Daring to be Different
Posted by Bob Reitsma on April 1, 2008
The game of survival is all about carving out your niche and being successful in it. Within the natural world, few birds play this game better than the American dipper (Cinclus mexicanus). As species upon species are faced with an imposing set of survival obstacles, one can only compliment the American dipper on a niche strategy that represents such simple strokes of genius.
Many birds search for insects around the edge of streams, but stop at the waters edge. Dippers are one of the few that have taken to plunge and become submariners of the insectivorous bird world. Dippers spend their lives searching for aquatic insects (mostly mayfly and caddisfly larvae) withstanding chilly, swift-flowing water and often diving down into frigid whitewater with a few quick wing beats.
They are capable of remaining submerged for up to 15 seconds by counteracting their natural buoyancy with powerful strokes of their wings. Often one can observe these intrepid creatures flying directly into chilling rapids too swift for people to stand in, then see them pop up somewhere upstream with a prey item in their mouth.
Although clearly at home and well-adjusted to the rigors of a mountain stream, occasionally dippers get into trouble in unusual ways. For example, the capture and consumption of a dipper by trout has been reliably reported. It is not know whether any other species has met such a fate. And imagine the surprise of the Utah fisherman who must have selected just the right fly to hook a wiley dipper instead of the flashing rainbow of his dreams.
For anyone watching (not catching) dippers, the question is: how do they stand their living conditions? Shaped by millions of years of evolution, dippers are exquisitely designed for live in and around cold streams. For starters, dippers have a preen gland containing waterproofing oils that is 10 times larger than that of any other songbird. Like a painter dabbing into a vessel of paint, dippers dab their bill in this gland for a thicker waterproof coat.
Secondly, dippers have a very thick undercoating of down feathers to give them the necessary insulation for enduring such frigid air and water temperatures. It?s because of this thick layer of down that dippers have been spotted as far as 14 kilometers above the Arctic Circle.
Further standard features that allow dippers to do their thing include a movable flap over the nostrils which closes when the dipper is under water, a highly developed nictitating membrane, or third eye lid, which acts as a windshield wiper when underwater, and higher concentrations of hemoglobin in the blood which allow greater quantities of oxygen to be stored for lengthy periods of underwater foraging.
Affectionately referred to as the Water Ouzel (or Water Thrush) by the early naturalists, the American dipper is North America?s only truly aquatic songbird. As such, the dipper does not fit the mold of an aquatic species, its gray chunky body and bobbed tail giving the impression of a cross between a thrush and a wren.
The North American species occurs in fast-flowing mountain streams and coastal streams along the Pacific coast of western North America and more sporadically in pockets along the Continental Divide into Mexico and Central America. Other dipper species are scattered around the globe,with two in South America and two more in Eurasia.
The dipper year starts with breeding, which must wait until at least March, when the ice breaks up and water flows abundantly from snow melt, and continues into the early summer. Ideal nest sites are often located on a streamside cliff ledge and less often behind a waterfall, or, when available, under a road bridge. The female builds a domed structure with a side entrance that faces the water, sometimes with a canopy that overhangs the entrance to protect from streamside spraying.
With such an early start to the breeding season, dippers commonly build their nests in freezing temperatures and weave coarse grasses and leaves together as a water repellant inner coating then adding a thick mossy outer shell to protect the cozy interior from frequent streamside water spray. To enhance the structural integrity of the outer shell, dippers materials in water briefly before weaving them into the nest.
The female does all the incubating and brooding while the male feeds her. She does most of the feeding of nestlings as well. Upon fledging, parents split the brood and the territory. After fledging, parents remove the inner lining of the nest to deter fledglings from using the nest and also to clean out and prepare the nest for the possibility of another brood in the same or following season.
Unlike most birds, dippers will reuse old nests or build on top of old ones because suitable nest sites are hard to find. It has been documented that a nest site of a European dipper in Scotland was used for 123 years! With a nest perched just above water level, it should not be too surprising to see just-fledged young dippers swimming in and hopping around the edge a stream.
