Hogan-Warburg A.J. (1966) Social behaviour of the Ruff, Philomachus pugnax (L.). ARDEA 54 (3-4): 109-229

1. Courtship and mating in the Ruff occur on a lek -a communal display ground. This study, based on four seasons of field observations, investigates the social organization of the lek community and the behaviour patterns that serve mutual communication among the birds; it also describes morphological features in their external appearance. 2. Very marked sexual dimorphism exists in both size and plumage. Males are larger than females and develop a distinctive nuptial plumage. The length and brilliance of the nuptial feathers, the size of the wattles, and the colour of the legs and bill are influenced by age. Extreme plumage diversity exists among males. The colours of the ruff and head tufts vary from black through brown, red, and yellow, to white. Both ruff and head tufts may be plain or show transverse striping, spots, or a pattern called the bib. The wattles show different shades of yellow and red. The values each of these dimensions assume can vary to some extent independently. Thus, completely identical males on one lek are very rare. Certain combinations of colour and pattern, however, are more frequent than others. Plumage types in individual males are constant from year to year. The plumage diversity among males makes individual recognition easy. 3. A lek site consists of a number of bare spots about 30 cm in diameter and about 1 meter from each other. The bare spots are called residences. Within the male community two groups can be distinguished: independent males and satellite males. The group of independent males can be subdivided into resident males and marginal males; the group of satellite males can be subdivided into central and peripheral satellite males. These distinctions are based on differences in territoriality and attachment to the lek and in the behaviour patterns shown by the various groups of males. Resident males possess a residence which they occupy nearly continuously during the day, and which they defend from other independent males. After their nocturnal absence, each resident male returns to his own residence. Marginal males stay at the edge of the lek outside the residences. They visit the lek only infrequently. Occasionally severe territorial fighting is seen among resident males, which may lead to the conquest of another one's residence. The resident male which loses his residence usually leaves the lek and returns only in the status of marginal male. A marginal male may acquire resident status by establishing a residence at the edge of the lek. In contrast to resident males, many marginal males visit different leks. Satellite males do not possess a residence of their own. Rather, they make use of the residences of the resident males in the presence of the owner. A satellite male is never seen to show any form of overt aggressive behaviour toward his host. The reaction of the visited resident male is highly variable: he may try to expel the satellite male by attacking him, or he may allow the satellite male to enter his residence and to remain there with him-in the latter case, the resident male shows behaviour toward the satellite male that is similar to the behaviour shown toward a female. The reaction of a resident male depends on the individuals involved; further, resident males on large leks are consistently less tolerant toward a visiting satellite male than resident males on small leks. One to three satellite males may visit the same residence at a time. Although an individual satellite male usually shows a preference for one or a few particular resident males, satellite males frequently change hosts; they also visit different leks. Satellite males spend more time on small leks than on large leks because of the intolerance of resident males on the latter. Central satellite males spend relatively more time on the lek and its residences than peripheral satellite males which are easily driven off to the edge of the lek by resident males. As one extreme, a specific satellite male which visited a particular small lek almost exclusively spent nearly as much time there as the resident males. In interactions among each other, resident males show fighting, attack, charge, and agonistic displays; this behaviour serves the function of conquest and defence of residences, and is predominantly aggressive. Most of the time, however, resident males stay peacefully together on the lek, each on his own residence. In this situation a display posture is shown which reflects balanced agonistic motivation. Toward, marginal males, resident males show behaviour similar to that shown among each other, but marginal males react with overt escape behaviour or with displays in which escape components are very marked. Among marginal males, interactions usually do not occur, though on rare occasions behaviour similar to that shown among resident males is seen. Toward satellite males, on large leks, resident males show attack, charge, and agonistic displays-behaviour which is predominantly aggressive; on small leks, resident males show behaviour with relatively balanced agonistic motivation. Toward resident males, on large leks, satellite males primarily show behaviour in which aggressive components are predominantly absent, but a few escape components are recognizable; while, on a small lek, the central satellite males frequently show behaviour in which both aggressive and escape components are predominantly absent, and which evidently has a strong appeasing effect on the resident males; the performance of such behaviour patterns evidently enables the satellite males to enter and stay on a residence in the presence of the owner. The distinctive reaction of resident males toward satellite males on small and large leks must be the result of a difference in the activation of aggression in the resident males. Transformations between independent male and satellite male occur neither within a season, nor between successive seasons. A very few males of both groups (less than 3%) occasionally show behaviour in some respects characteristic of the opposite group (anomalous behaviour). The status of individuals within the independent males and