(405) DISPLACEMENT-SLEEPING IN THE AVOCET AND OYSTERCATCHER AS A REACTION TO PREDATORS.

Transcription

1 (405) DISPLACEMENT-SLEEPING IN THE AVOCET AND OYSTERCATCHER AS A REACTION TO PREDATORS. BY K. E. L. SIMMONS AND R. W. CROWE. (Illustrations by Robert Gillmor.) MAKKINK'S studies (1936, 1942) on the behaviour of the Avocet (Recurvirostra avosetta) and the Oystercatcher (Hasmatopus ostralegus) have shown that, at certain points during intraspecific fighting', both these species may assume a "pseudo"-sleeping attitude in which the bill is placed in the scapulars, as in real sleeping, but the eye kept open (figs, a, b). Recently, Williamson (1950, 1952) has further noted that in the Oystercatcher this behaviour may also occur as a response to human disturbance, both on the breeding ground and in the flock. The present paper aims to record similar behaviour in the Avocet and to discuss the reaction, in both species, against the background of recent theories on the nature of predator-reactions in general. Observations on Avocets were made on Texel, Netherlands, mainly in June, 1952, but unavoidable delays in publication have allowed the inclusion of some observations made in May, We watched several pairs of these fascinating and attractive birds at various stages of breeding activity: some had eggs, some were at the hatching stage and others had small or semilarge young. A full, illustrated, account of our work will appear elsewhere in conjunction with studies on the Little Ringed and Kentish Plovers (Charadrius diibius and alexandrinus) and some other waders. We are indebted to Dr. N. Tinbergen for reading over the draft of the paper, and to Robert Gillmor for the illustrations. "PSEUDO "-SLEEPING AND OTHER DISPLACEMENT-ACTIVITIES PERFORMED BY AvOCETS AS PREDATOR-REACTIONS. As in the Oystercatcher (Williamson, op. cit.), the "pseudo"- sleeping shown by breeding Avocets to human intruders is a lowintensity reaction (see below); this seems generally the case for the displacement-activities of waders in such circumstances Simmons (1951, 1952). When the Avocets are really worked-up, they perform distraction-display and may even fly at the observer though this "attack" is seldom, if ever, pressed home, the birds swerving off without striking. We only definitely recorded "pseudo"-sleeping from those birds with eggs, though probably it may also occur when the young are large, as Williamson notes in the Oystercatcher. The usual procedure of Avocets with eggs or young, both on the ground and in the water, is to perform a type of crouch-run away from the observer (fig. c), as recorded

2 406 BRITISH BIRDS. [VOL. XLVI. AVOCET {Recurvirostra avosetta). DISPLACEMENT-SLEEPING AND SOME OTHER REACTIONS TOWARDS PREDATORS. (Drawn by ROBERT GILLMOR.) (See text.) in the Kentish Plover (Simmons, 1951), Little Ringed Plover and Ringed Plover (C. hiaticula) (Armstrong, 1952 ; Simmons, 1953), though in the Avocet this is a deliberate trot rather than a fast run. Birds with eggs then usually follow this up with a displacement-activity, either feeding, brooding, preening or sleeping, and rarely pass on to more elaborate behaviour as do those with

3 VOL. XLVI.] AVOCET AND OYSTERCATCHER. 407 young-. The "pseudo"-sleeping- attitude may be performed while standing- on one or both legs and also when sitting down. In this last instance we had the definite impression that the brooding- and sleeping attitudes were being combined, as is so often the case with undisturbed incubating birds (see Brown, 1950, p. 40). If the watcher is relatively inactive and does not follow the performing- bird, it may keep up its displacement-activities for several minutes at a stretch. The feeding actions used are the sideways sweeping ones ("food-mowing-," Makkink, 1936) functionally employed in the water, and correspondingly seem very inappropriate when performed out of context on land. Birds with small young also show the lower-intensity brooding and feeding movements (at least) but frequently these are "by-passed" and the simple crouch-run develops into moving distraction-displays with open wings (e.g. fig. d). These in turn may be followed by more elaborate, stationary or semi-stationary displays facing the observer (figs, e, f), including the curious "drifting" or "drunken" one (e), and by "attack" (fig-, g). Birds with eggs, when highly activated, may occasionally perform the more elaborate behaviour. A REVIEW OF THE NATURE OF ">SEUDO"-SLEEPING. It is now generally agreed (Tinbergen, 1940, 1952 ; Kortlandt, 1940; and others) that "pseudo"-sleeping, in both Avocet and Oystercatcher, is a displacement-activity in the true sense of the word, i.e. "an activity belonging to the executive motor pattern of an instinct other than the instinct(s) activated" (Tinbergen, 1952). In view of this, the term "displacement-sleeping" is preferable to "pseudo-sleeping" because some "pseudo" or "false" activities need not necessarily be displacements in the strict sense. Further, it is generally agreed that, in its fighting context, sleeping- occurs when the urges to attack and to withdraw are in equilibrium (Makkink, 1942), each drive inhibiting the expression of the other so that a displacement-activity results, as an outlet for these thwarted impulses. This last interpretation is opposed by Makkink (loc. cit.) who doubts the value of displacementsleeping "as a ventilation valve for energy, the more so as the sleeping attitude is preeminently suited for resting because requiring the least energy." Edwards et al. (1948) argue that "it is quite possible for the emotional tension to be dissipated merely by the forced assumption of a resting attitude." Both these views ignore one important point. Essentially we have to do with the dissipation of nervous not muscular discharge. The behaviour under review is thus an effective channel for the release of blocked motivation because sleep, as shown conclusively by the experiments of Hess (1944), "depends on activity of a strictly localized part of the brain," and "behaves exactly like movements generally recognized to be dependent on nervous activity" (Tinbergen,

