CHAPTER 12
Nutrition
BIRDS OF THE GROUSE family thrive for much of the year on abundant but coarse staple foods, which are fibrous and low in protein. This chapter explains how they cope by way of digestive and behavioural adaptations, but none the less depend upon seasonal foods of better quality. Weather and climate interact with nutrition to influence breeding success and numbers.
COARSE DIET
Heather is a poor food. Domestic sheep and cattle lose weight on a diet of heather, needing grass to supplement it. Red grouse, on the other hand, thrive mostly on heather year-round, processing it with a lighter and apparently simpler digestive apparatus than the ponderous four-stomached ruminant gut. A 650g red grouse
FIG 137. Grouse feeding on a sward of heather. (David A. Gowans)
FIG 139. Capercaillie cock taking a snack of needles from a fallen pine branch after a morning at the lek. (Kevin Cuthbert)
eats about 130g of heather a day,1 which would scale up to about 3.6kg or roughly 12 litres of heather shoots for a 65kg human being.2 In winter, birds of the grouse family typically subsist on coarse, abundant plant foods that are low in nutrients, especially protein, phosphorus and sodium, and high in indigestible stuffs such as fibre, tannins and resins. The ground-dwelling red grouse and rock ptarmigan eat largely dwarf shrubs, heather being the staple food of red grouse, and heather, blaeberry and crowberry that of ptarmigan. The classic winter food of the arboreal capercaillie is Scots pine needles, but in Scotland they also eat much Sitka spruce where it is prevalent.3 Black grouse have the most varied diet of the four British grouse, the majority feeding mainly on blaeberry and heather in winter. When snow covers the ground they take readily to trees, where they eat birch catkins, buds and twigs, or, where birch is scarce, Scots pine needles.
THE MECHANICS OF DIGESTION
The grouse gut is structured like that of the familiar farmyard fowl, though proportioned differently. Both these gallinaceous birds have a food-storage organ, the crop, and both produce two distinct kinds of droppings. The grouse, however, is better at digesting coarse foods.
FIG 140. Red grouse flapping its wings after voiding a dropping. (David A. Gowans)
FIG. 141. Red grouse roost heap: shiny, viscous, chocolate-coloured caecal droppings lying on fibrous, cylindrical intestinal droppings with white caps of urine. (François Mougeot)
The main divisions of the gut (Fig. 142) include the oesophagus, the crop, the first stomach (proventriculus), the second stomach (gizzard or ventriculus), the long small intestine and caeca,4 and the short colon.5 The colon exits at the cloaca, which also receives urine from the kidneys via the ureters. Undigested food is stored in the crop, a capacious, elastic, expansible thin-walled sac, which is an outpocketing of the oesophagus and lies at the base of the neck. In daytime, the crop contains little food, but just before going to roost at night a grouse fills its crop almost to bursting, emptying it gradually while asleep.
Directly, or via the crop, food particles pass through the glandular proventriculus and into the gizzard, a large muscular organ lined with leathery ridges and filled with particles of grit (see Fig. 146) that act as teeth, grinding coarse food to a pulp. The pulp is eased through the small intestine by peristaltic contractions until, partly digested, it reaches the colon. At the start of the colon are side entrances to the two caeca (see Fig. 147), tubular blind guts of roughly the same diameter as the small intestine and colon. Their narrow neck-like entrances act as valves, regulating the flow of partly digested food into the main body of the caeca.6
FIG 143. Hen ptarmigan eating sedge fruit, Svalbard. (Stuart Rae)
The force behind this flow comes from reverse peristaltic contractions of the muscular colon, which push part of the urine backwards from the cloaca and into the caeca. The reverse flow of urine helps to wash fluid and small particles from the contents of the colon into the caeca, where they ferment. Larger, more fibrous particles are prevented from entering the caeca by the valve-like entrances.7
The more fibrous fraction of the partly digested food pulp bypasses the caeca, and goes direct from the small intestine through the colon to the cloaca, to be excreted as firm, fibrous, cylindrical droppings, often tipped with white caps of urine. Such intestinal droppings are voided frequently through day and night, and roost heaps of these show where a bird has spent the night. A nesting hen, however, avoids soiling her nest by storing intestinal droppings in her colon and excreting them two to five times a day as outsized clocker droppings while feeding away from the nest.
FIG 145. Heather particles from the crop of a wild red grouse, framed by heather shoots browsed more or less heavily by captive birds. (Robert Moss)
FIG 146. Grit from the gizzard of a red grouse (top right) and a blackcock (bottom), compared with commercial grit from Cornwall (top left) sold for spreading on grouse moors. (John Phillips.)
