CHAPTER 9
Territory in Lagopus lagopus and Ptarmigan

BRITISH GROUSE SHOW TWO kinds of territorial behaviour. Blackcock and cock capercaillie defend small territories at communal leks, whereas cock Lagopus lagopus and ptarmigan defend spacious territories that divide up practically all the suitable habitat. The most obvious function of cock territorial behaviour is to attract hens for mating and to rear offspring.

This chapter describes territorial behaviour and how territory boundaries are established in Lagopus lagopus and ptarmigan. The average size of territories taken by birds in different places varies from area to area, and within areas the average size changes from year to year. We explain how male aggression affects territory size, and consider how food, cover and territorial behaviour limit grouse numbers.

If you watch Lagopus lagopus or rock ptarmigan on their territories you will see cocks advertising themselves and disputing with neighbouring cocks. On moorland with abundant red grouse, the cackling of many cocks makes a continual background sound, and is interspersed with the high-pitched yelping of hens, a noisy grouse society. On the Cairngorms in late winter, ptarmigan show a burst of territorial behaviour on the first calm sunny morning after a big thaw. The snow-white cocks rise and fall in their song flights with curved beating wings, shining against a deep blue sky.

Red grouse show territorial behaviour at all seasons except during late summer, when they are in heavy moult, and likewise Scottish ptarmigan. As in birds generally, they sing most frequently in the gloaming. At any population density, the first song comes on average more than an hour before sunrise. When both species are at low density, they usually show territorial activity, including calling, only at the dawn or dusk chorus, in light too dim for anyone to see them from afar.¹ An exception is in the Arctic, where birds have such a short time between arriving in packs and breeding that they show territorial behaviour in broad daylight, even at very low density.

Territorial behaviour has been studied more fully in Lagopus lagopus and rock ptarmigan than in most animals. Territorial behaviour limits spring numbers, and after years of debate it is now firmly established that variations in territorial behaviour cause variations in numbers of birds (see Chapters 3, 4 and 14).

TERRITORY IN LAGOPUS LAGOPUS AND ROCK PTARMIGAN

A territory is that exclusive area where a cock dominates other cocks and the hen paired with him dominates other hens.² It supplies the bulk of their food in spring and until their chicks hatch. Cocks divide up the ground and defend their plots by calling, attacking intruders, and meeting neighbours at mutual boundaries. Hens become territorial by pairing with such cocks.³ The cocks decide how big their territories will be each year, and their average territory size determines how many territories there will be per unit area – in other words, the population density. The main sex that determines spring density is the male,⁴ while hens decide with whom they will pair.

Hens show aggression to other hens that come near the cock or enter his territory. Although they call less often than cocks, their high-pitched notes of aggression towards other hens are sometimes frequent during bursts of territorial activity at dawn, and occasionally in daytime.⁵

How territories are maintained

Both Lagopus lagopus and rock ptarmigan maintain territories by showing aggressiveness in three broad ways. The first, most frequent, method is advertisement, where a bird stands or calls on a prominent spot such as a hillock, or makes flights with aerial songs called becks. Often a singing cock lands near the edge of his territory, whereupon his neighbours respond and a wave of calling by several cocks can follow. For most of the day there may be no calling, but each territorial cock spends much time standing erect and alert, on the lookout.

A second way to maintain territory is to threaten or attack others of the same sex, including ejection of strangers. A third way involves those actions that determine the precise positions of territorial boundaries, such as ritual border encounters between neighbouring territorial cocks.

Hens show the first two ways of maintaining territory but seldom the third, because their territorial behaviour emphasises exclusive pair bonds with cocks rather than geographical locations. In an exception to this at Chilkat Pass, British Columbia, Susan Hannon found that each monogamous hen willow ptarmigan defended the cock’s territory against other hens, and in the case of bigamous cocks each of the hens defended her own smaller sub-territory against the other bigamous hen, including boundary disputes.⁶ In red grouse this has not been observed, though each bigamous hen frequents her own favoured part of the territory, a part where she later nests. This part is not exclusive, for both hens are often with the cock, showing no animosity towards one another. None the less, the same individual hen red grouse consistently dominates the other hen when the two do interact, sometimes chases it, and accompanies the cock more often.

Territorial behaviour in hen rock ptarmigan is less common than in hen lagopus lagopus, and was not seen on the Ungava Peninsula and at Sarcpa Lake in Canada.⁷ However, it was seen further north in Arctic Canada at Bathurst Island.⁸ Scottish hens show it in autumn, winter and spring, especially when first pairing with cocks and in bursts of territorial or sexual behaviour during late spring.⁹

At high population densities in spring, birds of both species defend territories strongly throughout the day, and quickly expel intruders. At low densities, however, red grouse, willow ptarmigan at Dovrefjell in Norway and Scottish ptarmigan restrict territorial behaviour to dawn and dusk, and in daytime allow others to come onto ground that they defended at dawn.¹⁰

Encounters

The commonest encounter involves two neighbouring cocks ‘walking in line’ about 50-6ocm apart as they parade together along their common boundary. They face the same direction, and both show postures and calls of threat, attack intention and escape intention (see Chapter 7).¹¹ Occasionally this leads to an attack, which often results in one cock driving the other away, or a fight that makes one of them flee. Usually, the attacker wins, but occasionally the tables are turned and the attacker loses. Both cocks almost always retain their original territories and boundaries, but the disputes continually test their vigour.

One process at work here is that cocks compete for territories. High population density and consequent strong competition for territory tend to induce higher rates of song flights and territorial disputes per bird. There is then seldom silence in daytime, whereas birds at low density give spontaneous calls that are almost entirely restricted to dawn and dusk.

In any region or at any population density, cocks attack and fight frequently when first establishing territories, because those that manage to possess a domain

FIG 115. Two red grouse cocks displaying at their territorial boundary. (David A. Gowans)

FIG 116. Tracks in snow where two cock red grouse displayed at their territorial boundary for 20 minutes. (Adam Watson)

thereby gain in dominance and in their ability to attract hens. Cock willow ptarmigan at Chilkat Pass spent more time defending territories and fighting against newly established neighbours than against familiar ones.¹² If cocks of either species have little time to establish territories before the onset of nesting, as in the Arctic, territorial behaviour can induce many fights even at low density. On Bathurst Island in the Canadian high Arctic, for instance, neighbouring male rock ptarmigan were usually a mile (1.6km) apart, so the average territory covered a square mile (2.6km²).¹³ Despite this low density, they flew to meet one another in encounters that often turned into violent fights. One or both cocks lost feathers, with up to dozens pulled out in a single encounter and occasionally a piece of comb torn out. So, attacks and fights are frequent and intense even at low density in regions where birds have little time to establish territories between their arrival in wintry conditions and settling down to breed soon after.

If they have much time, however, as in Scotland, where territories are established in autumn, most encounters eventually tend to become ritualised posturing, which none the less tests the neighbours’ bottle on a daily basis. The familiarity of unchanging neighbours breeds relative tolerance, and many encounters between the cocks are brief

In contrast, a stranger cock is immediately attacked by the territory owner, and sometimes two neighbouring owners lay aside their differences by combining forces to attack the stranger, no matter whose territory he has invaded. In one instance in Alaskan willow ptarmigan, cock A allowed neighbouring cock B to continue chasing intruding cock C on A’s territory, until the intruder fled.¹⁴ At Sarcpa Lake, encounters of rock ptarmigan were longer and more intense when a non-territorial bachelor challenged a paired territorial cock than when paired neighbouring cocks met.¹⁵

During late spring, a territorial cock red grouse or Scottish ptarmigan will occasionally chase a non-territorial male stranger over several territories without stopping, even though the chased bird flees, and likewise with Alaskan willow ptarmigan.¹⁶ Such attacks can force the subordinate birds to fly to unoccupied ground, where they are sometimes found dead later, with wounds and congealed blood on their napes and heads.

The intensity of encounters does not depend simply on cocks, however. Each hen decides which cock she will join, and this sometimes provokes serious disputes among cocks. Often she flies out of her territory, whereupon her cock gives chase, sometimes joined by neighbouring cocks. Sometimes she returns to her former territory, but frequently she lands on a different one, along with the chasing cock beside her. If so, he is then immediately attacked by the owner of the territory upon which he trespasses. The hen also provokes disputes by walking into a neighbour’s territory, with similar results. By these means she tests the vigour of different cocks as a prelude to deciding which will be her mate. Also, disputes induced by a hen can sometimes result in territorial boundaries being changed.¹⁷

Territory maps and boundaries

Several studies have used mapping to measure the area of individual Lagopus lagopus and rock ptarmigan territories (Tables 20 and 21).¹⁸ In red grouse, territory maps were used to assess the positions of territorial boundaries in relation to landmarks and patches of vegetation, including heather classified according to age.¹⁹ Boundaries were not strongly influenced by the extent or position of vegetation patches. Physical features such as fences, walls or tracks had a big influence, however, a point noticed anecdotally before,²⁰ but now confirmed by statistical analysis.

TABLE 20. Range in annual mean territory size (ha) and extremes in individual territory sizes of red grouse (Kerloch) and rock ptarmigan (other areas).

^# Mapping by AW and Raymond Parr.

^* Mated cocks. Over a period of four years, unmated cocks had an overall mean territory size of 108ha, with extremes of 62-142ha (Holder & Montgomerie, 1993a).

^** Medians.

Boundaries often run along landmarks. Both Lagopus lagopus and rock ptarmigan use ridges or hillocks, and red grouse also use fences and the ground below electricity wires, where each pole is a landmark. Some territories are roughly circular, a few are triangular with a long extension at the most acute angle, more are approximately square or rectangular, and many are polygonal (see Fig. 176).²¹

TABLE 21. Annual mean territory size (ha) of cock Lagopus lagopus and rock ptarmigan in relation to number of mates. In the longer studies at Glen Esk and Dovrefjell, the range of annual means is given.

^# Mated, combining bigamous and monogamous.

^* Means based on combining all individual territory sizes over a three-year period. For Svalbard and Ungava, the two annual means are added and the total divided by two.

^** Two or more hens per cock.

Methods of mapping territories

Depending on the aims of study, four methods of territory mapping have been carried out, using large-scale maps. These showed vegetation based on vertical aerial photographs and ground checks, and other features added by observers, such as boulders, electricity poles and isolated trees.²² On uniform sections of Glen Esk and Kerloch with few landmarks, we inserted posts at regular intervals to help plot locations accurately.