satellite males is influenced by age. The independent male begins in the status of marginal male, with increasing age he acquires the status of resident male, and still later in life probably loses his resident status and becomes a marginal male again. Satellite males change status from peripheral to central as they grow older. Males with anomalous plumage (see below) appear to have a reduced chance to acquire resident status (for independent males) or central status (for satellite males). Of more than 200 males observed, about 38% were satellite males. Analysis of the plumage diversity within each group of males led to the conclusion that behaviour was highly correlated with the colour of the plumage: the typical plumage of independent males has black or dark-coloured ruff and head tufts or a white (or almost white) ruff with black head tufts; the typical plumage of satellite males has a white or almost white ruff and head tufts or, somewhat less common, a white or almost white ruff with coloured head tufts. Anomalous plumage among independent males and satellite males was seen in 11% and 9% of the cases, respectively. Females visit the lek for short periods. After landing they place themselves near a residence where they usually remain during their total stay at the lek. They may also move around in the interresidential area to visit other residences. They sometimes step on a residence and crouch and copulate. Only males on a residence (either resident male or satellite male) copulate. Females visit more than one lek. Females normally approach the lek by air. The males on the lek may react with a sequence of very conspicuous displays called the reception ceremony. After the females have landed, the resident males sink into the squat and freeze in that posture on their residence; satellite males, after having selected a residence, squat and freeze together with the resident male. Females may visit either a singly-occupied residence (resident male only) or a multiply-occupied residence (resident male and one or more satellite males). A resident male and/or satellite male may interrupt squatting from time to time by raising themselves slightly and turning from side to side; the resident male is usually oriented with his tail toward the female, while the satellite male usually faces the female. A resident may also interrupt squatting to attack or to show agonistic displays toward a satellite male on his residence; this may result in expelling the satellite male. A resident male may also interrupt squatting to attack or charge males outside his residence or to show agonistic displays toward such males, while staying on his residence. A satellite male interrupts squatting only to perform behaviour directed toward a nearby female. Females are evidently stimulated to step onto the residence and crouch during such interruptions of the squat. Copulation may then ensue. On a doubly-occupied residence, the satellite usually has an opportunity to copulate successfully in the short period that the resident male is absent from the residence during a charge or attack on outside males; the resident male usually copulates just after he has expelled the satellite male; when both males are present, interference frequently inhibits successful copulation. The presence of females increases the aggressive behaviour shown by resident males toward satellite males; females do not have a similar effect on satellite males. Fighting among resident males also increases in the presence of females, especially on small leks. Excessive fighting has a frightening effect upon females. On large leks, resident males alone on their residences are almost exclusively the copulating partners; on small leks, the presence of satellite males on a residence increases the chances for copulation for both resident males and satellite males. In one area with two leks, the frequency of copulations by satellite males was approximately proportional to their frequency; in another area with two leks, satellite males copulated relatively less frequently than independent males. Both males and females may copulate with more than one partner, although in the case of females promiscuity is probably less pronounced. Females choose the residence on which they will copulate. Certain residences (and thus particular resident males and/or satellite males) are chosen much more frequently than others. The choice of the female appears to depend mainly on individual characteristics in the behaviour and plumage of the males, especially rising from the squat; the previous experience of the female also seems to play an important role. Similar factors are involved in the host selection of the satellite males. Infrequently males without a nuptial plumage (naked-nape males) temporarily visit a lek. These birds are probably either young birds or birds from a more northern population which develop their plumage later in the season. 4. Except for the fighting activities, behaviour patterns were grouped according to the position of the body axis. Attacking activities and forward postures are shown only by independent males. Oblique, upright, and horizontal postures are shown by both independent males and satellite males; between the two groups of males, these postures differ primarily in that independent males hold their bill in a conspicuous position, whereas satellite males tend to conceal their bill in the ruff. 5. It is argued that the Ruff provides an example of balanced behavioural polymorphism. A model has been constructed of how the balancing forces operate to ensure the behavioural polymorphism. It is suggested that the survival value of satellite males lies in the facts that 1) their presence increases chances for copulation on small leks and 2) they promote the establishment of new leks and the maintenance of several leks in an area. It is suggested that the marked plumage diversity among independent males serves to facilitate individual recognition by both females and other males; the less marked plumage diversity among satellite males serves primarily to identify satellite males as such to both females and other males. The evolution of both the behavioural and morphological polymorphism is briefly discussed.