FIG 147. Diagram showing how digesta flows into and out of the caeca. The arrows indicate (a) the reverse flow of urine from the cloaca through the colon into the caeca, when the intestinal sphincter is closed and the caecal ones are open; (b) emptying of the caeca, mostly at dawn, to produce caecal droppings; and (c) normal flow of digesta with open intestinal and closed caecal sphincters, giving rise to intestinal droppings. (Drawn by Dave Pullan)
The main emptying of the caeca occurs once a day. Shortly after a bird wakes in the morning, it voids the six to eight soft, shapeless, semi-liquid caecal droppings that are typically found on, or close to, roost heaps. Caecal droppings, which are residues of the fermentation process that takes place in the caeca, are typically brownish yellow when first voided, but rapidly turn a darker brown as the surface weathers. Contrary to old keepers’ tales, the caecal droppings of red grouse are not grouse diarrhoea, nor do they signify an outbreak of grouse disease.
FIG 148. Red grouse clocker dropping lying next to normal intestinal droppings and mountain hare droppings. (George Reid)
CAECAL FUNCTION
Small particles of fibre are fermented in the caeca. Plant fibre comprises cellulose that is more or less encrusted with lignin.8 Cellulose from the thin walls of living plant cells, unadulterated with lignin, is ground to fragments in the gizzard, and small particles of it are easily fermented by microbes in the caeca.9 In contrast, the structural cellulose of thick cell walls, especially if lignified, resists grinding in the gizzard. This includes, for example, the woody cores and outermost parts of bark from twigs, structural parts of stems and leaves, seed husks and berry seeds. Such stuff constitutes much of the fibre that passes undigested through the colon without entering the caeca.
None the less, some particles of lignified cellulose (lignocellulose) from such sources do enter the caeca. Lignin is usually found to be quite indigestible in mammals and birds, and, bound to cellulose, protects the latter from fermentation. But ‘lignin’ is a loosely characterised chemical entity, some fractions of which are more digestible than others. Wild red grouse that are eating heather excrete only about half as much lignin as they eat, apparently digesting an unusually large proportion.10 Perhaps heather lignin differs chemically from the grass lignins usually studied, and may be more readily digested.
In chemical terms, lignin is easier to oxidise than to reduce. Lignin digestion requires oxygen,11 which is generally in short supply in the reducing environment of the gut. Perhaps oxygen, dissolved in refluxing urine while it passes through the cloaca, is transported to the caeca.12 If so, this could help to explain efficient digestion of lignin in grouse caeca. The possibility has not, however, been investigated.
The backwards flow of urine from the cloaca does more than wash small particles into the caeca, for it helps to retain water and to recycle scarce nutrients such as protein, phosphorus and sodium. The nitrogen compounds from refluxed urine are used by caecal microbes to make protein, and probably, as in ruminants, microbial protein is eventually broken down to its constituent amino acids and then absorbed by the birds.13 Also, ammonium nitrogen from the urine is absorbed from the caeca and probably used to synthesise non-essential amino acids and protein.14
Many textbooks make the general claim that birds excrete uric acid as the main end-product of protein metabolism. When eating coarse foods that are low in protein, however, grouse excrete digested protein-nitrogen mostly as ammonium salts and rather little as uric acid.15 Uric acid must be maintained in colloidal form in the urine to avoid crystallisation and consequent tissue damage.16 The colloidal spheres contain protein, which would seem wasteful when this is in short supply. So instead of excreting uric acid, grouse apparently economise on protein by excreting ammonium salts instead.
GUT SIZE AND FOOD DIGESTION
Birds on a coarse, natural diet have to eat large amounts. Also, the proportion of a coarse food that is digested depends upon the length of time it spends in the gut. Hence a bird’s ability to subsist on coarse foods increases with the size of its gut. But birds must fly, which dictates that guts must be as small and as light as possible. Grouse achieve this by rapidly processing the more fibrous fraction of their food, while retaining, for leisurely fermentation in the caeca, the most nutritious part of the material that remains after digestion in the small intestine. Gut size adapts to the coarseness of the diet.17 Grouse eating highly digestible foods have relatively small gizzards and short, narrow guts. On somewhat coarser diets they digest enough food by eating more and by increasing the size of their gizzards. On yet coarser diets, the bulk of food they can process is restricted by the capacity of the gut.18 As foods get coarser, the intestines and, especially, the caeca increase capacity and the microbes in the caeca change, thereby facilitating the fermentation of cellulose and the extraction of energy through the production of volatile fatty acids.19 Grouse eat bulkier foods in winter than in summer, and their guts are then longer and more massive. The caeca are typically 20 per cent longer and weigh twice as much in winter as in summer.
Interestingly, annual changes in gut size can occur without the stimulus of a bulky winter diet. The guts of captive red grouse, fed ad lib on highly nutritious pellets, enlarged between summer and winter.20 Rock ptarmigan begin to eat the winter buds of shrubs before the first lasting snowfall.21 These kinds of evidence suggest that grouse have a seasonal cycle in gut length, a cycle that anticipates enforced seasonal changes in diet.