The first method entails the time-consuming mapping of boundaries in daylight, by plotting the boundary parades of adjacent cocks. This is necessary if accurate data on territory size are needed to compare with data on the number of hens or the amount of food. It is feasible, however, only when spacing remains stable on successive days. With red grouse in eastern Scotland, where densities tend to be high, this method works from August until May. However, as territory boundaries are unstable in most autumns, in practice the method was used at Glen Esk and Kerloch mostly after November, with greatest effort in January-April. Early mornings before sunrise were best and forenoons good, but work was most productive during afternoons of fine weather in late winter and spring. This method has also been effective when used with willow and rock ptarmigan in spring.²³

Compared with early work at Glen Esk and Kerloch, predators have been more abundant in studies at Rickarton and Glas-choille, and cocks have shown less daylight territorial activity and particularly fewer boundary parades in daytime. It seems that cocks feel less confident and indulge in fewer prolonged conspicuous daytime encounters when diurnal predators are common. This led to a second mapping method, in which each cock’s daytime advertising display sites were plotted in addition to boundary disputes, because insufficient disputes were seen to allow the mapping of precise defended boundaries.²⁴ None the less, drawing a line around each cock’s outermost observed points created a polygon. The polygons showed which cocks had bigger territories than others, though not their exact boundaries or sizes.

The third method involves listening for calls at dawn or dusk, and works with birds at any density. It is especially useful at low density, when observers seldom see birds but can hear territorial calls and so can detect their locations.²⁵ It has been used for observations on red grouse at Glenamoy and Mull, red grouse and rock ptarmigan at Cairn Gorm, red grouse in years of low density at Kerloch and Rickarton, and willow ptarmigan at Dovrefjell.²⁶ By making repeated observations on successive dawns or dusks, an observer can map territories almost as precisely as in the second method. A lone observer cannot cover all cocks on an area of say 50ha accurately in a given dawn, as can be done in daytime with birds at high density, but extra observers who note times and directions can achieve this, through a triangulation.²⁷ Territorial hen red grouse can also be heard at dawn and dusk, at distances of up to 400m and sometimes even 1km.

The fourth method is to plot each cock and hen while one moves across a study area. This was often done with red grouse at Glen Esk and Kerloch from a vehicle used as a mobile hide, and with Scottish ptarmigan by observers on foot. It involves making observations of territorial and sexual displays, individual differences in plumage, tag colours and numbers, and coloured rings. For red grouse it was called a car census, and for ptarmigan a territory census.²⁸ When repeated on several days, it provides accurate counts and shows which territories are bigger than others, though data are cruder than in the first three methods. In addition, immediately after making observations we often searched small sections with pointing dogs. This resolved any uncertainties, by flushing the birds.

Pioneering work on the territorial behaviour of red grouse at Glen Esk in 1958-61 and 1961-78 at Kerloch involved detailed mapping of exact boundaries by the first method. The most precise mapping requires intensive near-daily effort by observers living close to the study areas, to make the most of fine weather. Good visibility is needed over the whole ground, from numerous viewpoints accessible to a vehicle.²⁹

This method can reveal an approximate spacing pattern within a few days. When birds are at high density, territorial cocks meet so often that the observer may be lucky enough to discover the main spacing on a 20ha area within an hour. On a Glen Esk area with excellent viewpoints, for example, about 300m of parades were observed amongst seven territorial cocks in 35 minutes on a fine afternoon in February 1961, including along most of one cock’s boundary.³⁰ However, accurate mapping of territorial boundaries on a larger area of, say, 1km² is feasible only when boundaries remain stable for at least several weeks.

Boundaries occasionally stay unchanged for many months. In an extreme case on a 7ha section at Glen Esk, the number, identity, territory size and boundaries of six territorial old cocks in August 1960 remained materially unchanged until May 1961. At Glen Esk, Kerloch and Rickarton, minor changes in the complement of territorial cocks occurred over winter when an occasional territorial cock left or died, to be replaced by a previously non-territorial cock. Observers could keep track of this by daily work, concentrating on the small area involved in each change.³¹ This was not possible in the more recent study at Glas-choille, however, because too many changes occurred when territorial cocks were killed by predators or left the area, to be replaced by other cocks. Indeed, some territorial cocks that left the study area returned a year or more later, indicating a more labile territorial system than in the earlier work at Glen Esk, Kerloch and Rickarton.³²

Annual reorganisation of territory in red grouse

This account is based on detailed studies of red grouse when spacing remained stable. It is at one end of a spectrum, the Arctic spring with willow and rock ptarmigan establishing territories shortly before the breeding season being the other end, and red grouse at Glas-choille somewhere in between. Red grouse give a useful insight into the establishment of territories because cocks establish them in autumn, months before the breeding season. This allows us to study territory establishment separately from territorial behaviour during the breeding season.

At Glen Esk and Kerloch, territorial cock and hen red grouse generally survived well over winter. At Glas-choille, however, a more labile situation prevailed, as described above. None the less, despite greater turnover there, the number of territorial birds each spring was close to that in the previous autumn.

Old cock and hen red grouse return to their spring territories in late July and August, and maintain these by territorial behaviour at dawn and dusk, and occasionally during the daytime. Then in September and October there comes a radical annual reorganisation of territory ownership and size, as young cocks take new territories and many old territorial birds lose their territories. At Glen Esk and Kerloch, this usually happened over a short period, sometimes within a few days, coinciding with an increase in aggressive calls and attacks, and often a decline in numbers.³³ Such a great change precluded accurate boundary mapping because details changed too much from day to day or even hour to hour. However, birds showed relatively stable boundaries in good snow-free weather later in the autumn, after the contest was over, thereby allowing effective mapping.

