Chris Knights and Terry Andrewartha have taken some remarkable. 440 [Brit. Birds 73: , October 1980]

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1 Chris Knights and Terry Andrewartha have taken some remarkable (pictures of displaying Capercaillies Tetrao urogallus which illustrate an obvious question: why are cock Capercaillies so much bigger than the hens? Body size and ecological niche An adult cock weighs over 4 kg, a hen about 1.7 kg. His bill is also very much larger than hers and is a prominent feature in his display (plate 219). Many studies on the idea of ecological 'niches' have shown that body size in birds is closely adapted to the life-style of the species; in particular, bill size and bill shape are closely related to feeding habits (Lack 1971). We might therefore guess that cock and hen Capercaillies should eat foods which differ at least as much as the diets of, for example, cock and hen Sparrowhawks Accipiter nisus (Newton 1979). In fact this is not the case. Cock and hen Capercaillies eat very similar diets, consisting largely of needles of Scots pine Pinus sylvestris in winter (Zwickel 1966) and a variety of other foods including bilberry Vaccinium myrtillus and heather Calluna vulgaris in summer. Although hens tend to occupy denser parts of a forest than cocks (Seiskari 1962), it seems unlikely that such a minor difference in ecological niches could explain such a major sexual difference in body size. The inference from this appears a paradox: either cock Capercaillies are too big for their ecological niche, or hen Capercaillies are too small for theirs. Surprisingly, the scanty evidence available confirms this suggestion: cock Capercaillies may be too big. Data from Norway (Wegge 1979) and preliminary results from Scotland (fig. 1) indicate that, in years when 440 [Brit. Birds 73: , October 1980]

2 Why are Capercaillie cocks so big? 441 Brood size Fig. 1. % cock chicks in broods of Capercaillies Tetrao urogallus and brood sizes (chicks per hen, excluding barren hens) in late summer in south Norway (dots, copied from Wegge 1979) and Glen Tanar, Grampian, Scotland (open circles, ). Tendency for % of cocks to decrease with decreasing brood size statistically significant at 1% level (combining probabilities (Fisher 1950) from Spearman rank correlation coefficients for the two studies) broods are small, cock Capercaillie chicks suffer heavier mortality than hen chicks. If further study confirms this, a reasonable explanation might start from the fact that cock Capercaillies have to grow more than the hens to reach their autumn weight. Hence, more cocks should die as chicks in years when broods are small and cock chicks are presumably under greater stress than hens. The nature of the presumed stress causing chicks to die is not known, but an obvious suggestion is poor food (Wegge 1979): this is possibly worse in years when the weather is colder and wetter than usual (Slagsvold & Grasaas 1979). Bad weather on its own is an unlikely cause of differential mortality between cock and hen chicks: the ratio of surface area to total volume is greater in the relatively small hen chicks and, hence, they should suffer relatively greater heat loss than the cocks. Cold, wet weather 219. Capercaillies Tetrao urogallus, cock A and his harem (note relative sizes of cock's and hen's bills), Grampian, May 1977 (C. R. Knights & T. Andrewartha)

3 442 Why are Capercaillie cocks so big? should therefore kill more hens than cocks (so long as the food is adequate) and this is the opposite of what is observed. Interestingly, only two cock chicks, but 17 hen chicks, were seen in Glen Tanar, Grampian, in August 1977 (the year of the smallest broods there, fig. 1) and one cock was smaller than his sister while the other was no bigger. Usually, cocks of this age are much larger than their sisters, both in the wild and in captivity. Even if the suggestion that cock chicks sometimes survive less well than the hens is confirmed, it fails to explain why cocks have evolved the seeming disadvantage of big size. The obvious answer is that there is some compensating advantage in being big, and the photographs of a cock surrounded by hens crouching to invite copulation (plate 220) give a strong hint as to what this might be. Perhaps big cocks have evolved because hens preferred them. A first step toward testing this idea is to ask if hens do prefer particular cocks Capercaillies Tetrao urogallus, hens crouching to invite copulation, Grampian, May 1977 (C. R. Knights & T. Andrewartha) Choice of mate There are several descriptions of Capercaillie display grounds in the literature and they differ considerably. Nonetheless, all agree that several cocks often gather together at a traditional site in the forest, where they display, sing and fight for a few weeks in the spring. Such sites are often in a relatively open part of the forest and on the top or side of a hill. The remarkable song of the cock Capercaillie is delivered with great vigour and with the aid of a part of his oesophagus that inflates like a toy balloon. Such inflatable organs are generally considered an aid to noise production and it has seemed paradoxical that a singing Capercaillie can scarcely be heard by a human listener at a distance of 200m. It appears, however, that the song includes a low infrasonic growling note which cannot be heard by human ears but which may be detectable by other Capercaillies at a distance of more than 200m (Moss & Lockie 1979).