In captivity, grouse fed on grain-based diets have very short guts. The intestines and caeca of a stock of captive red grouse shortened generation by generation as the stock became further removed from its wild origins.22 After two to three years in captivity, the length of the small intestines of these captive birds stabilised at about 70 per cent of that found in wild birds. The length of the caeca fell to about 50 per cent and was still decreasing after four years, when the experiment ended.
In the wild, the long guts of red grouse are presumably maintained by natural selection, and the wild population has enough genetic or epigenetic23 variation to allow guts to shrink rapidly when the selective pressures imposed by their coarse diet of heather are removed.24 No doubt other costly adaptations to coarse diets are similarly labile. If a population of grouse had been captive for as long as domestic fowl, it is likely that many of their adaptations to natural foods would have diminished or disappeared altogether.25
We can think of the guts of gallinaceous birds such as grouse as existing in two modes.26 In low-fibre mode, the intake of food is small and is not limited by its sheer bulk, the intestines and caeca are short and do not vary in size with intake, and surgical removal of the caeca has little effect on the birds’ ability to digest their food. In high-fibre mode, the bulk of coarse diets limits the birds’ intake and, therefore, the intestines and caeca enlarge with the increasing intake, and the caeca are probably essential, although this has not been tested by surgical removal. Almost all physiological studies of caecal function in gallinaceous birds (mostly on domestic fowl but a few in captive grouse) have been carried out in low-fibre mode. The results of such studies cannot safely be extrapolated to birds in high-fibre mode.
A bird can better digest coarser foods if it has more capacious guts and associated adaptations. Hence the proportion of a food that is digested – its ‘digestibility’27 – is as much a property of the bird as of the food itself. A natural winter food such as heather is typically 40-50 per cent digestible in wild red grouse with long guts, but only 20-30 per cent in captive birds with the short guts that come from a relatively concentrated artificial diet.28
The digestibility of natural foods varies widely with season. New-grown shoots of summer heather, for example, are about 80 per cent digestible to captive red grouse. Tables of the digestibility of staple agricultural foods are widely used to calculate the digestibility and metabolisable energy content of compounded diets produced in pellet form for domestic animals. For wild animals eating natural foods, however, an attempt to produce similar digestibility tables would be a will-o’-the-wisp.
Natural variations in gut size can be used to make inferences about natural variations in diet. In Alaska, individual rock ptarmigan eating more berries – a preferred and readily digested food rich in soluble carbohydrate (sugars and starch) – had shorter caeca than other birds in the same flock.29 Such evidence suggests that individual differences in diet persist long enough to influence caecal length. Evidence from several grouse species, including rock ptarmigan, shows that the more dominant sex and age classes have shorter intestines and caeca, indicating that they use their status to get preferred foods.30 This may be relevant to observations that grouse of different sexes or ages sometimes use different habitats in winter.
BEYOND DIGESTIBILITY – PLANT TOXINS
Birds raised in captivity on digestible grain-based diets lose weight when given natural fodder without a period of adaptation. For coarse winter foods, it is tempting to attribute this simply to the ill-adapted captives’ poor digestive abilities. Captive red grouse, however, lose weight even when fed highly digestible summer heather or blaeberry. The paradoxical evidence suggests that the new growth of their main natural food is toxic to red grouse.
Captive red grouse given fresh, newly grown shoots of either heather or blaeberry digested 70-80 per cent of each.31 Surprisingly, they lost weight rapidly, despite eating large amounts. The problem of weight loss was circumvented by giving the birds diets composed partly of either heather or blaeberry, and partly of standard maintenance pellets, which were 52 per cent digestible when given alone. The digestibilities of diets with small proportions of natural foods exceeded 52 per cent (Fig. 149), as expected. But, paradoxically, as the proportions of natural foods increased, so the mixed diets became less digestible, until, in some cases, they were less well digested than pellets alone. The birds lost little or
FIG 149. Digestion by captive red grouse of diets with various proportions of natural summer foods (heather • and blaeberry o) and artificial maintenance pellets. Paradoxically, the digestibility (percentage of dry matter) decreased as the proportions of natural foods increased, even though the digestibility of natural foods (80 per cent in heather and 74 per cent in blaeberry) exceeded that of maintenance pellets (52 per cent).32
no weight during these trials, and so the maintenance pellets had counteracted the toxic effect of new growth, while the new growth inhibited digestion of the mixed diets.
The captive birds compensated for the lower digestibility of heather-rich diets by eating more of them (see Fig. 150). They hardly did so, however, with the equivalent blaeberry-rich ones. This suggests that they were better adapted to heather than to blaeberry, and is consistent with wild red grouse eating much more heather than blaeberry, even where the latter abounds.