A temporary reorganisation of territories has been observed in a few springs, when heavy snow covers high moorland or Scottish Alpine land, but melts on nearby lower slopes.³⁴ The snow forces territorial red grouse or ptarmigan to move downhill. Although territorial residents can easily eject a few immigrants, many intruders will squeeze residents into small territories and eventually some intruders take small territories themselves. As soon as vegetation appears high up, however, the intruders return there, while those on the lower slopes resume the same territories and numbers as before. A single count during the temporary reorganisation can therefore be wildly misleading.

DAYTIME TERRITORIAL BEHAVIOUR

All territorial cocks and some hens call at dawn and dusk, but often not in daytime. Differences between areas are striking, with daytime silence on some, and noisy calling continuing on and off throughout the day on others nearby.

In Britain and Ireland

At high population densities, cock red grouse and Scottish ptarmigan spend much time in daylight on prominent hillocks or ridges, sitting or standing alert and occasionally giving ground calls or flying to give a beck. Each cock has several perches, which are often conspicuous because the extra dung deposited here favours the growth of herbs, moss and grass rather than heath (Fig. 117). Some ptarmigan have perches on cliffs, such as the 200m-high crag of Lochnagar, spectacular sites that mountaineers can reach only by severe rock- or ice-climbing.

When at low density, however, red grouse and Scottish ptarmigan have not been found to use such perches at any time of day. They confine territorial calling to twilight at dawn and dusk,³⁵ as do willow ptarmigan at low density on Dovrefjell in Norway.³⁶

When flushed by a human, a territorial cock of either species usually becks as it takes wing and again when landing, whatever the density. Seton Gordon noticed that ptarmigan on Mull were extremely silent, however, with not a single call even from cocks that he disturbed on their territories.³⁷ Ptarmigan density there is very low, associated with scarce heath that results from heavy rainfall and also overgrazing by sheep and deer. It has been claimed that the daytime silence of

FIG 117. A ptarmigan lookout on the Cairnwell in late winter, enriched by droppings so that sedges and moss grow instead of the surrounding heath. (Adam Watson)

grouse in the Outer Hebrides characterises Lagopus scoticus hibernicus,³⁸ but in fact it typifies red grouse and Scottish ptarmigan at low density anywhere.

Cock red grouse and Scottish ptarmigan call at dawn and dusk during all months, except when in packs on deep snow. At Glenamoy in spring, mated cock red grouse called at dawn more often than unmated ones.³⁹ The duration of the dawn period of calling was correlated with density. It lasted only 20 minutes at densities of about one cock per km², but 100 minutes at about eight cocks per km², when cocks continued so long after dawn that observers for the first time could see the last callers. The amount of calling also varied with population density, with just one or two becks per cock at low density.

When eggs from nests of wild hen red grouse at Glenamoy were taken for rearing in a nearby aviary, however, cocks in separate cages only a few metres apart called continually in broad daylight, while perched on high branches. Likewise, cocks raised from eggs laid by wild hens at Kerloch and Rickarton gave continual noisy calls when kept at close quarters in aviaries, as did cock willow ptarmigan from Dovrefjell eggs when in an aviary. When kept in aviaries at far higher densities than in the wild, birds from the same population therefore showed far more territorial behaviour throughout daytime than birds in the densest wild population.

In conclusion, British and Irish cocks at high density crow loudly in daytime and use prominent perches. Diurnal predators pose less risk to birds at high density, when there are many eyes on watch and many voices to warn of their approach.

In the Arctic and Amchitka

There is more to the intensity of display than density alone, however. Even at the very low density of a pair per square mile (2.6km²) on Bathurst Island in the Canadian high Arctic, male rock ptarmigan spent much time in late May and June on hummocks, from which they flew to meet one another in aggressive encounters. And despite a low density on Svalbard, territorial cocks behaved likewise, each with at least two to four favoured perches.⁴⁰

Arctic birds have only a few weeks between their arrival and subsequent nesting, and we think that this explains their frequent territorial behaviour in broad daylight, despite very low densities. An interesting case is Amchitka Island in the Aleutians, which at 51°N is much further south than Scotland, let alone the Arctic. The sole count there that we know of was in 1969, showing only 2.5-7.5 pairs per km².⁴¹ Cocks took territories early in May, far later than in Scotland and at about the same date as in Iceland. Despite the low density, they showed vigorous territorial activity in daytime. Hence daytime activity can occur without

FIG 118. Ptarmigan cock (left) with hen, making himself obvious in a low-density population (less than one pair per km²) in May near Reykjavik, Iceland. Purple saxifrage is in flower, right foreground. (Adam Watson)

high density in situations where the establishment of territories comes so late that little time remains before birds must start nesting.

TERRITORIES ON POOR OR PERIPHERAL GROUND

Some ground supports few or no territories (and mainly unmated cocks at that), because food or cover is sparse. Examples of such ground with red grouse include peat or gravel with little vegetation, wide rushy flushes, heather that is dominated by grass or bracken, and heather kept short by wind-scour, browsing or frequent burning.

