Behaviour of Black-winged Stilts

Paul D. Goriup

he family Recurvirostridae comprises four genera, two of which are T represented in Europe, by the Avocet Recurvirostra avosetta and the Black-winged Stilt Himantopus himantopus. Five subspecies of the latter are recognised, of which the nominate race occurs in the Palearctic and also breeds in Africa, India and Malaya. Black-winged Stilts have bred once in Britain: two pairs raised three young in Nottinghamshire in 1945 (Staton 1945). Benson (1950) observed copulation by a pair during a stilt invasion in Suffolk and Devon, and one overwintered in Lincolnshire in 1968/69 (Smith etal. 1970). Given its elegance of structure and plumage, vigorous nesting terri­ toriality, and ease of observation, the lack of detailed accounts of stilt behaviour is surprising. Hamilton (1975) published a comparative study of the American Avocet R. americana and the Nearctic race of the Black-winged Stilt known as the 'Black-necked Stilt' H. h. mexicanus in California, but no such information is readily available for Europe (Glutz et al. 1977 provide the most recent review of the European literature). The present study was undertaken during the period 5th May to 22nd June 1979, in Portugal.

Sites Most observations were carried out at the Caia reservoir, near the village of Santa Eulalia. The upper reaches of this waterbody comprise a series of shallow bays up to 2 m deep leading to a backwater where a stream and some drainage ditches entered. The shore vegetation was largely grasses, especially creeping bent Agrostis stolonifera, rushes Juncus and sedges Carex. Aquatic plants in the shallow water consisted almost entirely of filamentous algae, but curled pondweed Potamogeton crispus and common water-starwort Callitriche stagnalis did occur. Most of the observed activity was confined to the backwater and adjacent bays, where two pairs of stilts were generally present and a third sometimes seen. The second site, at Vedor, was an elevated, earth-embanked holding reservoir whose sides leaked, forming marshy terraces and pools in the stiff clay soil which supported a lush, herb-rich flora dominated by bulrush Typha latifolia. Many of the rivulets and pools were choked with evil-smelling stonewort Cham. A single pair of stilts nested at this site. The two sites were about 15 km apart, north of the town of Elvas.

Methods and observations I identified individuals by features of plumage, and sexed them according to the criteria given by Prater et al. (1977), confirming the sexes after copulation (see fig. 1). It became clear that the standard field-guide

12 [Brit. Birds 75: 12-24, January 1982] Behaviour of Black-winged Stilts 13 description of the plumage of stilts (e.g. Peterson et al. 1974; Heinzel el al. 1972; Bruun & Singer 1970) could not be applied to my birds: for example, pair 1 exhibited the exact opposite sexual plumages to those described in these guides.

Activity pattern During the period of observation, and until nesting occurred, the stilts used their time almost exclusively for feeding. Table 1 summarises the various watches, and particular activities seen.

Fig. 1. Head patterns oi'Black-winged Stilts Himantopus himantopus, Portugal, 1979: a male 1, b female 1, c male 2, d female 2 14 Behaviour of Black-winged Stilts Locomotion In flight, the stilts used a fairly rapid wingbcat and maintained a reasonable speed, well able to keep up with other waders such as Dunlins Calidris alpina and Redshanks Tringa totanus; speed could be adjusted by raising or lowering the legs, which trailed behind the body and acted as rudders for orientation. To land, stilts glide down with the wings held out horizontally, bringing the legs forward at the last moment for a running or hopping stop,

Fig. 2. Aspects of behaviour of Black-winged Stilts Himantopus himantopus, Portugal, 1979: a leaning into wind, b indirect head scratch, c upright posture, (/avoiding attack by Magpie Pica pica, e mobbing Raven Corvus corax Behaviour of Black-winged Stilts 15 or by dropping the legs during flight and hovering above the ground, descending vertically on to both feet. On the ground, the stilts usually employed a gently-bobbing walk, reminiscent of a sandpiper Tringa. Both sexes frequently waded when feeding, tending to stay in water not deeper than the knee, although wading up to belly-depth did occur. When troubled by a following wind, one female had to tack in a shallow zig-zag up the shore, leaning sideways into the wind (fig. 2a); but, when returning down the shore, she walked head-on to the wind.