These results, evidently the tip of an uncharted metabolic iceberg, indicate that the chemical defences of plants against grouse involve digestion-inhibiting toxins. Wild birds are adapted to nullify plant defences, but captives seem to lose some of these abilities. Adaptations lost so readily are evidently facultative, and probably physiologically costly.33 Also, the chemical defences of different plant species differ from each other. Hence the adaptations of different grouse species
FIG 150. Daily intake by captive red grouse of diets with various proportions of natural summer foods (heather • and blaeberry o) in relation to their digestibility. They ate more food as its digestibility declined, obviously so with the heather diets but barely for the blaeberry ones.
should also differ from each other, which in principle helps to explain their different food preferences. This topic is largely unexplored.
PLANT DEFENCES AND PROTEIN ECONOMIES
There is a vast scientific literature on plant defences against herbivores, and on how herbivores overcome these defences at physiological costs to themselves, often couched in terms of an evolutionary arms race. The topic has been little studied in grouse, but a winter diet of dwarf shrubs or conifer needles seems to make it difficult for the birds to get enough protein for their needs.
Polyphenols are the most widespread plant defence against herbivores. These include lignins, which inhibit digestion of the cellulose in plant cell walls (see ‘Caecal Function’, above), and tannins, which inhibit digestion of protein. Together, these polyphenols make up about a third of the dry weight of heather shoots in winter. During the course of digestion, red grouse absorb toxic phenolic molecules and detoxify their main breakdown product, benzoic acid, by conjugation with the amino acids ornithine and glycine, secreting the resultant conjugates into their urine.34
Conifer needles rely for defence partly on coarse fibre, but also on terpenes and other resinous compounds. The end result is comparable with red grouse eating heather, for blue grouse eating conifer needles in North America excrete the same amino acid conjugates.35 The nitrogen costs of detoxification to red grouse on a diet of heather and to blue grouse eating conifer needles seem similar, accounting for up to a quarter of the nitrogen in their food.
Protein nitrogen is in short supply in winter fodder and a quarter seems a large metabolic cost. Ornithine and glycine, however, are non-essential amino acids, which can probably be synthesised from urine nitrogen that is refluxed into the caeca and absorbed by the birds as ammonium ions. If so, detoxification of the harmful products created from the digestion of polyphenols and resins might use recycled, non-essential nitrogen, and so be less costly than it seems.
FOOD SELECTION
Metabolic adaptations are one way of coping with coarse or toxic foods. Another method adopted by grouse is selecting what they eat, the strength of selection varying with a bird’s physiological state, the season and the time of day.36 Selection can be seen as a hierarchical process. At the basic level, a grouse chooses to eat certain plant species – for example, capercaillie prefer Scots pine to Norway spruce. Within the chosen species, it prefers particular individual plants, and thus arboreal grouse typically use certain individual trees heavily, but others of the same species little or not at all. And within the chosen plant, a bird prefers particular structures – for instance, a black grouse in a birch tree prefers catkins to twigs. If eating twigs or shoots, a bird decides whether to peck small, delicate morsels from the tips or to take bigger, coarser portions.
Birds pack their crops with food before going to roost at night. They are pressed for time and hence less selective than earlier in the day. Thus, red grouse get better food during the day by feeding more slowly and taking smaller particles of heather than at dusk.37 Similarly, during relatively mild days in the Arctic winter, willow ptarmigan in northeast Asia feed slowly as they walk through shrub willows or dwarf birches.38 In the evenings or during periods of extreme frost, they browse in the crowns of bigger willow species, thereby getting coarser particles at a faster rate.
At least three other factors determine which plant species a bird decides to eat.39 First, part for part, some species are more nutritious than others, having more nutrients, being more easily digested or being less toxic. Willow, for example, is generally more digestible, contains more protein and phosphorus, and is less heavily defended than birch. Again, berries are a much-favoured food because they contain high levels of soluble carbohydrate. But not all berries are equal. Blaeberries soon disappear from Scottish hillsides in autumn, but some crowberries remain uneaten until spring. This is because blaeberries are more digestible and contain more protein, phosphorus and soluble carbohydrate than crowberries. It is easy to identify with the birds’ choice in this case, for to us blaeberries are sweeter and juicier than crowberries.
A second factor is that grouse species that live together tend to develop different dietary preferences. In Iceland, the rock ptarmigan is the sole naturally occurring vertebrate herbivore, and in winter it prefers willow to birch. In interior Alaska, the willow ptarmigan (the same species as red grouse) also prefers willow to birch, but in winter the rock ptarmigan eats much more birch than willow. Presumably the rock ptarmigan has adapted to birch in Alaska owing to competition from the bigger willow ptarmigan, which has won the better diet.
FIG 151. Blaeberries in autumn, after most leaves have fallen. (Robert Moss)
In Fennoscandia, willow ptarmigan again prefer willow to birch, but willow is less abundant here and so the birds eat mostly birch.40 In Britain, scrub willow and birch have largely disappeared from moor and hill, and so red grouse must get by mostly on heather, while ptarmigan subsist largely on heather, blaeberry and crowberry.
A third factor determining a bird’s dietary preferences is its adaptations. In Alaska, the rock ptarmigan has adapted to a winter diet of birch, and prefers birch to willow even when there are no willow ptarmigan nearby.41 In autumn, however, rock ptarmigan occupy higher ground than willow ptarmigan and, secure from competition, eat willow and birch in proportion to their availability.
Grouse make much finer distinctions than crude categories such as ‘willow’ and ‘birch’ convey. The many species of willow and birch vary in size from dwarf shrubs to trees, and, within each species, the different parts of the plant vary in nutritive value. Birds generally take catkins of birch, for example, in preference to the less nutritious buds and twigs. They prefer blaeberries and crowberries to leaves and shoots of the same plants. When eating heather, they take the tips of shoots in preference to the less digestible, woodier parts.
Wild red grouse eat heather shoot tips that are more digestible, and richer in protein and phosphorus, than other tips of the same size picked nearby by humans attempting unsuccessfully to emulate the birds. Faced with a sward of heather, red grouse tend to feed on richer patches and, within patches, choose the more nutritious shoots, being more selective when feeding on poorer patches. They can identify patches of heather that have been experimentally fertilised to increase their protein content, and prefer these to unfertilised areas. Even in captivity, caged red grouse can distinguish bundles of fertilised from unfertilised heather, but eat more from the fertilised bundles only when breeding, growing or in poor condition.42
FOOD ABUNDANCE
The staple foods of grouse are usually abundant and the birds eat a tiny proportion of them. One can hardly imagine red grouse devouring a noticeable proportion of the heather on a grouse-moor, for example, or capercaillie depleting a pinewood of its needles.
But the quality of different food items varies, the birds are selective feeders, and preferred foods can be in short supply. In the extremely cold winters of northeast Asia, for example, willow ptarmigan face a progressive depletion of their willow food during the winter, partly because mountain hares and moose
FIG 152. Willow (Salix pulchra) in Porcupine Creek, interior Alaska, during spring 1970. In April (top), when willow ptarmigan arrived on their snow-covered breeding grounds, these catkins were a favoured food. By the first few days of May (bottom), the birds had eaten all the available catkins, the only ones left being at the ends of long twigs out of reach. (Robert Moss)
use the same willow thickets. In one study,43 birds in autumn ate fine willow twigs some o.8-1.5mm in diameter. During winter, the twigs eaten became thicker and heavier. Finally, the remaining twigs were too thick for the willow ptarmigan to cut and so the birds turned to willow bark. This in itself is fairly nutritious, but eating it is laborious and, to avoid losing too much heat, birds had to return to their snow-holes before they had eaten enough. Consequently, they often lost weight, until the snow melted and better foods became available. Indeed, it seemed that a shortage of good-quality winter food could affect their condition in spring and so influence their breeding success in summer.
Crucial high-quality spring foods can also be in short supply, due perhaps to late growth or because a scarce food is overexploited. The newly growing shoots of cotton-grass, for example, are favoured by several species of grouse. In the basin of the Ilexa River near Archangel in Russia, they are eaten by capercaillie and also by small rodents, hares, moose, reindeer, bears, wolverines, cranes and other birds, and even pine martens. A shortage of cotton-grass shoots, it is argued,44 could affect the quality of the capercaillie’s diet and depress their breeding success. In Britain, sheep or deer also compete with red and black grouse, ptarmigan and capercaillie for cotton-grass shoots, perhaps with similar consequences.
FIG 153. Cotton-grass flowers. (Desmond Dugan)
If the best food is in short supply, an increase in grouse density might cause a decrease in diet quality. On Rickarton moor, for example, the proportion of digestible protein in the diet of breeding red grouse was lower in years of higher grouse density.45 This might have been due to a shortage of cotton-grass shoots or of the most nutritious heather shoots.
Weather can also reduce the amount of food available to grouse. Winter snow often covers dwarf shrubs. This has little effect on capercaillie, which in winter eat mostly conifer needles, or on black grouse, which can use birch or pine trees. Red grouse and ptarmigan dig through loose snow, or more often move to windswept ridges where food plants protrude above the snow. Icy conditions, however, due to freezing rain or to the melting and refreezing of snow, can prevent birds from foraging. Such conditions are common on Svalbard, where rock ptarmigan carry unusual amounts of winter fat, so insuring themselves against starvation in icy periods. Occasionally, ice can prevent even arboreal grouse from feeding. In one example, freezing rain encapsulated the twigs of aspen, the main winter food of ruffed grouse, and many birds died.46
Heather thrives best in relatively mild, oceanic climates and is vulnerable to ‘browning’ in late winter and early spring. Browning occurs in weather that is normal in continental climates but less frequent in oceanic Britain, during which the ground freezes, the sun shines and humidity drops. Unless covered by a protective blanket of snow, heather plants faced with such conditions lose water from their leaves faster than they can replace it through their roots, and as a result their leaves turn brown and so become unappetising to grouse. Some heather damaged in this way recovers and some dies. Either way, the amount of food available to breeding grouse can be much reduced. For example, about three-quarters of the heather was damaged in this way at Rickarton moor in March 1985, and breeding success that summer was only 1.2 chicks per hen, the poorest observed during a ten-year study.47 Much of the heather that survived this event undamaged was protected under patches of snow.