Ground with poor food or cover tends to be vacant at low densities of birds, even though it may hold territorial pairs or unmated cocks at high density. At Kerloch, small areas that had little food or cover owing to overgrazing by cattle and sheep remained unoccupied in springs of low density. On Derry Cairngorm, cock ptarmigan took territories on exposed wind-scoured heather only in years of high numbers.⁴² In both species, the vegetation on unoccupied areas appeared no worse than in years of high density when it was unoccupied. Hence poor habitat may be sufficient for an unmated cock or pair at high density, but not at low density.

On good habitat, peripheral ground often supports breeding pairs successfully at high density, yet may be unoccupied at low density. Breeding willow ptarmigan on the Norwegian island of Tranøy showed this, at low density avoiding isolated areas where they had few neighbours.⁴³ A similar example on grouse-moor was White Hill at Rickarton, a fairly isolated triangle with farmland on two sides and holding red grouse only in years of high density, despite abundant heather and cover.⁴⁴ Scottish ptarmigan also show this frequently.⁴⁵ We infer that birds avoid taking territories where there are few or no others nearby, a likely reason being that a bird on a solitary territory would have no neighbours to give early warning of predators.

There are other sites that red grouse tend to avoid. One is the toe of a slope with a conifer plantation at the bottom, and another the steep slopes of a gully or glacial melt-water channel with rocks on its sides. Avoidance of predators may again be involved.

TERRITORIAL BEHAVIOUR AT DIFFERENT SEASONS

Autumn

In northeast Scotland, young and old cock red grouse and ptarmigan begin to show daytime territorial behaviour usually in late September or October.⁴⁶ Male rock ptarmigan in the Swiss Alps defend territories in autumn,⁴⁷ and willow ptarmigan have long been known to show autumn territorial behaviour on Newfoundland’s Avalon Peninsula as early as September, and at Nueltin Lake west of Hudson Bay.⁴⁸ At Chilkat Pass, British Columbia, all adult cocks and about half the adult hens returned to former territories in late October or November during the first snowfall, and left about mid-December when deepening snow reduced food and cover.⁴⁹

It used to be thought that Norwegian willow ptarmigan do not show territorial behaviour in autumn,⁵⁰ but Hans Pedersen and others found a resurgence of autumn territorial calling by cocks at Dovrefjell in Norway.⁵¹ Because cocks confine this to twilight, observers who visit in daytime would overlook it. It may often be overlooked elsewhere for this reason.

Winter

On subarctic and Arctic lands, willow and rock ptarmigan experience a long snowy winter and show no territorial behaviour at that time. Patches of food that might be defended are often soon buried by snowfall or drifting. Instead, the birds roam in packs for months, postponing territorial behaviour until the spring thaw.⁵² This applies also to rock ptarmigan in the Swiss Alps, a region with heavier snowfall and consistently longer snow-lie than the Scottish hills. In some regions, populations have wintering grounds that are distinct enough from their summering grounds for the movement between them to be called migration (see Chapter 3).

During winter, most Scottish red grouse and rock ptarmigan stay in packs throughout the 24 hours on days when there are strong winds, heavy rain or snowfall, or deep snow cover. Because fewer gales and snowfalls occur on moorland, red grouse have more opportunity to show territorial behaviour than ptarmigan. During mild winters with little snow and much calm weather, however, both species show territorial and pairing behaviour during every month, usually in the morning but occasionally on and off through the day until mid-afternoon.⁵³

Early spring

When the main thaw comes to snowy regions in spring, cocks of both species arrive back from their wintering grounds, or if they are resident in winter (as in Scotland), they disperse out of the flocks. Cocks then take territories on tiny patches of snow-free ground, provided that some food plants are exposed. Scottish ptarmigan do this even at 99 per cent snow cover, and maintain it as long as the food plants on the territories remain snow-free. When fresh snowfalls cover the plants, however, as commonly happens with both species in Scotland and further north, birds abandon their territories and usually form packs until the next thaw.⁵⁴

In the snow-free conditions that often prevail on low Scottish moors, red grouse after midwinter increase the frequency and intensity of their territorial behaviour, at dawn, dusk and in the morning. More notably, they frequently increase them throughout the morning, and often throughout the daytime. The onset of this change usually comes in late January or February, but can be as late as March or even towards the end of April on high moorland in severe winters. In Scottish ptarmigan, the main burst of all-day territorial behaviour usually comes in late February or early March, although it started as early as the beginning of January in the very mild year of 1964, and not until the end of April in 1951, when deep snow covered almost all ground.

At Porcupine Creek in Alaska, the more dominant cock willow ptarmigan established territories in the creek bed, where tall willow scrub offered abundant food.⁵⁵ The hill slopes lacked tall vegetation, so the less dominant cocks had to wait until some snow-free patches appeared there before they could establish territories. Meanwhile, rock ptarmigan stayed in packs that broke up later, with new territories and pairs forming on the slopes as fresh higher ground became snow-free, and the territories of three marked cocks moved uphill as the snow melted. Hence territories were defended, even though they were not fixed in location during the early stages.⁵⁶

Late spring and summer

In the few weeks before laying and during incubation, cock and hen feed in the territory, and usually she nests in it. Occasionally a hen red grouse nests just outside, in which case her cock extends his territory to include an extra small slice around the nest. After the eggs hatch, most hens with chicks leave the territory in the first few days (sometimes on the first day), usually accompanied by the cock in Lagopus lagopus, though less so in rock ptarmigan.⁵⁷ We infer from this that staying on the territory is not necessary for rearing chicks, and that birds often incur an advantage by leaving it.

