CHAPTER 10
Plumage

TO THRIVE in the cold, grouse have evolved a dense plumage, and in this chapter we discuss the insulation provided by it. Because grouse are often abundant, they attract many predators, but they reduce the risk of attack through camouflage and strong flight, both of which are provided by their versatile plumage. Like stoats, ptarmigan turn white in winter, a phenomenon that has long interested naturalists, and they are also unusual among birds in having three seasonal plumages and corresponding moults. We discuss plumage and moult in willow ptarmigan and rock ptarmigan, which turn white in winter, and in red grouse, black grouse and capercaillie, which do not. All four species show changes in colour as individual feathers grow, in relation to changes in hormones. Finally, we describe the bills, claws and striking red combs of the four species, and mention recent research showing that the combs reflect ultraviolet light, which is unseen by man but visible to grouse.

THE NEED FOR MOULT

Feathers suffer wear and tear, both of which reduce insulation, waterproofing and flight ability. Each year, grouse grow new feathers of glossy bloom. In late summer and early autumn, adults undergo the main moult of the year, starting with the innermost primary, and including the primary and secondary wing-quills and main tail-quills, along with most feathers on the body.¹ This poses risks as the birds cannot fly well until new flight-quills have grown, but they accomplish the change in the warmth of late summer, when plumage insulation is least important and good food is readily available. Feather growth requires protein, which is frequently in short supply in habitats on poor soils. If you hold a moulted main wing- or tail-quill up to the light, you can sometimes see crosswise fault bars, mute evidence of checks in growth when a bird was short of protein or otherwise stressed nutritionally.

Birds with chicks must be able to fly well, and so they delay their moult. In red grouse and Scottish ptarmigan, unmated cocks moult first, next the failed hens, then paired cocks with young, and lastly hens with young. Mated male rock ptarmigan usually abandon their families sooner than mated cock Lagopus lagopus, and also moult earlier. Blackcock, cock capercaillie and unmated cock red grouse shed so many feathers in a short period that they skulk in tall vegetation, unwilling to fly and sometimes tail-less.

In addition to the main annual moult, rock and willow ptarmigan renew most body feathers to form a white winter plumage, and shed it to grow coloured spring feathers. Red grouse are in a class by themselves, as they are in almost perpetual moult on some part of the body, a slow process that should reduce nutritional stress.

There has been an attempt to classify avian moult and plumage objectively.² However, it is not easy to apply this to the complex generations of feathers in Lagopus, especially the protracted moults of southern races.³

PLUMAGE IN YOUNGSTERS

Young red grouse soon escape danger by fluttering at a week, flying 4-5m at ten days, and covering 100m by three weeks – or further with a tail breeze. Rock ptarmigan fly sooner, and at three weeks can scatter up to 400m when disturbed.⁴

Fluffy down covers day-old chicks. Wing-quills show more prominently in day-old capercaillie than blackgame, and in blackgame more than Lagopus lagopus or rock ptarmigan. These differences tally with incubation periods, as capercaillie embryos are longest in the egg, blackgame intermediate and the others shortest. Down covers the legs of all day-old grouse chicks, and the toes of Lagopus lagopus and rock ptarmigan. Newly hatched red grouse have shorter down and more bare skin on their toes and legs than willow or rock ptarmigan,⁵ a subtle adaptation to mild conditions.

The age of red grouse can be judged by their plumage development from a day old until they appear fully grown at 12 weeks.⁶ Rock ptarmigan grow slightly faster, and at ten weeks are hard to tell in flight from adults. Fully grown young grouse have pointed tips to their outermost two primaries, which they keep until the following summer. Older birds have rounded tips. Also, if the tip of the second outermost primary has more speckling than the tip of the third, the bird is young. This applies to willow, rock and white-tailed ptarmigan (dark speckling on white feathers), and to red grouse, blackgame and capercaillie (pale brown or creamy speckling on dark feathers).⁷

When blackgame or capercaillie reach six weeks, cocks can be told from hens by their larger size and, more reliably, by the first patches of black feathers. At three months, young cocks still have narrower tails than old birds. Young hens at three months are less bulky than old hens, but this is hard to judge unless a nearby adult hen affords easy comparison.

In autumn and early winter, youngsters have a ‘bursa of Fabricius’, a blind pouch inside the cloaca, obvious on dissection as an opaque creamy-coloured sac up to 1cm across, which helps develop their immune system. In old birds the sac is much smaller.

PLUMAGE OF BLACK GROUSE AND CAPERCAILLIE

Black grouse undergo their main annual moult from June to October or November, sometimes starting in May and ending in December. Greyhens with young start and finish later than hens without young, and later than blackcock. In June-July, blackcock moult some feathers on the head and neck, and grow new shorter, rounder feathers there with white or rufous edges or bars, and without after-shafts.⁸ These rather resemble greyhen feathers, and may indicate a low level of testosterone. Blackcock retain these feathers for a month, before shedding them during the complete autumn moult, when they replace them with characteristic dark blue glossy feathers that last until the following June.

Capercaillie also have one complete annual moult, cocks starting in May after their displays at the lek are over, and hens from late May to early July after chicks hatch. Both adult cocks and hens shed their body feathers from July until September or October. Like greyhens, capercaillie hens with young start and finish moulting later than broodless hens. Like blackcock, capercaillie males shed some feathers on their heads and necks in May-June, and grow new short feathers there with a brown colour or narrow grey bars, which are more like hen feathers. Capercaillie cocks shed these summer feathers in July during the main body-moult, replacing them with glossy dark feathers that last until the following May-June.