4 Why are Capercaillie cocks so big? 443 Several species of grouse have communal displays and most observers think that there is usually a 'master cock' which tends to occupy the central position on the display ground and to mate with the majority of the hens that attend for this purpose. This kind of observation is usually rationalised by saying that the master cock is dominant over other cocks, and that hens prefer him for this reason. There is, however, an element of circularity in this explanation, as no unambiguous measure of dominance usually exists that is independent of the individual's location on the display ground and independent of the number of his matings. An additional complication is that not all displaying groups of cocks conform to this pattern (Johnstone 1969; Hjorth 1979). The Capercaillie is perhaps the least formally organised of the communally displaying gamebirds. The Blackcock Tetrao tetrix is more typical: each Blackcock has his own well-demarcated territory to which he returns each morning. Territory boundaries are usually well defined and are the same morning after morning. In such cases one may ask: do hens prefer the master cock because he is the best cock, or because he occupies the central territory? Three cocks attended regularly at the display ground where Chris Knights and Terry Andrewartha took notes and photographs; from lack of imagination, we called them A, B and C (fig. 2). Other cocks were seen occasionally. Cock A was invariably surrounded by hens, up to 15 on one morning (table 1); although B and C displayed vigorously, they usually attracted no hens. The sketch map illustrates how A, B and C were dispersed on a typical morning, but this was by no means the same every morning. For example, B often displayed on ground that A occupied on other mornings or at other times on the same morning. On occasion, A walked out of sight, with all the hens following him, well away from his usual display area. Evidently, they were more interested in A than in any particular piece of ground. Steep NNE facing slope about 1 in 3 Road No ground visible below road Fig. 2. Sketch map of Capercaillie Tetrao urogallus display ground Glen Tanar, Grampian, 1977

5 444 Why are Capercaillie cocks so big? Table 1. Calendar of observations of Capercaillies Tetrao urogallus at a display ground at Glen Tanar, Grampian, in 1977 * = including four in trees. Compiled from notes by T. Andrewartha, C. R. Knights, R. Moss and D. Watt. Date Cock A Cock B Cock C Cock D Total hens April 23 April 24 April 25 April 26 April 27 April 28 April 29 April 30 May 1 A beat B 8 hens A beat B and C 12 hens 11 hens A beat B 15 hens A beat B 12 hens 4 copulations 5 hens 5 hens A chased C 5 hens 2 hens (briefly) 2 hens (briefly) (on A's ground) The hens surrounded A in a dense crowd (plate 221). They often chased, displayed at and pecked one another (plates 222 & 223), while competing to get close to him. Sometimes, a disputing hen adopted the same posture as a displaying cock, with head and neck held vertically, beard erect, and with tail raised and fanned. As A sang and displayed, some of the hens in front of him squatted in the pre-copulatory posture. He would then strut past them, 221. Capercaillies Tetrao urogallus, hens crowding in front of cock, Grampian, May 1977 (C. R. Knights & T. Andrewartha) 12*

6 222 & 223. Capercailyes Tetrao urogallus, hens sparring in front of cock, Grampian, May 1977 (C. R. Knights & T. Andrewartha) whereupon hens that had now been left behind stood up (plate 224) and ran round to the front of the crowd, to squat again, and so on in a continuing procession. Occasionally, a hen flew on to the branch of a tree to rest for a 224. Capercaillies Tetrao urogallus, hens standing up after cock has strutted by them, Grampian, May 1977 (C. R. Knights & T. Andrewartha)