Hence it is not surprising that vigorous territorial behaviour in daylight almost ceases during the weeks when the broods are being reared. The most that happens is that some cock Lagopus lagopus and Scottish ptarmigan briefly return to their territories to crow at dawn and, to a lesser extent, at dusk.⁵⁸ If a hen red grouse or Scottish ptarmigan loses her clutch, however, the cock resumes territorial behaviour and courtship on his old territory in daylight, and a second nest often follows.

By late July and early August the broods have been reared, and territorial activity increases markedly. Cock red grouse usually return to their old territories to call in twilight at dawn and dusk, and afterwards the mated cocks then rejoin their hens and young. In several cases in Scotland and Ireland, cocks called at dawn on their former territories, but counts with dogs shortly afterwards revealed no birds.⁵⁹ Subsequent dawn visits revealed that hens and fully grown broods were on higher ground outside the study area, and that the cocks deserted them briefly to call on their old territories before returning to their families before daylight. Immediately before they returned, the hens gave contact calls, doubtless informing the cocks of their locations. The cocks then gave the last becks of the morning when landing beside the hens.

SHIFTS OF TERRITORY FROM SPRING TO SPRING

Intensive studies with large numbers of colour-tabbed red grouse at Glen Esk and Kerloch showed that individual territory sizes often altered between one spring and the next, roughly corresponding with a change in population density. None the less, where a cock had a territory two or more years running, his territory in later years usually included some ground that he had occupied in the previous year. At Kerloch, for example, out of hundreds of cocks that had territories in the previous year, only two shifted to a different part of the study area.⁶⁰ Hens were more footloose.⁶¹

Cock willow ptarmigan at Chilkat Pass, British Columbia, showed more shifting from one spring to another (known as breeding dispersal) than cock red grouse at Glen Esk, Kerloch or Rickarton. For instance, 9 per cent of cocks (mainly unmated) shifted territories at Chilkat between one spring and the next.⁶² The three Scottish moors are at low altitude and typically get mild winters, so birds were on their territories during the day in every winter month, often for most of the winter in mild seasons. By contrast, birds at Chilkat Pass did not show territorial behaviour in the depths of the months-long winter, and many wintered outside the study area. It seems that a long period of territorial behaviour between autumn and spring allows birds to maintain their territorial boundaries more precisely.

This may not be quite the whole story, however. Red grouse at Glas-choille took territories in autumn and maintained them during winter as at Glen Esk, Kerloch and Rickarton. Nevertheless, the territorial system was more flexible, involving more turnover of territorial owners. During studies carried out between 1992 and 2003, about half the old cocks on the 53ha core study area were immigrants.⁶³ Within that core, many cocks shifted their territories hundreds of metres (many territory radii) from one spring to the next. It might be argued that, because Glas-choille stood at a higher altitude and hence had snowier winters than the three lower moors, red grouse there behaved more like the willow ptarmigan at Chilkat Pass.

Most old hens paired with the same cocks in successive autumns if these had kept their territories, but paired with nearby cocks if their cocks from the earlier year had lost territories or died.⁶⁴

TERRITORY SIZE

Average size differences between areas and years

Studies that have continued for some years show that the average territory size differs from one area to another. This broadly reflects counts of grouse numbers on the same areas, so the higher the average density, the smaller the average territory. Territories on study areas have been mapped in Scottish red grouse, willow ptarmigan at Chilkat Pass in British Columbia and Dovrefjell in Norway, and rock ptarmigan in Iceland, Svalbard, Arctic Canada and Scotland (see Tables 20 and 21).⁶⁵ Because the population fluctuation at Chilkat Pass lasted for more years than the number of years of territory mapping, any inference must be tentative. However, it seems that average territories of willow ptarmigan were larger at Dovrefjell than at Chilkat Pass and the Scottish moors, and that rock ptarmigan at Svalbard had larger territories than birds at Hrisey, Ungava and the Cairnwell.

It has often been suggested that changes in food supply from one year to the next cause fluctuations in the average territory size of birds generally.⁶⁶ Thus, in red grouse the abundance and nutritive value of heather might affect aggressive behaviour and territory size. Certainly, extreme impacts such as severe winter browning of heather are followed by changes in territory size. However, these changes usually occur many months later at the next annual reorganisation of territories, when other factors have also changed, including fewer young birds competing for territory, after a summer of poor breeding that was consequent upon the poorer winter-browned food.⁶⁷

Experiments throw some light on this. An application of ammonium nitrate fertiliser at Kerloch boosted the growth and nitrogen content of heather, which in turn resulted in smaller territories during the increase phase of a population cycle, but another application during the subsequent decline failed to prevent cocks taking successively larger territories.⁶⁸ The evidence is that short-term alterations in food cannot explain all year-to-year changes in average territory size, and hence are not necessary for such changes (see Chapter 14).⁶⁹

Territory size in a given year and area

In a given year, territory sizes of individual cocks on an area vary considerably (see Table 21). There are three influences here: (1) territories are larger on patches with less food or poor-quality food, or with less physical cover provided by vegetation height; (2) territories on patches with hillocks are smaller than those on more open ground nearby; and (3) some cocks are more aggressive, take larger territories and attract more hens than other territorial cocks on the same area, even though influences (1) and (2) are broadly similar.