4 408 BRITISH BIRDS. [VOL. XLVI. 1952). This shows "that there is no inconsistency in the occurrence of sleep as a displacement activity." In its fighting' context then, displacement-sleeping is an outcome of the joint stimulation of the urges to attack and withdraw. Is the activity in its second role as a predator-reaction a result of similar drives? Williamson (op. cit.) thinks not and suggests that here it is due to a "state of low emotional value in which the impulse to carry on normal routine (feeding, preening, etc) was inhibited, and nicely balanced, by the declining impulse to remonstrate against my presence." However, we would suggest that in displacement-sleeping, as in so many other predator-reactions (distraction-display, intention-movements, displacement-activities, "mobbing," etc.), basically the same dual motivation is in operation as in intraspecific fighting; this concept has recently been reviewed in some detail by Simmons (1952), and by Hinde (1952) and Tinbergen (1952). One can see movements of this kind alternating with escape and aggressive behaviour; in the case of the Avocet, flying away and crouch-running on the one hand, and aerial "attack" on the other. The theory gains further support when one considers that many of the displacement-activities possessed by Avocet (brooding, sleeping, feeding) and Oystercatcher (brooding, sleeping) are common to both the situations under question, as they are in the Little Ringed, Kentish and Ringed Plovers (Simmons, 1953). It is unlikely that in all these instances the underlying causes are different in the two contexts.* Finally, a word on possible function. Williamson (op. cit.) records displacement-sleeping from Oystercatchers disturbed when in flock, outside the breeding-season. K.E.L.S. has a single record, from Egypt in the winter of , of a rather "tame" Avocet which, as he stalked it, initially performed displacement food-mowing, first on land and then in the water, before flying away. When the bird was again approached, it displacement-slept in shallow water. These instances support the view that sleeping, and the other displacements mentioned, are low-intensity reactions in which neither of the two drives that produce them are strong ones. With breeding and non-breeding birds alike, when the escape element grows in intensity the birds move off; in those Avocets and the more northern Oystercatchers with young, an increase in the strength of the aggressive element produces distraction-display. By including displacement-sleeping in the list of the Oystercatcher's distraction-displays, Williamson (op. cit.) implies that the activity has been selected for a deflection function. If this were so, as N. Tinbergen points out (in litt.), then the sleeping posture shown as a predator-reaction should be different * In a very recent contribution (antea pp ), appearing since the present paper was accepted for publication, Williamson has changed his views, without reference to his former ones, now holding that in all cases displacement-sleeping appears "when the impulse to attack or remonstrate against intrusion is balanced by the desire to withdraw from conflict."

5 VOL. XLVI.] AVOCET AND OYSTERCATCHER. 409 from that appearing- in intraspecific fighting:. We believe that sleeping- and other displacements performed as predator-reactions (with the probable exception of brooding in Oystercatchers) are merely by-products of the internal state and are not distractiondisplays in the strict sense. No one would claim, for instance, that the less spectacular head-scratching and defaecating movements shown by Little Ringed Plovers, when disturbed with eggs or young, had an evolved deflection function. It is because so many of the activities performed by breeding birds to predators cannot be classed under such restricted terms as "diversionary display" (Armstrong, 1949) or "distraction-display" that the more general term "predator-reaction" has been suggested (Simmons, 1952). The context in which this last term is used will indicate whether breeding or non-breeding birds are concerned. SUMMARY. The displacement-sleeping activity of the Avocet and Oystercatcher is reviewed in its role as a predator-reaction, especially as shown by breeding birds. The view is put forward that this behaviour, both as an element of intraspecific fighting and of predator-reaction, is a result of the joint stimulation, at relatively low-intensity, of attack and escape. Some other displacementactivities of a similar nature are also dealt with. REFERENCES. ARMSTRONG, E. A. (1949). "Diversionary display. Part 1. Connotation and terminology." Ibis, 91: (1952). "The distraction displays of the Little Ringed Plover and territorial competition with the Ringed Plover." Brit. Birds, 45: BROWN, P. E. (1950). "Avocets in England." Occas. Pub. Royal Soc. Protect. Birds, 14: EDWARDS, G., HOSKING, E., AND SMITH, S. (1948). "Aggressive display of the Oyster-Catcher." Brit. Birds, 41: HESS, W. R. (1944). "Das Schlafsyndrom als Folge dienzephala Reizung." Helv. physiol. Acta, 2: HINDE, R. A. (1952). "The behaviour of the Great Tit and some other related species." Behaviour, Supplement 2, pp KORTLANDT, A. (1940). "Wechselwirkung zwischen Instinkten." Arch. Nee"rl. Zool., 4: MAKKINK, G. F. (1936). "An attempt at an etho'gram of the European Avocet..." Ardea, 25: (1942). "Contributions to the knowledge 6f the behaviour of the Oyster-Catcher." Ardea, 31: SIMMONS, K. E. L. (1951). "Distraction-display in the Kentish Plover." Brit. Birds, 44: : (1952). "The nature of the predator-reactions of breeding birds." Behaviour, 4: (1953). "Some aspects of the aggressive behaviour of three closely related plovers." Ibis, 95: TINBERGEN, N. (1940) "Die Ubersprungbewegung." Zs. Tierpsychol., 4: i-4".

6 410 BRITISH BIRDS. [VOL. XLVI. TINBERGEN, N. (1952) " 'Derived' activities; their causation, biological significance, origin and emancipation during evolution." Q, Rev. Biol., 27: WILLIAMSON, K. (1950) "The pseudo-sleeping attitude of the Oyster-Catcher." Brit. Birds, 43: 1-4. (1952). "Regional variation in the distraction displays 0/ the Oyster-Catcher." Ibis, 94: