Dirty little secrets: "dress for success" is the key to the mating, game among Arctic ptarmigan

Natural History, June, 2004 by Bruce Lyon, Robert Montgomerie

Most birds act as if cleanliness really is next to godliness. Watch any bird for a while, and you will see that it spends a lot of time preening its feathers and bathing in water or dust. Feathers are essential for flight, waterproofing, and insulation, so it is not surprising that maintaining them is a vital part of a bird's daily routine. How to explain, then, the bizarre sartorial metamorphosis we have observed in the male rock ptarmigan, a species of grouse? In just a couple of days in early summer the male ptarmigan suddenly transforms himself from an immaculate, pugnacious white bird that stands tall on large boulders, to a filthy, bedraggled creature that skulks about on the tundra. Why do these birds get so dirty? Equally intriguing, why are their feathers so white and conspicuous to begin with?

Charles Darwin was one early naturalist who took an interest in such plumage changes. Probably referring to the willow ptarmigan, which winters in the boreal forest and flies north in the spring to breed in the Arctic, he argued that the species" superb camouflage--white in winter, brown in summer--supported his idea that natural selection shapes the traits that increase an animal's ability to survive. To buttress his case, he noted that the birds often suffer intense predation in the spring, when the snow melts and the once-camouflaged white plumage stands out dazzlingly against the brown tundra. Our own study of rock ptarmigan in the Canadian High Arctic, assisted by Karen R. Holder, currently a lecturer in biosciences at Loyalist College in Belleville, Ontario, confirms that the bird's strategy as a camouflage artist follows a predictable pattern, at least in females. As it happens, though, the story for males is more complicated than Darwin realized.

For thirteen springs in the 1980s and 1990s, we headed for Sarcpa Lake, on the remote Melville Peninsula at the top end of Hudson Bay. To get there we had to hop and skip from place to place on commercial and chartered airlines. Some years we could take a six hour commercial flight from Montreal that would land more than 1,800 miles to the north, at Hall Beach (population about 625), in what is now the territory of Nunavut. >From there we would charter a Twin Otter to fly us the final sixty-mile leg of our journey, transporting all the gear and food needed for a six- to eight-week stay. Our field station was a former Distant Early Warning Line radar site, abandoned in the 1960s when satellites became the method of choice for watching for Soviet invaders from over the pole.

We usually arrived at the end of May, when the treeless tundra was still covered with snow. At that time, flocks of migratory shorebirds and waterfowl are yet to arrive, but small coveys of rock ptarmigan are already roaming about, looking for exposed seed heads to eat. Dressed in white ever since the preceding September, they have spent the winter on the snowy tundra as far south as the tree line and are all but invisible against the snow.

Once settled on their territories, ptarmigan were spread thinly over our five-square-mile study area. A typical day of fieldwork involved walking for miles as we followed and watched the birds. Our first priority each year was to find out which banded birds had returned from the preceding year and which birds needed to be captured and banded for the first time. The ptarmigan at our site were ridiculously tame, making it easy to catch them with a noose at the end of a twenty-foot-long pole. We found it hard not to giggle while slipping a wobbly noose over a walking but oblivious target. Once birds were individually color-banded, we spent our days recording the color and condition of their plumage and documenting the birds' daily activities to see which males were successful at attracting females. The hard part was that the birds are active twenty-four hours a day, since the summer sun never sets in the High Arctic.

The Arctic spring is brief and intense--the transition from a snowy winterscape to the brown tundra of summertime seems to happen overnight. When the color of the landscape changes, females shed their white plumage as brown replacement feathers grow in. This transformation makes good sense, because the females are entirely responsible for incubating the eggs and tending the chicks. The low-growing vegetation on the treeless tundra provides nothing in the way of protective cover, and the Arctic has many predators that find ptarmigan tasty. From overhead, nests and chicks are vulnerable to ravens and jaegers, and adults are exposed to gyrfalcons and peregrines. From the ground, Arctic foxes, ground squirrels, and people are a persistent threat. The mottled brown summer plumage of females helps conceal the nest and chicks from those unfriendly eyes.

The effectiveness of the female's spring camouflage is as uncanny as that of her white plumage in winter. We once found ourselves crawling on hands and knees in a small patch of tundra to rediscover a ptarmigan nest we had found only a few minutes earlier. Motionless and blending almost perfectly into the surrounding heather and lichen, the female was as close to undetectable as an animal can get. Her mate, though, nervously watching us from atop a nearby boulder, was anything but cryptic. His white plumage practically glowed against the dark tundra.

The males, in fact, delay their molting until about a month after the females molt. Their spring plumage has two unusual features. In most bird species the males sport conspicuous, even gaudy plumage during the breeding season, often changing their colors through a feather molt. Although the male ptarmigan also acquires special breeding colors, in this case the colors come tree of charge, because it is the background that changes. Then, a few weeks later, well after the snow has melted, the male's color finally does change, but not just by molting. All the males in our study population changed color during a two-week period simply by getting dirty: literally bathing in dirt. Dirt-bathing precedes their molt into the brown summer plumage by a couple of weeks or more.