During his lonely wanderings in the wilderness of the Sierra Nevada, naturalist John Muir was often cheered by an encounter with the Water Ouzel, a tiny passerine that we now call the American Dipper. The bird he considered "a singularly joyous and lovable little fellow" and "the mountain stream's own darling, the humming-bird of blooming waters" became the subject of Chapter 13 of his "Mountains of California" . Thought to be most closely related to wrens or thrushes, this smallest of avian divers (only 2 oz!) in North America has some interesting adaptations not shared by its non-aquatic cousins. Let's have a look at the behavior that so endeared the Ouzel to Muir, and at the adaptations of the bird that "leads a charmed life beyond the reach of every influence that makes endurance necessary."
It would be amongst the rocky, turbulent stretches of a mountain stream that Muir would most often encounter the Dipper, for this bird rarely strays more than a few meters from the water's course. For it is along these creeks that the Dipper dives and swims underwater to glean aquatic insect larvae and small fishes from the pebble-strewn bottom. Unlike many diving birds, the Dipper does not have webbed feet, but rather propels itself underwater with powerful strokes of its wings. Compared to the typical thrush, the American Robin for instance, the Dipper has several anatomical modifications for diving. These include nasal flaps to exclude water from the nostrils as well as a musculature that facilitates a swimming motion of the wings . Studies suggest that the Dipper has emmetropic (i.e., normal) vision both above and underwater, due to a much better development of muscles (iridial sphincter) that change the shape of the lens to accommodate the differing refractive indices of air and water . These adaptations, along with the fact that they can store more oxygen in their blood than most non-diving birds because of a higher hemoglobin content per red blood cell, allow the Dipper to remain underwater for as long as 30 seconds for dives typically of 6 inches to 2 feet in depth . On the subject of eyes, the "winking" of the dipper, that flashing of white across the eyes so commonly observed, was often attributed to the nictitating membrane (a translucent third eyelid that can be drawn across the eye to protect it while still permitting vision). However Goodge  showed that it is the upper eyelid, with its complete covering of small white feathers (lacking in its passerine cousins), that becomes visible when the bird blinks (often 40-50 times per minute!).
In addition to diving, the Dipper is often seen wading with its head underwater while foraging for larvae in the shallows. Muir poetically described how during such wading "the swift current is deflected upward along the glossy curves of his neck and shoulders in the form of a clear, crystalline shell, which fairly incloses him like a bell-glass". This brings us to another adaptation: an enlarged uropygial gland that secrets special waxy substances called "uropygiols", commonly referred to as preening oils, that waterproof the feathers .
Dippers have unusually dense plumage compared to their passerine cousins. While a typical passerine will have on the order of 3000 contour feathers (the largest feathers of a bird, giving it the characteristic shape; includes flight feathers), Googde  counted up to twice that number on Dippers. More importantly, Dippers have a very heavy coat of down between the feather tracts (areas between rows of feathers that consist of naked skin in most other birds). It is this dense plumage and lofty insulation that allows these non-migratory birds to remain in the mountains over winter, even in Montana where the Dipper must endure air temperatures as low as -40 ºF . But this insulation becomes a liability during the warm season, when air temperatures can soar above 90 ºF. At first glance the bare legs of the Dipper would give the impression of cold feet, especially when the bird stands for prolonged periods in icy waters. But it is through controlled thermal conductance that the bird is able to regulate under these temperature extremes. During the heat of summer, as long as the Dipper has access to cool water, it can regulate its temperature by vasodilation of blood vessels in water-immersed legs. During the cold of winter, these blood vessels can be constricted to reduce heat loss .
Perhaps it was these physiological adaptations that allowed the Dipper to endure the icy torrents of a winter steam, giving the bird the undaunted character that Muir admired. For a creature well accustomed to wet and cold of swift currents and waterfalls, inclement weather has no effect on the Dipper, though it subdues the other mountain birds and has them cowering in the shelter of the trees. Physiological adaptations aside, it was undoubtedly the indomitable and cheery song of the Ouzel that lightened Muir heart. "He must sing though the heavens fall", wrote Muir. Look for this tiny streamside denizen on your next trip to the backcountry and delight in its cheery spirit.
References and Further Reading:
 Muir, J., "The Mountains of California", The Century Co., New York, (1894).
 Grinnell, J. and Storer, T., "Animal Life in the Yosemite", University of California, Berkeley, (1924).
 Googde, W. R., "Adaptations for Amphibious Vision in the Dipper (Cinclus mexicanus)", J. Morphol., 107, 79-91, (1960).
 Murrish, D. E., "Responses to Diving in the Dipper, Cinclus mexicanus", Comp. Biochem. Physiol., 34, 853-858, (1970).
 Googde, W. R. "Structural and functional Adaptations for Aquatic LIfe in the Dipper (Cinclus mexicanus)", thesis, University of Washington, (1957).
 Bakus, G. J., "Observations on the Life History of the Dipper in Montana", The Auk, 76, 190-207, (1959).
 Murrish, D. E., "Responses to Temperature in the Dipper, Cinclus mexicanus", Comp. Biochem. Physiol., 34, 859-869, (1970).