In the spring and early summer of 2003, much heather in northeast Scotland was damaged or killed by unusually dry conditions. At the same time, the largest outbreak of heather beetle in living memory was ravaging heather moors. This plague of pests began in the west of Scotland in 1997,48 reached Speyside in 2003 and was still active in 2004. The outbreak caused much worry among landowners, but there is no reliable account of how much it influenced grouse numbers.
Browsing by domestic stock is a less natural form of damage to food plants. Such browsing of heather over winter can be so heavy that a good autumn stock of red grouse almost disappears by spring. Much heather may remain on the ground, but is too short to provide the birds with cover from predators. Food without cover is of little use to grouse and so, though present, becomes unavailable.
In brief, the staple foods of most grouse populations seem to be in large excess of the birds’ requirements. High-quality foods that provide crucial supplements to the diet can, however, be in short supply. Also, occasional catastrophes can make much of the staple food unavailable. Such events might not occur in a short study lasting only a few years, thereby leading to incomplete conclusions about the importance of food abundance to grouse populations.
NUTRITIONAL BOTTLENECK – SPRING HENS
In British conditions, the nutrient requirements of grouse are most critical twice a year: in spring, when hens lay eggs; and in summer, when the young chicks are growing fast. Hens put on weight before breeding, but reserves alone are insufficient for a clutch of eggs. Most of the energy and protein required for a clutch must come from the food eaten during the week or two of laying.
As the staple food plants begin to grow in spring, they become more digestible and richer in nutrients. Red grouse hens, however, begin to lay so early in the year that the shoots of their main heather food have barely started to swell at the tips. Hen and cock feed close together, she taking smaller pecks than him and so getting a higher proportion of new growth.49 But this apparently is not enough, and hens seek other, earlier-growing foods.
Grouse generally supplement their spring diet with leaves of early growing herbs and other digestible, nutrient-rich plants. Ptarmigan on Scottish hilltops, for example, take spring leaves of common mouse-ear, heath bedstraw and sheep’s sorrel. On lower land, red grouse, black grouse and capercaillie are attracted to boggy ground where they find new shoots of cotton-grass, while black grouse and capercaillie are seen in larch trees eating buds and flowers.
Spring growth can be erratic, starting with warm weather and checked by cold. During a check, the nutrient content of growing shoot tips can fall. Even when new growth looks unchanged, it may be less nutritious. It is therefore too simple to think that early springs provide good nutrition and late springs poor. A prolonged spring that starts early but is then checked can leave hens in poor condition. In one study, for example, breeding success in ptarmigan was related not to the date of first growth, but to the number of days of unchecked growth before the hens started laying.50
On the heather-dominated, dry, infertile, granitic grouse-moors of northeast Scotland, however, early growing herbs and cotton-grass are scarce and red
grouse resort to picking out the growing shoot tips of cross-leaved heath and bell heather.51 This seems to be solely because these two plants begin to grow earlier than heather, for they contain less protein, phosphorus and soluble carbohydrates than heather, and at other seasons the birds largely ignore them. A greater proportion of the energy and nutrients in such new growth is presumably available to the birds than in the slower-growing heather. New growth also contains a hormone-like factor that stimulates egg-laying.52
The importance of newly growing plant material to the spring nutrition of red grouse and ptarmigan is well documented. They select such food out of proportion to its availability, and tend to rear more young when the spring flush is early, such that laying hens have had access to unchecked new growth for longer. Less work has been done on capercaillie and black grouse, but similar generalisations probably apply to them too.
A well-fed hen lays good-quality eggs, and these in turn hatch into vigorous chicks that survive well. This was discovered when clutches of red grouse and ptarmigan eggs were taken from the wild, and the chicks hatched and reared in standard conditions in captivity. Almost all chick mortality occurred in the first
FIG 155. Brood size of wild rock ptarmigan in Scotland and the proportion reared in captivity from eggs taken in 1968-71 from a hill with base-rich soil (•, Cairnwell hills) and one with acidic soil (o, Derry Cairngorm).53
two weeks. Chicks hatching from the poorest clutches sometimes died within a few hours of hatching, well before the yolk sac was exhausted. The proportion that died varied markedly between years and study areas. The example given here (Fig. 155) is from ptarmigan. Breeding success in the wild populations, from which the eggs were taken, was also measured. Survival in captivity and in the wild varied together, and so it was inferred that both were determined by a common antecedent cause, namely egg quality.
