Winter white, summer brown

patterns of change in a seasonal world

This winter I’m seeing snowshoe hare tracks very commonly, in a variety of locations. Sometimes, repeated travels created well-packed hare highways through the brush. But I certainly don’t see the track makers very often, perhaps partly because I’m not out there at the times that they are most active. Another reason is that they are usually quite well camouflaged, with white fur in winter and brown fur in summer. Sometimes, however, the timing of the molt is not in synchrony with the background colors of the environment, and there’s a serious mismatch between hare and background: the hares become quite conspicuous as white patches on brown background or brown patches on white background.

There are at least ten mammal species in North America that show this kind of seasonal coat-color change, as well as some in Eurasia. In Alaska, we have the snowshoe hare, the Alaska hare, the short-tailed weasel or ermine, the least weasel, a species of collared lemming, and the Arctic fox. The ermine and snowshoe hare are the only ones in Southeast. Some of these species have large geographic ranges extending southward, where snow is less common, and the change to white doesn’t happen there; in contrast, hares in the Canadian High Arctic may not change to brown. In some cases, there is variation among individuals within a single population, some showing the seasonal change and others not. The general consensus seems to be that the observed coat-color changes are adaptive principally in providing seasonally appropriate camouflage and protection from visually hunting predators. Winter coats are often thicker and better insulated as well, but apparently the color itself does not greatly affect heat gain or loss.

As far as I can determine, in all species in which this has been studied, the physiological control of the coat-color change is driven mainly by photoperiod, or the relative lengths of day and night during a twenty-four-hour period, mediated of course by hormones. So if climate change brings less snow, but the physiological control of color change is still regulated by day length, there can be some serious mismatches between coat color and background. Seasonal timing may differ slightly for males and females, and for breeding animals vs nonbreeders. In some cases, temperature is thought to have a modifying effect: warm temperatures in fall, for example, may delay the molt somewhat, or hasten it in spring, but this effect is generally less than that of photoperiod.

In the bird world, ptarmigan engage in seasonal shifts from brown plumage in summer to white plumage in winter, and back again. Here too, the principal driver of change is photoperiod, and there can be timing differences between male and female. For example, willow ptarmigan females get their brown summer plumage earlier than males.

The most intriguing species is the rock ptarmigan. Males sport a showy white plumage in spring, for weeks after the snow has melted and their females have molted to a cryptic brown (which makes them very inconspicuous while incubating eggs in the nest). Males keep their conspicuous white coat until well after their female consorts have laid eggs and are no longer fertile. However, when egg-laying is underway, the males do something very unusual: they start to make their white plumage dirty, and by the time egg-laying is complete, they are very dirty—and much less conspicuous– indeed. But if the clutch of eggs is destroyed by a predator, or if the female is killed, suddenly the bereft males clean their plumage to a brilliant white again, and go a-courting once more. Most rock ptarmigan mate monogamously, but a few males are polygynous (with more than one female) and some may not obtain mates in any given year. The bachelors and polygynous males stay clean white longer than monogamous males, in keeping with their protracted potential for mating. Eventually, all the males turn brown, until it is time to turn white for the next winter.

Male and female rock ptarmigan in spring. Photo by Bob Armstrong

The rock ptarmigan use the seasonal pattern of plumage change for camouflage, like all the other animals that do the white/brown shift. But in addition, the males apparently use their white winter plumage as a sexual advertisement in spring and early summer, and female rock ptarmigan choose their males in part on the basis of this studly self-advertisement. There is no doubt a cost to showing off this way; rock ptarmigan suffer heavy predation, and those studly, conspicuous males are taking risks and paying a price.

Willow ptarmigan

gallant males and choosy females

One nice day in September, I walked with a few friends in a subalpine area with small shrub thickets scattered throughout a meadow of no-longer-blooming wildflowers. A subdued clucking sound in the brush caught our ears and we stopped to look.

A little family of about six well-grown willow ptarmigan chicks wandered out of the brush and into the meadow, heads down, busily searching for bugs, seeds, berries, buds, and catkins. The brown female was nearby, clucking gently and keeping watch. As they all searched and sampled whatever looked good, we noticed another bird, standing on a small rock not far away.

Photo by David Bergeson

This bird was resplendent in a plumage of rich brownish red, with white legs and belly. He was overseeing the foraging efforts of his mate and chicks, alert for predators. He stood there like a proud papa wearing a cut-away coat with tails and white breeches. Very handsome! And the family was still intact, chicks shepherded by both parents.

Willow ptarmigan are unusual members of the taxonomic group of grouse: males participate in parental care until the chicks become independent in the fall. Males of all the other kinds of grouse are intent upon courting females and showing off to each other and have no role in chick-rearing.

In early spring, male willow ptarmigan begin to establish territories, which can be over ten acres in extent. Territory borders are defended vigorously. Neighboring males may march, rather peacefully, side by side along a shared border; and territories are advertised by vocalizations (the ‘rattling’ call) and flight displays. More intense competition involves charging at each other, knockdown-dragout fights, and long-distance chases, often well outside the area of contention. Females arrive a bit later than males, and they may be aggressive against other females.

Females are choosy when selecting mates. They like males with large red ‘combs’ over the eyes, large territories, and vigorous displays. Although sexually mature at age one, the yearlings are less likely than more experienced males to attract a female. A male courts a female by fanning tail and wings, strutting, stomping and bowing, and ‘waltzing’ around her with his fans facing her. If she likes what she sees, they form a pair. Most pairings are truly monogamous, with little extracurricular activity (unlike many other birds). If both members survive, the pair may stay together several years (annual survival in northern B. C. was reported to be roughly thirty to sixty percent (slightly higher for males than females).

Although the female builds the nest and does the incubation, the male guards her and the nest, sounding an alarm if a predator approaches. He tries to protect the family by distracting the predator: feigning injury, leading the predator away. An intensive defense includes loud vocalizations.

Females generally lay seven to nine eggs, sometimes as many as fourteen. During the egg-laying period, before incubation begins, females often leave the nest and go foraging. They commonly cover the eggs with grass or leaves while they are away. During three weeks of incubation, she covers the nest and eggs herself at least ninety percent of the time, seldom leaving to find food. Both male and female defend the eggs and, later, the chicks. A study in northern B. C. found that, on average, about fifty-four percent of females successfully raise at least some chicks; predation on eggs can be more serious than predation on chicks, but even so, about half the chicks that hatch don’t survive more than a few months.

Chicks can feed themselves soon after hatching and can regulate their own body temperatures quite well after a week or so. But the female broods even two-and three-week old chicks if the weather is cold and wet. Males sometimes brood, too, and if the female dies, he can rear chicks by himself. Chicks can fly at age ten to twelve days, and they move around together with the parents, often ranging beyond the territory borders. The family stays together until fall. Occasionally, parents may adopt chicks from another family.

Photo by Bob Armstrong

As winter approaches, ptarmigan molt into the white winter plumage that camouflages them on snowy backgrounds. Not quite all-white—there are some black tail feathers. Come spring and the breeding season, male plumage turns reddish brown on head and neck, leaving the body white, while female plumage gradually turns brown (with white in the wings). As summer progresses, the reddish brown of the male spreads over the chest and back, leaving the belly (and most of the wings) white. That is why these birds are called red grouse in Scotland, and this was the plumage sported by the elegant fellow we saw.

All the birds will be white when winter comes. Winter plumage is denser than summer plumage and that, together with a seasonally greater metabolic tolerance of low temperatures and the habit of burrowing into the snow blanket, helps keep them warm.