7 446 Why are Capercaillie cocks so big? 225. Capercaillies Tetrao urogallus, copulation, Grampian, May 1977 (C. R. Knights & T. Andrewartha) few minutes before returning to join the living carpet which surrounded A as he continued his regal progress. We presumed that A was there to copulate (plate 225), but were astonished to see how few hens he actually mated in a morning. We saw a maximum of four copulations on any one morning, although there were 12 or more hens on several mornings. We may have missed seeing a few copulations, but we are certain that A did not achieve 12 matings on any morning that we were there. Therefore, many hens left without having been mated on many mornings, presumably to try again the next day and until mated. This seeming disinterest was all the more surprising by contrast with Black Grouse and other lekking gamebirds where a hen that invites copulation usually gets served at once. David Lack once watched a Blackcock mounting a stuffed hen Black Grouse 56 times in 45 minutes (Lack 1939). A, however, was not unique among Capercaillie. In the spring of 1967, I watched a single Capercaillie hen offering herself to a solitary displaying cock for four hours. She eventually gave up and left him displaying by himself while she flew off to feed on newly growing needles of larch Larix. On a wet and miserable morning (29th April), one of the last that hens were seen in 1977, two hens did leave A and walk over towards B. He displayed vigorously while trotting forward impetuously to welcome them. At this, they flew up into the trees and settled there, not returning to the ground that morning. C, however, was a little more successful. On two occasions, he approached the group of hens while A and B were fighting violently. On the first occasion (on 24th), the fight lasted six minutes: A and B rolled together 20m down the steep hill, away from the hens and into ground previously occupied by B. When A returned, he found C displaying to the hens, chased him off after a fight lasting three minutes, and then mated with two of the hens. On 26th April, C tried the same ploy with more success. B had approached to within 5 m of A, who left his harem to deal with the intruder. Meanwhile, C approached the 12 hens and, although nine of them flew off into the trees, two did mate with him in the ten minutes that A was fighting B.

8 Why are Capercaillie cocks so big? 447 These fights were serious matters Two fighting cocks pecked vigor ously at each other's heads an< buffeted one another with their wings, sometimes rolling head over heels downhill while locked in combat. B was bleeding from the head after the fight on 26th, and even A was somewhat battered (plate 226). It is by no means unusual to see such battle marks on cocks in the spring, and there have been several reports of cocks being killed in fights (e.g. Hjorth 1979). We saw A beat B in four fights; A beat C in one fight and chased him away from the hens on another occasion. The hens, it seems, preferred the best fighter: A Capercaillie Tetrao urogallus, cock A, note feathers missing from head following fights, Grampian, May 1977 (C. R. Knights & T. Andrewartha) Here we have a possible resolution of our paradox. If hens prefer the best fighters, and if, in the past, big size has conferred an advantage in fighting, then natural selection sexual selection is likely to have favoured big cocks. This can, however, be only part of the story: although the sexes are dimorphic in other polygamous species of grouse, none of them shows such a big difference in body size as do Capercaillie cocks and hens. Acknowledgments J. Oswald showed us the display ground; I. B. Trenholm drew the figures; and Dr A. Watson and Dr D.Jenkins criticised the manuscript. References FISHER, R. A Statistical MethodsforResearch Workers. 11th ed. Edinburgh. HJORTH, I Discussion following MULLER, F., A 15-year study ofa Capercaillie lek in the western Rhon-Mountain (W. Germany). In LOVEL, T. (ed.) Woodland Grouse. World Pheasant Association, Bures, Suffolk. JOHNSTONE, G. W Ecology, dispersion and arena behaviour of Black Grouse, Lyrurus tetrix (L) in Glen Dye, N.E. Scotland. PhD thesis, University of Aberdeen. LACK, D The display of the Blackcock. Brit. Birds 32: Ecological Isolation in Birds. Oxford & Edinburgh. Moss, R., & LOOKIE, I Infrasonic components in the song of the Capercaillie Tetrao urogallus. Ibis 121: NEWTON, I Population Ecology of Raptors. Berkhamsted. SEISKARI, P On the winter ecology of the Capercaillie, Tetrao urogallus, and the Black Grouse, Lyrurus tetrix, in Finland. Pap. Game Research 22: SLAGSVOLD, T., & GRASAAS, T Autumn population size of the Capercaillie Tetrao urogallus in relation to weather. Ornis Scand. 10: WEGGE, P Status of Capercaillie and Black Grouse in Norway. In LOVEL, T. (ed.) Woodland Grouse, pp World Pheasant Association, Bures, Suffolk. ZWICKEL, F. C Winter food habits of Capercaillie in north-east Scotland. Brit. Birds 59: Dr R. Moss, Institute of Terrestrial Ecology, Banchory Research Station, Blackhall, Banchory, Kincardineshire AB33PS