Issue (1) can be regarded as analogous to the observation that the abundance of food and cover vary between widely separate moors. The sole difference is that in this case the variation can be highly localised within areas as small as 25-50ha. On such areas at Glen Esk and Kerloch, cock red grouse on poorer patches such as grassy bogs had large territories close to, and in some cases adjacent to, cocks on better patches where heather predominated and territories were smaller.

At Kerloch, Art Lance found that the cocks’ territory size in a given year was not related to the area or proportion of young heather on the territory.⁷⁰ Instead,

FIG 119. Flat moorland supports lower densities of red grouse than hillocks because cocks take larger territories where they can readily see each other. These pictures show adjacent stretches of ground at Glen Esk. (Adam Watson)

territory size was related negatively to an index of the nitrogen content per hectare in the heather shoots. One component of his index, the abundance of green shoots (as measured by their percentage cover of the ground), was important only on territories with sparse heather. Otherwise, the average content of nitrogen was more important – in other words, the richer the content of nitrogen, the smaller the territory. This shows that cocks adjusted their territory size on sparse heather according to food quantity, and on abundant heather according to food quality.

Let us turn to issue (2). At Glen Esk, where territories did not differ significantly in the productiveness of their green heather shoots per hectare,⁷¹ cocks took larger territories on flat ground than among hillocks.⁷² Because neighbouring cocks can see each other more readily on flat ground, we infer that this would be stressful, and so they compensate for this by taking larger territories so as to be further apart. This did not happen in years of very high density, however, when cocks crowded onto flat ground as well as hillocky ground. On crowded flat ground, a cock could see all his neighbours if all cocks merely raised their heads. We think that this would be particularly stressful to cocks restricted to territories on flat ground with little seclusion, and would be likely to lead to population decline.

Issue (3) involves some cocks taking larger territories than others on the same type of ground. During studies in Scotland and the Arctic, more aggressive male rock ptarmigans took larger territories than less aggressive cocks.⁷³ In Scotland, studies with much larger numbers of marked red grouse showed that the more aggressive cocks occupied larger territories. Cocks implanted with testosterone at Kerloch became more aggressive and took bigger territories at the expense of their neighbours, and an experiment at Glen Tanar confirmed this on a much larger scale, at the level of the population and not just the individual bird.⁷⁴

To conclude, average territory size varies from year to year. Within years, cock red grouse take bigger territories where heather is sparser or has poorer nutritive value. On ground of similar heather abundance and nutritive value, more aggressive cocks take bigger territories.

Territory size, number of hens and breeding success

Average territory size varies much between years, with no evidence that the annual variations are generally related to food quality. Hence the variations discussed in this section are not of absolute territory size but of relative territory size within years. If relative territory size affects breeding success but absolute territory size does not,⁷⁵ then relative territory size may well reflect pair quality.

In both Lagopus lagopus and rock ptarmigan, cocks with two hens have bigger territories than monogamous cocks on the same area, and unmated cocks have yet smaller ones (see Table 21). This was the case in Scottish red grouse at Glen Esk and Kerloch, willow ptarmigan at Dovrefjell and Chilkat Pass,⁷⁶ and rock ptarmigan at Ungava and Svalbard.⁷⁷

Having noted that aggressive cocks take big territories and pair with more hens, we turn to what characterises such cocks. On Svalbard, male rock ptarmigan that arrived early on the breeding grounds took bigger territories and subsequently paired with more hens than cocks that arrived late. Likewise, in a study of rock ptarmigan for one breeding season at Colville River in Arctic Alaska, more hens paired with cocks that arrived early.⁷⁸

In the Alaskan study, moreover, cocks that had big combs attracted more hens, and a cock had more hens if his territory held many willow twigs, the main food.⁷⁹ Since a hen favours a territory with a good cock and with much food, and a good cock usually takes a territory with much food, it is impossible to tell from such cases whether she prefers the home, the owner or both. After a study of vegetation on the territories of willow ptarmigan at Dovrefjell, for instance, it was suggested that the owner was more important, but this was dubious.⁸⁰

Some element of male quality does seem to be involved, however. At Dovrefjell, mated cocks outweighed unmated ones. Bigamous cocks at Chilkat Pass tended to be older than monogamous ones, and old male rock ptarmigan at Ungava took larger territories than young cocks. During later work at Chilkat, the ‘condition’ of individual willow ptarmigan was estimated by weighing them and then adjusting for their body size as indicated by wing length. Hens in good condition settled no earlier on territories and chose territories no bigger than average, but they were more likely to pair monogamously than hens in poorer condition, which frequently paired bigamously.⁸¹ In another study at Chilkat, cocks and hens were shot on their territories, and birds that replaced them were then shot.⁸² The combs and body-weights of the replacement hens were no smaller than those of the residents, but replacement cocks were in poorer condition than resident cocks.