With the cessation of nesting, dippers (like so many other birds) completely shed and regrow their feather coat—a process known as molting. In this activity, they are much more similar to waterfowl and other aquatic birds, then they are to their songbird relatives. Whereas most songbirds molt their wing and tail feathers two at a time, dippers and ducks shed these feathers all at once rendering the bird flightless for a short period of time.
Also remarkable is that instead of the process taking weeks, the dipper puts on its new feather coat in only 4 to 14 days! The method in this madness lies in the fact that American dippers are heavy relative to their wing size due to their thick feathered coats. Dippers need every ounce of power their wings can produce to create needed lift so even the loss of a couple of feathers can impair their flight. So it pays for them to shed and regrow the feathers as quickly as possible by doing it all at once.
After molt, most dippers journey farther downstream to escape the ice build-ups of fall and winter. Some individuals may travel several hundred kilometers to a rocky sea coast or the mouth of a large river. In the absence of freezing, howeer, some birds remain on the breeding grounds for the winter. For all dippers, freezing and thawing cycles within a wintering season can prompt short trips up or downstream.
Much larger densities of dippers occur on the wintering grounds, compared to breeding grounds, making competition for food resources stiffer and territorial behavior more intense. As the ice breaks, dippers return upstream to nest and start the dipper year anew.
Anyone who has camped along a rushing stream knows that communication (or lack thereof) is an enormous problem. Therefore, the song of the American dipper is especially loud and carries over large distances. Yet it isn?t just the volume of the song that sets the song apart. Its sweet, bell-like tones can go on for 10 minutes or more. In addition, like few other species, both the male and female sing indistinguishable songs. Perhaps the early naturalist John Muir put it best:
“The more striking strains are perfect arabesques of melody, composed of a few full, round, mellow notes, embroidered with delicate trills which fade and melt in long slender cadences. In a general way his music is that of the streams refined and spiritualized. The Ouzel never sings in chorus with other birds nor with his kind but only with the streams.”
Sometimes vocal communications do not cut it in very noisy places. Therefore it may not be surprising that the dipping behavior (which has other functions as we will see) combined with white-eye lids allows dippers to catch the attention of other dipprs without raising their voice.
Which brings us to the most essential question concerning the dipper: why do they dip?
We do know that the dipping (or bobbing) gets more intense when the bird is agitated and we also know that dipping behavior occurs in other birds that forage along streams (for example, spotted sandpipers and waterthrushes). A couple of theories have been put forth: dippers are better able to pinpoint locations of aquatic prey items by taking visual information from more than one vantage point; and, dippers sometimes communicate with each other by dipping instead of vocalizing when stream noise makes movement more detectable than song.
Streams are under environmental threats everywhere, from damming and channeling to human disturbance and the downstream flow of contaminants. How these have impacted overall numbers of dippers in North America is not known, since the most comprehensive surveys of birds are tied to roads or study plots that are not conducive to streamside monitoring.
In general, the consensus of dipper biologists hold that the higher elevation populations are holding there own, but the downstream dippers are suffering. Perhaps the gibbest culprit is development and changes in stream hydrology. However, on the positive side, nest sites appear to be the most limiting to breeding dippers, and the advent of bridges and other structures have provided new nesting opportunities for some populations.
Food is limiting for most of the year, as cold-fast streams are often relative unproductive to begin with. Decreases in streamside shading, siltation from agricultural and logging areas, and the input of fertilizers and toxins from cities and farmfields can reduce the abundance of macroinvertebrates and fill them with detrimental chemicals.
We do not yet know the true impact of development and pollution, but American dippers are used now in toxicology studies as indicator species how stream health is affected by everything from mining waste pollutants in Colorado streams to atmospheric pollutants in British Columbia. One study found that dippers living year-round in lower elevation streams had higher levels of atmospheric pollutants than migratory individuals breeding in higher elevation streams. Interestingly, the difference was tied to the relatively greater amount of salmon eggs eaten by the lower elevation dippers.
To be sure, all bird species, be they waterfowl or warblers, are unique. But one must admit that the American dipper is an organism of so many vivid surprises, and that it ranks among the highest in piquing the curiosity.