MOULTS IN LAGOPUS SPECIES THAT TURN WHITE

Rock, willow and white-tailed ptarmigan have three annual moults and subsequent plumages, including a white winter dress for camouflage, which occasionally has a rosy flush in live birds.⁹ The timing of the moults varies with climate.¹⁰ Summer dress in Norway begins to appear a month earlier than in colder, snowier Canada at the same latitude. The rate of moult and plumage growth also varies with climate, being fast in Arctic regions where summers are short, and slow in southern regions with long summers, such as Scotland.

PLUMAGE OF WILLOW PTARMIGAN

One annual moult in willow ptarmigan would suffice for plumage maintenance, but three are required for camouflage. Camouflage in spring and summer, however, does not require the physiological expense of a full moult (unseen white winter feathers can be retained in spring and summer). Also, especially further north, seasons are short and the requirement for camouflage can conflict with the need for cocks to display and for hens to retain the ability to fly well while rearing chicks. These conflicting pressures can lead to incomplete spring or autumn moults, and to differences between the sexes in the timing of moult (earlier in cocks than hens in spring, so that they can display, and later in hens than cocks in autumn, so that they can rear chicks).

Cocks show spring plumage before hens, growing a reddish nuptial plumage like a hood on the head, neck and breast while hens are still mostly white (see Fig. 11).¹¹ On Hitra and nearby islands off west Norway, cocks sprout the first of these feathers by late February, on the more snowy south Norwegian mainland during mid-March, and in the yet more snowy interior the feathers appear a fortnight later.¹² Subsequently, some hen-like feathers appear on the cocks’ heads, necks and breasts. Although these have been regarded as signifying a fourth moult,¹³ they involve only some of the feather follicles on the head, neck and breast,¹⁴ so in fact there may be only three moults.

Hens in spring and early summer finally catch up with cocks, and then surpass them by growing a more complete summer plumage, with less winter white retained on the underparts and no white on the back. Their summer feathers are heavily barred black and buff, with some creamy or yellow spots, and are much paler and more barred than male feathers. The cocks’ summer plumage is less dark than the spring dress, with some pale narrow barring. Cocks in southern races grow a fairly complete summer plumage, but high-Arctic cocks keep the chestnut hood and white back of the spring dress until about mid-July.

The sexes are much more alike in autumn than in summer. Autumn feathering has a richer brown hue, often blackish brown in cocks, and hens show darker upperparts than in summer, with finer barring and more vermiculations. Most hens start their autumn moult later than cocks and develop a less complete autumn dress, retaining more of the barred summer feathers.

Winter plumage is white, except for a black tail that is out of sight until the bird takes flight. High-Arctic birds already show many white feathers on their upperparts in late August and some are mostly white at the start of September,¹⁵ whereas in south Norway they moult later, the first white feathers appearing on the back at the end of September, leading to a white plumage by early November. Young birds become white later than old, and young from renest broods moult much later.¹⁶

On Hitra, where snow lies ephemerally and the mean temperature of the coldest month remains above freezing, birds delay winter moult until October, dark feathers still cover much of the back in January, and the white wing-quills carry much dark pigment.¹⁷ Snow in Newfoundland and Kazakhstan lies much longer than on Hitra, but in winter here the birds usually have some coloured feathers on the head and neck, and rarely on the back. In most Kazakh birds the central pair of greater upper tail-coverts is also coloured in winter,¹⁸ as in red grouse and, occasionally, Scottish rock ptarmigan. Although Kazakh winters are short, ending in March, midwinter temperatures drop to -45°C.¹⁹ The winters are also very windy, the shallow snow blowing about, to uncover a variegated landscape with patches of snow-free ground and withered grass. Winters in oceanic Newfoundland are far less cold than in continental Kazakhstan, but frequent thaws combined with a windy climate often create a speckled landscape with dark patches. In both places, a few coloured feathers afford camouflage in a variegated landscape with patches of darker, snow-free ground.

PLUMAGE OF ROCK PTARMIGAN

Seasonal plumages

In summer plumage, Scottish ptarmigan cocks are mainly grey, and hens golden brown although some have much grey. Cock feathers are finely barred with many fawn vermiculations, whereas hen feathers carry wide bars of black on a buff or orange ground colour, peppered with some white. The lower breast and legs remain mostly white, especially in cocks.

Autumn feathers on the belly and wing are white, but grey on the rest of the body, with fine vermiculations and a salt-and-pepper appearance. Scottish birds are a very pale race, silvery grey in colour. This camouflages them at a season when they spend much time resting among boulders that are grey from the colour of rock lichens growing on them. The hens’ feathers have slightly wider bars than those of cocks, far narrower than the wide bars of summer.

The winter plumage is white, save for black tail feathers and a black stripe from the cock’s bill to behind his eye. Hens occasionally have a few black dots around the eye, and rarely a black patch, though this is far smaller than in cocks.²⁰

Dark feathers in winter plumage

In countries with mild, relatively snow-free winters, a few dark feathers occur in the rock ptarmigan’s white plumage. Two documented examples are in the Cairngorms and on Amchitka Island in the Aleutians,²¹ where birds do not attain the fullest winter plumage until January or February. Even then, many have a few dark feathers on the back, neck and head, some retained from the autumn plumage and some growing in winter, the latter often with dark or black bases and white tips.

