supervisory biologist at Kenai National Wildlife Refuge. You can find more information about the refuge at http://kenai.fws.gov or http://www.facebook.com/kenainationalwildliferefuge.