The viability of ptarmigan chicks from the Cairnwell hills exceeded that of chicks from the Cairngorms. The Cairnwell soils, derived mainly from relatively base-rich schists and other base-rich rocks, were more fertile than the granitic Cairngorms soils, and the Cairnwell birds had a richer diet. It seems that the effect of bedrock upon soil fertility was reflected in the quality of the laying hens’ diet, and that this in turn influenced the chicks’ viability and the number of chicks reared per brood.
On the infertile granite, egg quality tended to be better in years when springs were early and the laying hens had longer access to unchecked new growth of their main foods, heather, blaeberry and crowberry. However, on the richer schists this relationship was not clear, possibly because the boost to nutrition provided by spring growth was less critical to hens already on a relatively rich diet. Of course, early growth of the birds’ staple food plants might simply have indicated early springs, and other newly growing plants were probably an important part of their diet.
Well-fed hens lay better-quality eggs partly because they are in good condition. In Sweden, the diet of willow ptarmigan was found to be dominated by blaeberry, bog whortleberry and new shoots of cotton-grass.54 The hens were in better condition and reared more young after earlier springs, when they had been eating more cotton-grass shoots, and the diet was consequently more digestible as well as richer in protein and phosphorus. The hens’ condition in spring was also related to their condition in late winter, suggesting that winter circumstances might influence breeding success too.
Hens in good condition may also lay more eggs, but average clutch size varies little from year to year, and the main determinants of annual variations in breeding success are nest losses and chick mortality. A hen in poor condition is more likely to desert her nest,55 but most nest losses are usually attributed to predation. Interestingly, a few studies and many anecdotes suggest that a hen’s condition influences the chances of her nest being taken by a predator. Perhaps a hen in good condition sites it with greater care, is more thorough about covering her eggs with vegetation when leaving the nest after laying, sits tighter and so is less likely to reveal the nest to a passing fox, or is more wary of disclosing her nest to watching crows when leaving to feed.
As in many other birds, individual hens that begin to lay earlier in the year also lay larger clutches. This is something of a gamble. In good years, more eggs should result in more chicks. But in years when spring comes late, an early bird’s diet can be insufficient to support good-quality eggs. In such years, red grouse chicks hatched from large early clutches can be unviable and die soon after hatching.56
NUTRITIONAL BOTTLENECK – SUMMER CHICKS
Grouse chicks grow rapidly for about 12 weeks on a largely vegetable diet. Although summer plant foods are more varied and nutritious than winter ones, the chicks need a supplement of invertebrates for their first few weeks to provide them with protein and other nutrients. The chicks of bigger species have to grow faster and may need a larger supplement, although the evidence is confusing. For example, red grouse chicks can get by on a diet containing as little as 5 per cent of insect food,57 typically the flightless crane-fly Molophilus ater and the larvae and nymphs of the spittle bug Philaenus spumarius. The diet of very young chicks of the same species (willow ptarmigan) in a study in northern Norway, however, comprised some 60 per cent insects, mostly caterpillars, bugs, aphids and flies.58 None the less, capercaillie chicks seem to eat more invertebrates for longer than any other grouse species,59 consistent with the idea that bigger and hence faster-growing species need more invertebrates.
It seems inevitable that year-to-year variations in the availability of invertebrates must affect chick survival. Good evidence to this effect is surprisingly scarce. Studies on red grouse and willow ptarmigan have shown that chicks grow faster when they have a greater proportion of invertebrates in their diet,60 and that the slowest-growing broods survive poorly.61 But no study has shown that the broods on a given area survive better in years when they eat a greater proportion of invertebrates.
Part of the answer to this puzzle is probably that the quality of plant food fluctuates from year to year, such that the chicks’ need for invertebrates also varies. In addition, chicks may simply eat less food overall in years of poor survival, with no change in the proportion of animal food taken. Finally, invertebrates must vary in their value to growing chicks – a big juicy caterpillar is evidently more nutritious than a small dry beetle.
On the island of Tranøy in Norway, the summer of 1974 was warm and dry, but 1975 was cold and wet. In 1975, the chicks of willow ptarmigan spent less time foraging and avoided wet areas with dense vegetation. Sweep-netting for invertebrates on the ground revealed few differences between the years, and in each year the chicks’ crops contained the same proportion of insects. In 1974, however, the chicks ate half as much food again as in 1975, and fed selectively on caterpillars (80 per cent of their insect food), which occurred in wet areas with dense vegetation. In 1975 they ate mainly smaller insects such as aphids and flies. From each brood, an average of four chicks survived in the summer of 1974, but only half a chick in 1975.62
In a Scottish study,63 the breeding success of capercaillie was related more strongly to the size of the caterpillars available than to their abundance on the ground. In general, it seems that chick survival can be related to the qualities of the invertebrates available, that the most nutritious invertebrates can occur in particular habitats, and that accessibility of invertebrates in these habitats can be influenced by weather.