Amchitka hens have more dark feathers than cocks. This corresponds with cocks wintering on the snowier upland (highest point 373m) and hens on the lowland, where snow-lie is usually patchy and more coloured feathers afford better camouflage.²² Birds studied on the Cairngorms had fewer dark feathers during cold, snowy winters than in mild ones with little snow. This applied also to captive birds that had been reared from eggs laid by wild Scottish hens. They were kept in outdoor cages on lowland Deeside, which gets far less snow and warmer winters than the hills where the birds’ wild parents lived.²³ The captives also grew many more dark feathers in winter than their wild counterparts. It was not clear, however, whether these variations were due to differences in snow or in temperature.

Timing of summer plumage

Already in late February, male rock ptarmigan in the Cairngorms begin to grow the first dark spring feathers underneath their white dress. They show them externally in March, and are ahead of hens until mid-April (see Fig. 5), when the hens overtake them to form a more complete summer plumage.²⁴ The timing of the onset of spring moult changes little between years, but subsequent rates of moulting and plumage growth do vary. Birds darken later on Derry Cairngorm than on the lower, less snowy Cairnwell, and on a given hill they stay white later in a cold, snowy spring than in a mild one.

Amchitka cocks are also ahead of hens in March and until mid-April, when the hens pass them, and photographs of pairs in the Alps and on Attu in the Aleutians also show cocks ahead of hens in spring.²⁵ Cocks in such areas eventually have a dark summer plumage that is almost as complete as in hens.

On the Cairngorms in spring, territorial cocks with hens have more summer plumage than territorial unmated cocks, the latter have more than non-territorial cocks, and mated hens have more than unattached hens.²⁶ Also, mated cocks occupy larger territories than unmated ones, and are more aggressive and vociferous.²⁷ Hence aggressive birds get larger territories and grow summer plumage earlier, presumably because of higher hormone levels.

In the far more snowy subarctic and Arctic regions, a very different order of moult prevails after wintering birds return to their breeding grounds and take territories. Cocks are far behind hens in developing coloured plumage. Still white in early June, high-Arctic cocks appear conspicuous on snow-free ground, while hens soon grow a dark plumage that camouflages them when sitting on eggs or tending chicks. In late May, high-Arctic hens start to show coloured feathers, and they have a camouflaged summer dress by the time that they start incubating, when about half the snow cover has gone.²⁸ Cocks start to grow coloured feathers on the head at about the same time as hens, but this becomes so suppressed that they remain almost white in early summer after all snow has vanished, and are visible to the naked eye at a distance of 2km.

White Arctic and subarctic cocks in early summer

Naturalists have wondered why male rock ptarmigan in Arctic and subarctic lands keep their white winter plumage until after the snow has vanished, so becoming extremely conspicuous (see Figs 39 & 118), while the hens don a cryptic dark summer dress. The white cocks may well be more vulnerable to predators, and it has even been suggested that they thereby divert predators from the hens.²⁹

Ornithologists have long reported that the immaculate white plumage of the rock ptarmigan becomes dirty in midsummer.³⁰ Bob Montgomerie found that Arctic cocks intentionally soil their plumage as soon as their hens begin laying eggs, and the amount of soiling peaks by the time that incubation starts, making cocks six times less conspicuous to observers.³¹ In contrast, two cocks of medium dirtiness became immaculate again within 24 hours of their hens’ clutches being lost to predators. Hence the unsoiled white plumage may confer an advantage to cocks in sexual display. Scottish cocks are also white during their most intense period of sexual display, usually in February or early March.

Keeping a white male dress for display may involve what biologists call the ‘handicap principle’, where an individual carries a character that is a handicap (for example, is conspicuous to predators), and yet this is selected during evolution because it shows to a female that the carrier would make a good father for her offspring. One might put it: look at me, I avoid predators despite my handicap.

The question still arises why Scottish cocks in late March and April develop a partially dark plumage. This is less conspicuous than white plumage, even though it appears striking in sexual display to hens. The answer may lie partly in the fact that the spring display period in Scotland lasts much longer than that in the Arctic. Also, the period of snow-lie in Scotland is much shorter than in the Arctic. Retaining a white plumage throughout the long Scottish spring might simply be too dangerous to be worth the risk. Hence only the initial sexual display involves a white dress. By the time that the cocks begin to develop their partially dark plumage, most have already bonded with a hen and the dark plumage may suffice to maintain this pair bond, although some hens still change their partners. By late March-April, there may be less need for handicap-type displays and these might be more dangerous.