Broadly uniform heather swards on study areas at Glen Esk and Kerloch were found to have a similar amount and age of green heather per unit area. On such ground, mated cocks had larger territories than those that remained unmated, and hence had a larger amount of green heather.⁸³ At Kerloch we tried to find what controlled the hen’s choice in a spring when territorial cocks outnumbered hens two to one, by measuring the quality of food at sites where we had seen hens feeding.⁸⁴ On a section with poor food due to overgrazing by sheep and cattle, hens took longer to decide which territory to choose for nesting. As usual, mated cocks had bigger territories than unmated cocks. Although territories with richer feeding sites tended to be smaller, the hens favoured large territories, so ‘some factor other than food quality, but related to territory size, must have affected the hens’ choice’. Other studies had shown that aggressive cocks take large territories and pair with more hens, so a hen’s choice of cock and territory may depend on his aggressiveness or on an associated factor such as his courtship, attentiveness or comb size.

When three territorial cocks were implanted with testosterone in late winter, they expanded their territories, sang more frequently, paired with more hens, and chased hens more often.⁸⁵ One of the three had been mated before the experiment, but from two weeks after the implant he was paired with an extra hen that had previously been with a neighbouring cock. By two weeks he had expanded his territory and sang more. Hence the extra hen might have been attracted by his increased territory, or by his greater territorial and sexual activity, including his comb size, or by both.

In experiments at Glenamoy and Kerloch, applications of fertilisers boosted the heather’s growth and nitrogen content.⁸⁶ This resulted in territories with more hens per cock than on unfertilised control areas, and one experiment at Kerloch led to the unusual situation of more hens than cocks on the territories. This might suggest that territory quality influences female choice. Cocks may still be involved, however, because better cocks take better territories. Also, a hen’s decision whether to leave or stay with a courting cock depends on his relative vigour. In short, a hen choosing a good cock is also going to have a good territory.

Another aspect of territory size is whether it influences breeding success. At Glen Esk and Kerloch, hens whose territories held larger amounts of green heather reared more young, although not significantly so, which indicates that breeding success depended little upon the quantity of potential food.⁸⁷ A drawback of this study, however, was that it did not include chemical analysis of the heather, to distinguish quality from quantity. A later study at Kerloch did include such measurements. It revealed that hens on territories where heather had a higher content of nitrogen reared more young.⁸⁸ Our experiments with fertilisers also showed that hens on enriched heather reared more young than those on control areas.

DOES TERRITORIAL BEHAVIOUR LIMIT SPRING NUMBERS?

Much has been written about this subject in birds generally.⁸⁹ Some authors have proposed that territorial behaviour does limit numbers, others that it merely determines which of the individuals that survive until spring get the best territories, after their number has already been reduced by food shortage over winter.⁹⁰

Studies of marked red grouse at Glen Esk and Kerloch showed that territorial behaviour in autumn limits numbers the following spring.⁹¹ After some of the cocks on an area at Glen Tanar were implanted with testosterone in spring, they enlarged their territories at the expense of neighbours, some of whom left, so territorial behaviour limited numbers and caused a local population decline.⁹² At Glas-choille, territorial behaviour in autumn again limited spring numbers.⁹³

During two decades, Peter Hudson and colleagues often claimed that behaviour does not limit numbers of red grouse, though their arguments were fallacious.⁹⁴ More recently, a large-scale experiment led by François Mougeot on two English and two Scottish moors has shown that increased aggressive behaviour, caused by implanting old cocks with testosterone in autumn, limited numbers in the following two springs and led to population declines.⁹⁵ Since Hudson is one of the authors, evidently he no longer maintains his earlier claims.

On marginal areas with poor habitat, net immigration may be needed to maintain numbers, and limitation of spring numbers by territorial behaviour in autumn may be less frequent. Red grouse were studied at one such poor area, Glenamoy. Here it was found that territorial behaviour in autumn limited spring numbers in two years out of five.⁹⁶ In the other three it did not, because territorial birds suffered heavy winter losses that were not fully replaced, so the number of territorial birds in autumn exceeded that in the following spring.

Three experiments with willow ptarmigan elucidated this problem. After territorial cocks and hens at Chilkat Pass in British Columbia were removed in spring, others that had not held territories on the study area took replacement territories; the same also occurred at Dovrefjell in Norway.⁹⁷ Hence territorial behaviour limited numbers on both areas. At Porcupine Creek, Alaska, a small proportion of the male spring population was non-territorial,⁹⁸ which suggests that limitation by territorial behaviour may occur more widely.

In rock ptarmigan, two studies provide evidence. Territorial cocks and hens that were shot in a corrie of Derry Cairngorm during spring were replaced, and despite many territorial birds being shot on half the study area, the two halves held similar numbers in late April.⁹⁹ On Svalbard, about half the territorial cocks and hens that had been shot in spring were replaced.¹⁰⁰ Territorial behaviour limited numbers in both studies.

SUMMARY

Cock Lagopus lagopus and rock ptarmigan defend territories against other cocks, and hens pair with cocks on the territories and dominate intruding hens. A territory provides food and cover for the pair until their chicks hatch. Its average size over the years varies between different areas, being smaller where food and cover abound. The average territory size on an area fluctuates between years, and annual changes in food or cover are not necessary for these fluctuations. None the less, on a given area in any one year, the size of individual territories is negatively related to the food and cover on them. In addition, aggressive cocks take large territories and pair with more hens, while unmated cocks have small territories. Autumn territorial behaviour in red grouse often limits density in the following spring. In willow ptarmigan in Canada and Norway, and in rock ptarmigan in Scotland and Svalbard, spring territorial behaviour can limit spring numbers.