Over the years, we have watched and handled many ptarmigan, red grouse, black grouse and capercaillie chicks, leaving us with an anecdotal but definite impression. The smaller species are more vigorous, the larger more lethargic. On the ground, ptarmigan run about and forage more quickly than red grouse, which in turn are more active than black grouse, and these are more active than capercaillie. In the hand, all chicks have quiescent periods punctuated by bouts of struggling. Chicks of ptarmigan and red grouse tend to struggle most often and most vigorously, the other species less so, in order of size. At the same age, ptarmigan and red grouse chicks fly most strongly when flushed, capercaillie most weakly. Within a species, male capercaillie chicks have to reach twice the weight of females and so grow much faster. After their difference in size and plumage has become obvious, at about six weeks of age, males fly less readily than females and are more easily exhausted.
In short, chicks of the bigger species and the bigger sex grow faster, probably need more invertebrates in their early weeks, and seem to be less active. They may also have less energy for activities such as foraging and escaping from predators. It is as if chicks that devote more energy to growth have less for other activities.
These differences between species translate into broad differences in breeding success. Clutch size differs little between the four British grouse species,64 but red grouse usually rear more chicks per brood than greyhens, and greyhens more than capercaillie. Scottish ptarmigan do not fit this pattern, but most of them live on poor soils and experience catastrophic weather, such as summer snowstorms, more often than the other three species.
Males, being bigger, must grow faster than females, and so, if chick survival is inversely related to growth rate, we expect a difference between sexes within species. Indeed, female black grouse and capercaillie chicks survive better than males. In clement years with good breeding success, roughly equal numbers of male and female chicks are reared. But, when times are hard and breeding success is poor, more females than males survive. Furthermore, the difference in survival is greater for capercaillie, in which the difference in size between sexes is greater.65 Hence survival seems inversely related to growth rate, especially in hard times. The bigger, more lethargic chicks must find more food, especially invertebrates, to support their faster growth.
NUTRITIONAL BOTTLENECKS – DOUBLE BIND
We have discussed how nutrition can affect breeding success, indirectly through the condition of laying hens in spring, and directly through the chicks’ diet. In each case, available food is at its best for a short period. In spring, this is just as the hens’ food plants begin to grow (see Chapter 6), and in summer it is when the invertebrates eaten by chicks are most abundant. Chicks thrive if they come from good-quality eggs and hatch when invertebrates are at their peak.
The number of days between the onset of egg-laying and chick hatching is more or less fixed. But the length of time between the onset of plant growth and peak invertebrate abundance is much more variable. Spring in Britain has advanced in recent decades and birds generally have been breeding earlier. If grouse lay earlier, the chicks might hatch too early to benefit from the peak of invertebrate abundance.66 For capercaillie, there is evidence from the 1990s that moth larvae, their favoured insect food, pupated and became unavailable while the chicks were still relying on insects as a dietary supplement.67 There is, however, no good evidence that this represented a change from earlier years, or that it reduced breeding success.
NUTRITION AND DISTURBANCE
Disturbance is a topic of increasing importance in grouse conservation. Despite lack of firm evidence, it therefore seems useful to speculate about likely effects of disturbance upon grouse breeding success and survival. In the wild, grouse chicks generally grow more slowly than in captivity and, in some studies of wild grouse, growth rate has varied in relation to invertebrate abundance. It seems likely that continued disturbance will affect the chicks’ ability to forage, especially for invertebrates, and that this might slow their growth to a point where it imperils survival.68 This seems particularly likely in faster-growing chicks of bigger species, notably capercaillie, for these are intrinsically more sluggish and depend more heavily upon invertebrates.
The grouse family probably evolved in response to global cooling and occupied the newly created niche for a large bird that could subsist on coarse foods through long winters (see Chapter 1). Days are short in winter and the time available for foraging correspondingly limited. Grouse minimise the amount of food they must gather and digest by resting for much of the day, often in snow-holes.69 Disturbed birds must eat more food to compensate for the energy used in flight and, with increasing disturbance, there must come a point where they simply cannot get and digest enough food to maintain the fire of life. If so, the amount of disturbance they can tolerate without succumbing to slow starvation may well be limited. Such an effect is likely to be more critical in bigger birds, especially the capercaillie, which are more cumbersome and expend more effort when taking flight.
SUMMARY
Grouse subsist over the winter on plant foods that are abundant, but coarse, fibrous and low in protein. Their adaptations include well-developed caeca, in which they digest fibre and conserve protein by recycling urine. They are also selective about what they eat, choosing a good-quality diet from the food available. Food shortages in winter, due to browsing by larger herbivores or to catastrophic weather, can reduce the birds’ survival and breeding success. Even so, effects of diet upon survival and reproduction are more commonly seen in laying hens and growing chicks. Hens supplement their spring diet with early growing nutritious vegetation. When this is not available, as in late springs, they get into poor condition and lay deficient eggs that hatch into less viable chicks. Young chicks depend upon a supplement of invertebrates for a few weeks after they hatch. If they cannot get enough, as in wet weather, they grow more slowly or die.