There is some evidence that white Arctic cocks incur heavier predation than hens. On Hrisey in Iceland, gyrfalcons killed a higher percentage of cocks than hens during the egg-laying and incubation periods.³² At Windy Lake in Canada, however, an area with one of the highest known densities of gyrfalcons in the world, they and other predators took cocks and hens equally during the incubation period and also in each month of the breeding season.³³ Furthermore, the prey taken to gyrfalcon nests at Windy Lake comprised similar proportions of each sex in May-June and July-August.³⁴ Also, those who studied rock ptarmigan during the breeding season in other Arctic or subarctic habitats at Eagle Creek, Bathurst Island and Ungava did not mention heavier losses of cocks to predators, or found no losses to predators (Svalbard).³⁵

Hence, heavy predation on cocks may not be typical of Arctic or subarctic habitats, and Hrisey may be an exception. In spring there, ptarmigan are found on dark-coloured heath with hardly any boulders, so the white cocks look starkly more conspicuous than cocks on nearby mountains with many boulders. Hrisey means ‘scrub island’, but the scrub that once existed and that would have provided better cover has long vanished owing to overgrazing by sheep. Also, ptarmigan density on the island is exceptionally high for Iceland, making it a magnet for falcons. None the less, the habitat affords such good conditions for breeding hens that it is a magnet for cocks, too, despite their heavy losses. It seems that sheep may therefore ultimately be responsible for the hen-skewed sex ratio there.³⁶ It would be enlightening to know whether ptarmigan in areas with better cover from boulders on the mountains only a few kilometres away, on either side of Hrisey, also show this skew, and whether cocks suffer heavier losses to predators than hens, but this has not been studied.

Although high-Arctic cocks are whiter in summer than those on subarctic Hrisey, they occur at such low density that it may be less profitable for predators to hunt them. Hence the unusually high ptarmigan density on Hrisey may contribute to the difference in the sex ratio of preyed birds between it and Windy Lake.

Timing of autumn and winter plumages

Arctic rock ptarmigan do not have time to develop complete autumn and summer plumages in the same way as Scottish birds. High-Arctic cocks grow coloured summer feathers on their necks and backs in late June, but only briefly and partially before starting to develop a coloured autumn plumage. Their autumn dress is less complete than in Scotland and Amchitka, and includes a few dirty white feathers from the previous winter. High-Arctic hens with chicks do grow a summer plumage, but they have little autumn plumage before starting to develop a new white dress for the rapidly coming winter. As a result, their autumn dress is as suppressed as the cocks’ summer dress. Hence both sexes in the Arctic largely sacrifice one plumage, the cocks dropping most of the summer plumage and the hens most of the autumn plumage.

High-Arctic cocks and hens are already growing white feathers in August, leading to immaculate whiteness by late September or early October. An adult cock in Baffin Island was sprouting winter feathers on 1 August,³⁷ and birds at Thule in north Greenland on 5 August.³⁸ By contrast, cocks in Scotland and on Amchitka in the Aleutians grow a complete autumn plumage, although Scottish hens with broods begin one or two weeks later than broodless hens and also cocks, and in the end attain a slightly less complete dress. Scottish and Amchitka cocks show the first white feathers on the back and nape in October, and hens grow winter plumage sooner than cocks. Young birds whiten later than old ones in the same year.³⁹

PLUMAGE OF RED GROUSE⁴⁰

Because red grouse are in almost continuous moult throughout the year, seasonal changes in plumage are gradual. They do not develop the white winter dress of rock and willow ptarmigan, and so there are only two moults and plumages.

Seasonal differences

Cocks begin their autumn moult in June, grow a set of new primaries and tail feathers by the end of August, and continue a slow moult of body feathers until midwinter. From August onwards, both sexes sprout darker body feathers than in early summer, with less barring, although hens remain paler than cocks. Sprouting feathers in hens become increasingly barred, until by late November or December the hens differ greatly from the cocks, being much paler and more barred.

By midwinter, most cocks are largely dark red, dark brown or blackish brown, with feathers that have few or no bars but fine vermiculations, whereas hens have a brown plumage with many feathers carrying conspicuous golden bars and spots. After December, cocks almost stop moulting for a month or two, but hens continue, albeit more slowly, growing feathers that become more barred with each month.

From March, cocks start to grow hen-like feathers, barred with light buff or yellow on the head and neck, but this proceeds very slowly and the new feathers do not appear in large numbers until May.⁴¹ During June, the whole head and neck may have these feathers, and the back a few. Hence the spring moult is only partial, gradually becoming heavier in June-July and merging with the autumn moult.

Hens, by contrast, show a fairly heavy body-moult in March-April, and grow feathers that are banded with wide yellow and blackish-brown bars. This slows down when they start to incubate, by which time the barred plumage provides good camouflage. Their body-moult then rises to a peak in July, when the new feathers resemble those growing on cocks quite closely, a characteristic associated with shrinking of the ovaries and testes to their smallest size for the year. Some hens that have lost nests or young, and that have not relaid eggs, can grow autumn plumage almost as early as cocks, whereas those with late broods still retain a few old primaries in October. Hens in poor condition with many worms delay moulting even more.⁴²

Seasonal differences relative to willow and rock ptarmigan

Some differences in moult timing between red grouse and willow and rock ptarmigan can be ascribed to behaviour. The equivalent of the spring moult and plumage of willow and rock ptarmigan takes place in red grouse during autumn. In all three, the timing coincides with a big resurgence of territorial behaviour. The equivalent of the heavily barred plumage of hen willow and rock ptarmigan in late April-early June occurs among red grouse in March-April. In all three, this plumage occurs when hens lay down material for eggs, lay eggs, and start to incubate them.

Variation among individuals

Good illustrations of the main varieties of red grouse plumage have been published.⁴³ Cocks in winter vary, some being reddish chestnut, dun-brown or blackish. Blackish males usually have some reddish feathers or white spots or both, and birds with mixed plumage are commonest.

Most cocks and hens have some white feathers or white-tipped feathers on their heads, flanks and wings. The whiter cocks have white bellies, legs and lower breasts, heavily white-spotted upper breasts, wings and tail, and large white patches on heads, necks and backs. Such cocks are characteristic of northeast Scotland, the UK’s coldest, snowiest region in winter, but examples have occurred in most of the range.

A completely white bird occurs rarely.⁴⁴ Several keepers in different regions told us they had once seen a white grouse during their lifetimes on the moors, and said it was so conspicuous that they could see it on heather a mile (1.6km) away. Occasionally a pale ‘leucistic’ grouse appears.⁴⁵

Variation between regions

One reason for the paler, more yellowish appearance of grouse in Ireland and the Outer Hebrides is that, in all seasons, the sprouting feathers of birds there are paler than on birds in eastern Britain. Another reason, involving plumage in winter and spring, is that grouse in the far west shed more old feathers and sprout more new feathers in winter than eastern birds, and winter-grown feathers in any region are more yellowish than autumn-grown ones, because they carry more yellow or creamy pigment on the bars and speckles.⁴⁶ Hence, for instance, west Irish grouse in winter develop a more yellowish winter plumage. In late spring, however, grouse in Ireland and northeast Scotland show less difference. By then, the latter have caught up to some extent, by shedding more old feathers in spring than Irish birds, and correspondingly they sprout more new ones with some yellow pigment, albeit later.

Many birds from Wales and west Scotland are also pale in winter, such as on Mull, other islands in the Inner Hebrides and on the west mainland coast. This camouflages them against the pale western moorland, which is dominated by grass, sedge and rush, in contrast to the dark heathery moors of east Scotland and England.

These differences in plumage colour constitute what taxonomists call a cline, changing gradually from dark in the east to pale in the west. Occasionally, a pale individual occurs on low moorland in east Scotland, and a dark one in the west. However, out of hundreds of birds seen by us in Mayo in Ireland and in west Scotland, not one was blackish. Also, none had much white on the upperparts or on the underparts, apart from the legs and feet.

CHANGE WITHIN PIGMENTED FEATHERS OF LAGOPUS

Finn Salomonsen found that the colour and pattern of pigment on individual feathers alters even as they grow, with the tip (which grew earliest) differing from the middle of the feather, and this in turn differing from the latest-growing part at the base. He suggested that this change (for example, in spring from an unbarred dark grey tip to a heavily barred hen-like base with some orange pigment) coincides with gonad development. Such changes are widespread in red grouse and Scottish ptarmigan at every season, and also occur in other races of willow and rock ptarmigan, and in white-tailed ptarmigan.⁴⁷

Ray Hewson monitored changes in the individual feathers of captive-reared Scottish rock ptarmigan kept in an unheated building.⁴⁸ Each feather growing in winter reflected a brief cold snap by developing a whiter part during the cold period. The cause of such fine-tuning may be hormonal changes while the feather is growing, in response to a drop in temperature or a snowfall. This partial whitening occurred in association with lower temperature, which in natural conditions would often be associated with snow. This does not rule out the possibility that birds respond to snow itself, and even birds inside a building with no snow might have been responding to snow they could see outside the windows. Nor does it exclude the obvious point that whiteness is an adaptation to snow, not cold.⁴⁹

PHYSIOLOGY OF PLUMAGE AND MOULT IN LAGOPUS

Day-length, local climate and moult

In commercial egg farms, poultry that are given longer day-length with electric lighting lay eggs in midwinter; conversely, they stop laying in summer if they are given a shorter day than they would experience outside. When Per Høst gave extra duration of light to captive willow ptarmigan in winter, cocks developed the spring nuptial plumage and hens the dark summer plumage, while both came into breeding condition and one hen laid eggs.⁵⁰ Having brought a cock into spring nuptial plumage in February, he then gave it a short seven-hour day-length, whereupon it moulted and grew a white winter plumage without first developing a dark autumn one. When he gave both sexes a much shorter day-length during the summer months, they developed a brief dark autumn plumage, before prematurely moulting it and growing the white winter dress.

Lagopus lagopus and rock ptarmigan use the local light cycle as a trigger to start moulting and then to start growing a new summer or winter plumage. However, the point at which declining light triggers the moulting and subsequent growth of winter plumage varies between regions with different climate. Hence birds turn white earlier in cold, snowy northern Canada than at the same latitude in milder Iceland. Also, birds in Scotland and doubtless elsewhere have evolved fine-tuning, turning white faster in cold, snowy autumns than in mild ones with little snow.

Hormonal influences

In one study, cock and hen willow ptarmigan grew their summer plumage at the usual time, despite their gonads having been removed in winter, a result that apparently ruled out any influence from gonad hormones. Later research, however, showed that castrated cocks do not develop the spring plumage of dark chestnut on the head and neck, so this does depend on testosterone.⁵¹

Arctic male rock ptarmigan have almost no distinct spring plumage, unlike southern races and willow ptarmigan. They retain almost all of their winter plumage for territorial and sexual display in spring, and remain almost white until midsummer, when they begin to grow a partial brown plumage. After testosterone was injected into a territorial cock in Arctic Canada on 23 June, he remained almost white while the upperparts of other cocks became completely brown.⁵² Hence the development of dark plumage was blocked by a high level of testosterone.

When small patches of feathers were plucked from captive willow ptarmigan during winter, new white feathers grew on the bare patches. However, after the birds had been injected with pituitary hormones, dark feathers grew instead. It was concluded from this that the white plumage requires no hormonal stimulus,⁵³ that testosterone and pituitary hormones are necessary for the growth of the cocks’ dark spring plumage, and that declines of both hormones induce more hen-like summer feathers on cocks (see Table 22). Because cocks of all British grouse grow such feathers in early summer, this explanation may apply to all four species.

After a low point in late summer, the combs of red grouse enlarge in autumn. This coincides with the onset of vigorous territorial and sexual display, and with a reddish-bronze plumage on the cocks’ heads, necks and breasts, resembling that of willow ptarmigan in spring. They keep this plumage through winter and spring, and then grow feathers with pale hen-like bars in a summer plumage from April until June, when the comb has stopped increasing and has declined slightly. Red grouse have adapted to mild British winters by extending their spring territorial behaviour and associated plumage back into the autumn.

Scottish cock ptarmigan begin to grow dark spring feathers on the head and neck in late February-early March, when their testes are increasing in size and

TABLE 22. Seasonal plumages of grouse in relation to hormonal state.⁵⁴

^# Starts later in northern populations.

^* Hannon & Wingfield (1990) confirmed that less testosterone and LH in cocks in late May induce moult and then growth of new feathers.

^** The apparent lack of a hormonal cause for the white winter plumage of willow ptarmigan and rock ptarmigan is in line with Tickell’s (2003) suggestion that feathers in all bird species are white by default.

^{^} All dark plumages also involve melanophore-stimulating hormone (MSH).

^† Long spring days stimulate gonads to enlarge and produce gonad hormones, ova and sperm. In summer, gonads come out of breeding condition despite the long days and so are called ‘photo-refractory’. This coincides with the end of breeding and the annual moult of wing and tail.

their combs are rising very rapidly to full size,⁵⁵ and before hens show their first summer feathers. In April, the cocks’ growth of spring feathers slows down while their testes increase greatly, and hens then overtake them in the extent of their summer dress. By this stage the cocks are growing hen-like barred feathers, unlike the black unbarred feathers that typify their partial spring plumage. We regard this male spring plumage, with dark feathers on the head and neck, as the equivalent of the hooded spring plumage on the head, neck and breast of cock willow ptarmigan.

Variation in relation to climate

In Iceland and Alaska, thawed snow-free patches of ground where territorial behaviour and pairing can take place do not occur before late April or early May. Compared with Scotland, far less time remains before hens start incubation, and the cock’s combs and testes increase more synchronously. It might be argued that male rock ptarmigan do not have enough time to grow a partial spring plumage with many dark feathers as in Scotland, but cock willow ptarmigan in the high Arctic are equally short of time and yet do grow a partial spring plumage.

High-Arctic male rock ptarmigan are even later than birds in Iceland, for they do not take territories before the end of May. Although they show a few summer feathers on the crown, they remain mostly white while the hens lay eggs and incubate. Only when their combs and testes decline in June do they shed many white feathers and grow a partial summer dress other than on the crown.

Common to willow and rock ptarmigan in all areas is that territorial behaviour and associated pair formation begin when both sexes are mostly white. This coincides with a big increase in comb sizes, and varies from late February during an average winter in Scotland, to late May in the high Arctic. Later, cocks of both species (except for Arctic male rock ptarmigan) develop a partially dark spring plumage that looks spectacular during courtship display. Hence, although the first stages of pairing in Scotland begin early, hens are willing to shift partners at any time before laying. As we suggested above, maintaining the pair bond is a likely reason for cocks growing a display dress with many dark feathers. It might be too risky to do this earlier in the season, when there is often a complete snow cover. More thawed ground in their habitat could also explain why Arctic willow ptarmigan develop hoods, whereas Arctic rock ptarmigan do not develop a spring plumage.

Insulation properties of Lagopus plumage

The winter plumage of rock ptarmigan contains more feathers than in summer or autumn,⁵⁶ with extra follicles coming into development in preparation for cold

FIG 120. After-shaft on autumn ptarmigan flank feather. (Robert Moss)

weather. Winter feathers are longer and have denser, thicker bases, and the downy lower part of each extends further towards the tip than in summer or autumn plumage. In addition, the after-shafts of sprouting winter feathers in rock ptarmigan are longer and usually more downy.⁵⁷ It seems likely that this also occurs in other grouse species.

In Norwegian and Svalbard rock ptarmigan, the combined weight of skin and feathers does not differ between winter and summer,⁵⁸ yet captive Norwegian rock ptarmigan and Alaskan willow ptarmigan have a lower metabolic rate in winter than in summer.⁵⁹ The summer plumage is thinner and looser, however, and hens have more bare skin and blood vessels on their incubation patches.⁶⁰ Hence the weight of the skin and feathers may not be a good measure of insulating capacity.

Although the insulation of different seasonal plumages has not been measured, winter dress is doubtless superior. Some of this may be provided by the barbules⁶¹ in the white feathers of willow and rock ptarmigan, which contain tiny air-filled cavities.⁶² These should reduce conduction and convection of body heat. Also, the barbules should retain heat inside each air cavity by a greenhouse effect, receiving incoming radiation from the sun but reducing outgoing radiation from the body.

In willow ptarmigan and other grouse species in Siberia, winter feathers are denser than in summer because their barbs and barbules are longer and also more numerous per given length of feather.⁶³ The dense feathers can be raised to form an umbrella-like outer canopy, which can be puffed out or retracted to enclose more or less air for insulation.

The plumage of rock ptarmigan is better insulated than in tropical birds, but conducts heat more rapidly than in larger Arctic birds such as snowy owls.⁶⁴ Despite having large, naked, webbed feet, Arctic glaucous gulls can walk on snow at -50°C without harm, and can tolerate very low temperatures in their legs and feet for hours, keeping their feet just above freezing. It seems likely that rock ptarmigan have similar mechanisms for reducing blood flow to the extremities without incurring the risks of frostbite, for only in hard frost do they raise their metabolic rate to keep warm in winter. Tropical mammals, such as humans, respond to cold through ruinously costly shivering and increased blood flow to the extremities, whereas large Arctic mammals such as Arctic wolves or huskies do not increase their metabolic rate even in extreme cold. Intermediate between these extremes, the relatively small rock ptarmigan would be encumbered by heavy plumage like that of a snowy owl, and survives extreme cold by roosting in snow (see Chapter 8).

REFLECTANCE OF WHITE PLUMAGE IN LAGOPUS

Fresh snow has a reflectance of 80 per cent, like that of white feathers with solid barbules, such as in gulls. The air cavities in white Lagopus barbules increase the feathers’ reflectance by 5 per cent above that of the airless white barbules of seagulls. This makes a white grouse slightly more conspicuous against fresh snow, and should reduce camouflage slightly, a possible cost to set against any benefit from reducing heat loss. A potential advantage of the higher reflectance might be that birds may find it easier to maintain social contact or to display more effectively if they stand out slightly against the snow. White ptarmigan in sunlight do seem to shine more brightly than snow. Stu MacDonald wrote, ‘To appreciate what a splendid creature the Rock Ptarmigan is, one must see it in life in its Arctic environment. The winter plumage, so flat and chalky white in museum skins, is vivid in the living bird.’⁶⁵ Thus, the birds’ barbule structure may affect their appearance.

LAGOPUS SNOWSHOES

The willow ptarmigan’s snowshoe takes a track pressure of only 12-14g per cm², the lowest in the grouse family.⁶⁶ This must cut energy loss from sinking into soft snow, as well as allowing the bird to walk around more easily while feeding. Wading in deep snow soon saps the energy of even the fittest mountaineers and can be fatal unless they have skis or snowshoes. The feathers on the legs and feet of willow ptarmigan are longer and denser than in other ptarmigan, and curl round to the sole, reducing heat loss from the toes and making the feet good snow shovels as well as effective snowshoes.

Red grouse have less luxuriant snowshoes than willow ptarmigan.⁶⁷ Likewise, the winter snowshoes of Scottish rock ptarmigan have less dense and shorter feathers than those of Arctic rock ptarmigan. Rock ptarmigan have narrower snowshoes than willow ptarmigan, befitting the generally wind-packed snow of their range, as opposed to the softer snow of willow-ptarmigan country. When both species were tested on soft snow, their depth of penetration showed no material difference, however, presumably because the willow ptarmigan’s heavier weight counteracts the floating effect of its snowshoes.⁶⁸

In their adaptation to travelling on snow, rock ptarmigan could be said to resemble the Inuit, whose Arctic country in winter usually has drifted snow that affords easy walking and also speed for the Inuit sledge, with its narrow runners. In contrast, willow ptarmigan could be said to resemble the subarctic and boreal North American Indian, whose country has deep, soft snow that makes travel almost impossible without the Indians’ superb snowshoes and their toboggan, with its flat, smooth bottom and upward-curving front.⁶⁹

BILLS, CLAWS AND TOE SCALES

Bills and claws suffer wear and tear, and all four species shed the old coatings once a year. Of red grouse it has been stated:

Old grouse shed their toe nails in July-September and young do not, so an old nail in the process of becoming detached is also a sure sign of an old bird. A transverse ridge or scar across the top of the new nail, showing where the old one was formerly attached, may last for a few weeks, indicating an old bird. Young grouse have long smooth sharp claws, but old birds show faint transverse corrugations on their new claws, which tend to be thicker and blunter.⁷⁰

FIG 121. Cock ptarmigan strolling up icy snow. (Derek McGinn)

New claws are hard and strong, with sharp points and edges that afford a good grip on wet rocks, hard snow and icy patches. During sudden squalls, rock ptarmigan grip the snow with their claws, crouch low with heads down facing the wind, and duck lower during violent gusts. In gales, AW has seen standing and crouching men blown over, but not a resting red grouse or Scottish ptarmigan. Also, friction from the claws and the transverse ridges of rough skin on the underside of the toes allows ptarmigan to climb steep, hard snow, ice or rock faster than the most skilled mountaineers.

Blackgame and capercaillie lack feathers on their toes, but they do have a row of hard elongated scales called pectinations along each side of the toe, like short teeth. These help them grip wet or icy branches. Birds shed them in early summer and grow new ones by early autumn. Both species in colder regions of Russia have longer pectinations, and white-tailed ptarmigan have very small ones, even though their toes are feathered as in rock ptarmigan and Lagopus lagopus.⁷¹

COMBS

A fowl has a red comb along its crown and red skin below each eye, extended in cockerels and turkeys to drooping wattles. In severe cold, these bare patches of skin would lose much heat and risk frostbite. Overcoming this, grouse have evolved an erectile comb above each eye, which they can quickly make flaccid and hide under the feathers (Fig. 122).⁷² As in fowls, the upper rim of the grouse’s comb has serrated projections like the teeth of a human comb. The projections are more numerous but smaller than in a fowl’s comb, and can be extended vertically upwards, in cocks with the teeth above the bird’s crown almost like small red horns. Cocks have larger, thicker and darker red combs than hens, with taller teeth and bigger papillae.⁷³ Combs of rock ptarmigan and Lagopus lagopus have a relatively flat surface, whereas those of blackgame and capercaillie have papillae with a coarser grain than in Lagopus, resembling tiny frilly projections.

For most of the time, birds of all four species keep their combs down, raising them only in sexual or aggressive display. Richly supplied with blood vessels, the

FIG 122. Ptarmigan hen on a nest with its comb completely hidden by feathers. (Stuart Rae)

FIG 123. Cock red grouse with combs erect. (David A. Gowans)

combs can almost instantaneously engorge with blood, dilate in thickness, stiffen to full extent and darken in colour.⁷⁴ An erect comb in both hen and cock grouse signals aggressive, territorial and sexual behaviour. In cocks it is a secondary sexual character, its size dependent on testosterone.

The combs in all four species reach maximum size in both sexes during spring. In red grouse they are already large in late autumn and early winter, and remain so throughout midwinter. They usually begin to increase again in late winter, well in advance of the testes reaching their full size in April.⁷⁵ Scottish ptarmigan enlarge their combs rapidly in late February-early March, again long before the testis and ovary reach full size in May at the time when most hens start incubation.⁷⁶

In Canada, Stu MacDonald noticed that combs of male rock ptarmigan become less fleshy, small and pale by the time their hens are incubating, and that the upper portion of each projection ‘shrivels and seems to be sloughed off’.⁷⁷ Although the combs of Scottish cock ptarmigan and red grouse begin to decline when hens start incubation, they regress far less than the rapid decline of their testes, and remain quite large through the summer and autumn. In winter, red grouse and Scottish ptarmigan have much bigger combs and gonads than their Arctic counterparts, and they also show vigorous territorial and sexual display in winter, unlike Arctic cocks. In line with this, the blood of red grouse in winter contains more luteinising hormone (which stimulates the testis to produce testosterone and the ovary to produce oestrogen) than that of Norwegian willow ptarmigan.⁷⁸

Comb sizes in cock red grouse are related to the birds’ aggressiveness and dominance, and implants of testosterone increase all three aspects of maleness.⁷⁹ Three studies, one of willow ptarmigan in British Columbia and two of rock ptarmigan in Arctic Canada and Alaska, showed that cocks with small combs paired with fewer hens than cocks with large combs.⁸⁰

The erect combs of Lagopus lagopus and rock ptarmigan glow red in sunlight.⁸¹ François Mougeot and colleagues have found that the combs of all four British grouse species also reflect ultraviolet light of a frequency invisible to humans but probably visible to grouse.⁸² Although the combs of cock red grouse are redder than those of hens, the hens’ combs show a more intense ultraviolet coloration than those of cocks. Comb colour, both red and ultraviolet, is probably a signal during aggressive and sexual encounters.

Comb size and redness in cock red grouse vary with body condition, such that vigorous cocks have large red combs. The colour of grouse combs comes from carotenoids, a group of yellow, orange and red plant pigments. Recent research has revealed that comb redness in cock red grouse is related to the level of carotenoids in their blood plasma and to the level of testosterone. In addition, comb redness and plasma carotenoids decline when birds carry many threadworms in their guts. The redness and the size of the comb may therefore act as a combined signal to hens that a cock is vigorous and in good body condition, likely to make a good father for her offspring. The reflectance of ultraviolet light by combs may be an extra signal, for in both sexes the ultraviolet declines as threadworm burdens increase.

SUMMARY

Adults renew their main flight-quills in late summer. Birds that have bred successfully moult later than birds without young. Willow and rock ptarmigan have three main moults, red grouse two, and black grouse and capercaillie one. Willow and rock ptarmigan occurring in southern regions where there is some snow-free ground in winter show some dark feathers in their winter plumage. The summer feathers of hen Lagopus lagopus and rock ptarmigan have wide bars of dark and yellow, while those of cocks have narrower bars. Blackcock and male capercaillie grow some hen-like feathers after their lek displays end.

Changing day-length triggers the start of winter and spring moults in willow and rock ptarmigan, but local climate determines when the moult starts and the rate of change thereafter. Scottish ptarmigan can fine-tune this by altering the rate of change, according to whether the season is milder and less snowy than usual. In willow ptarmigan, increases in gonadal and pituitary hormones induce the growth of coloured spring plumage, and declines in them induce the growth of hen-like barred feathers on cocks in summer. White feathers contain air-filled cavities that should enhance insulation. Above each eye, grouse have an erectile comb of red skin that also reflects ultraviolet light, unseen by humans but visible to grouse.