Feeding Stilts were observed feeding in all habitats from dry land to belly-deep water, but mostly at the silty margins of the water. There appeared to be a number of discrete feeding territories, comprising some 20 to 200 m of shoreline, used by pairs or individuals and defended from intrusion by other stilts. The birds present in these territories, however, changed from time to time, or were absent altogether: there were more feeding sites than stilts, and they seemed to be utilised on a first-come, first-served basis. Hamilton (1975) has already described three feeding techniques for Black-necked Stilts: pecking, plunging and snatching. Two new methods were adopted by the Portuguese stilts, while bill-pursuit (previously recorded only for Avocets) was also seen. PECKING This was by Car the commonest way that stilts fed. They walked or waded with a declined body, and a slight bobbing action caused by Hexing the legs, while short jabs were made with the bill (fig. 3a). The items taken by males were visible more often than those taken by females, and particularly large, grub-like prey were dunked and washed before being swallowed. At nightfall, and by moonlight, the pecking-gait altered completely, becoming remarkably plover-like: the stance became quite erect, with the head held well above the shoulders. Locomotion was now a series of short runs followed by pecks, rather than an endless walk. The intervals were spent relatively motionless, with the head cocked to and fro, and it seemed that prey-detection had switched from visual to audial. PLUNGING AND BILL-PURSUIT Plunging appeared to be largely opportunistic, taking advantage of especially clear water or locally abundant submerged prey. A male once used this method lor over 20 minutes, wading energetically in complicated figures and gyrations, kicking water up all around him, before plunging his head and neck into the water and propelling his bill in all directions, then emerging to swallow an item. The travel of head and neck was often so vigorous that a shower of droplets and a sizeable 'bow-wave' was produced (fig. 3b). Nearer the surface, bill-pursuit was used: the bill was half-immersed, and rapidly dabbled while chasing prey. PROBING This technique, and the semi-scythe (below), were performed by a male in addition to the usual pecking method, during an evening feed. When probing, he moved forward step by- step along the shoreline, delving his bill vertically down in a series of short jabs, rocking his body to help provide the necessary thrust. When prey was caught, it was brought to the surface lor swallowing. The bill could be sunk up to the hilt, with the head becoming partly submerged in surface water. The stilt would sometimes twist around to one side and behind in order to probe at an angle (figs. 3c, 3d). SEMI-SCYTHE Hamilton (1975) described the single scythe technique for Avocets, where a partly-open bill is swept horizontally in a wide arc from one side of the body to the other, collecting prey on the way. The stilt, however, swept its bill through silt only from directly in front to a little on one side of the body before swallowing: I could not see if the bill was held partly agape, or if it was dabbled in a sifting action. In order to get the bill more or less horizontal, two postures were adopted, which 1 termed the 'Avocet' and the 'Flamingo'. In the former (fig. 3e), the body was declined, with the legs deeply flexed. The 'Flamingo' involved 16 Behaviour of Black-winged Stilts

keeping the legs nearly straight, with the head and neck curved under the body (fig. 3f), and, when the sideways sweep was made, the silt just in front of the toes was sampled. None of the stilts was ever seen drinking, nor was drinking recorded by Hamilton (1975), but Glutz et al. (1977) do mention it. It seems likely that sufficient moisture is usually obtained during swallowing items taken from water.

Fig. 3. Feeding techniques of Black-winged Stilts Himantopus himantopus, Portugal, 1979: a typical pecking posture, b plunging, c probing (direct), (/probing (to the side), esemi-scythe— 'Avocet* posture,/semi-scythe—'Flamingo' posture Behaviour of Black-winged Stilts 17 Maintenance and comfort Hamilton (1975) gave a detailed analysis of these activities for the Black- necked Stilt; all of his categories were witnessed in the case of the Portuguese stilts and are briefly reviewed below. PREENING This took place on land or water, usually facing into any breeze. In water, males often employed bill-dipping, where the bill was haif-iminersed, shaken to remove excess drops, and then used to work the breast feathers. Females did not appear to bill-dip, a sexual difference probably related to the role of bill-dipping in copulation. Fig. 4 illustrates the elegant postures adopted by one female while preening. SLEEPING Remarkably, throughout the time of observation, sleeping was recorded only once: by a female in two separate bouts between 09.00 and 11.00 GMT, for a total often minutes. The stilts at this time kept relatively dispersed, and did not congregate to roost. BATHING The most prolonged examples of bathing were observed in females. Since one female spent over nine minutes at her toilet, during which time she performed a comprehensive sequence of the comfort movements described by Hamilton (1975), this is worth recording in some detail. After wading into a patch of shallow, open water, she proceeded to flex her legs repeatedly, wetting her belly. Then she dipped her head under the water several times, coming up each time to allow the drops to trickle down her neck; at each dip, the wings were flapped, splashing water over the back. Water was worked into the 'shoulders' by telescoping the neck into an S-shape, and rubbing the back of the head on the mantle. Next, standing just clear of the water, she preened her back, breast and underwing (fig. 4). This whole procedure was repeated twice, so that she became quite bedraggled. Excess moisture was then thrown offby jumping into the air, flapping her wings vigorously, but allowing her legs to dangle, to a height where her toes just cleared the water surface. Immediately after landing from the first leap, she repeated this hop-and-flap. She then: preened her breast; flapped her wings; shook both legs in turn; scratched head and bill indirectly, on both sides, using the respective foot; shook wings and body; preened neck, breast, mantle, back and underwing; began moving towards dry land; paused to scratch directly both sides of the head; ruffled feathers; at shoreline shook feet; indirectly scratched head and bill on both sides; and finally resumed feeding. Interspecific behaviour I have assumed that an animal can adopt one of three basic responses towards its neighbours: aggression, tolerance or escape. These attitudes, however, are not necessarily discrete, and an animal's reaction usually results from one of these responses predominating. AGGRESSION Most aggression occurred in the nesting area as soon as a territory had been established. Within about 100 m of the nest, human beings, dogs and sheep were met by an incessant mob display where one or both of a pair rose up and circled the intruder at a radius of 3-15 m, 4-10 m high, calling excitedly; at about 50 m from the nest, a peak frequency of about 200 notes per minute was counted. The intensity of the display varied: one of either sex mobbed individually, or a pair mobbed together, alternately. When I was within about 10 m of the nest, the stilts landed, but they continued to call, walking around in a very agitated manner, and making erratic pecking and preening movements. Further indications of ner­ vousness were frequent upright postures (fig. 2c) and bouts of head-bobbing. If only one bird was displaying at this point, the other incubating eggs, then the latter crept away from the nest and hid, but then joined in the calling. When I reached the nest, both stilts became silent and apparently vanished, or one approached very close, calling rapidly. The distraction displays described by Hamilton (1975), however, were not observed. When I retreated, the stilts resumed the mob display, going through the above sequence in reverse order. An angler was mobbed for over 20 minutes, with complete indifference on his part, before the stilts finally gave up and accepted his presence, but when a second angler walked past, ten minutes later, they began mobbing again. When a Raven Corvus corax flewove r a nest area when eggs were being incubated, the female stilt flew up and chased it, homing in as though trying to spear the Raven with her bill (fig. 2e); at the last instant, the Raven rolled and ducked, while the stilt soared up about 5 m, hovered briefly, and then descended on the Raven again. This procedure was repeated four or five times 18 Behaviour of Black-winged Stilts

Fig. 4. Series of postures adopted by a female Black-winged Stilt Himanlopus himantopus during a preening session, Portugal, 1979 until the Raven was about 500 m from the nest, but no physical contact was made. The male stilt remained at a distance, once or twice joining in to reinforce the female's attack: both stilts called continuously. On a subsequent date, a Buzzard Buleo buleo was mobbed at this nest site, solely by the male. His technique was quite different: calling constantly, and flying behind and just above the Buzzard, his legs dangled, and he attempted to strike them on the raptor's back; the latter, however, avoided being hit and quickly flew out of the vicinity. Away from the nest, the only aggression seen was by a male stilt against a Redshank. The stilt made repeated attacks, in the crouch-run posture (Hamilton 1975: the stilt runs in a very hunched posture) and flapping his wings; each time, the Redshank retreated a few metres. The stilt had been engaged in prccopulatory activity with his mate, and his pugnacity spoiled the first mating attempt: when he returned from chasing the wader, the female had temporarily Behaviour of Black-winged Stills 19

lost interest. The stilt seems to have regarded the Redshank as a possible rival, perhaps owing to its similar shape and bare-part coloration. After copulation, the male stilt accepted the Redshank's presence quite readily; the female never showed any signs of discontent. On a later date, another pair of stilts successfully copulated while a Ruff Philomachus pugnax fed only a metre away. TOLERANCE The stilts seemed relatively docile, ready to share feeding and nesting grounds with a number of other species, apart from predators and potential rival males. If another bird approached within a metre or so, the stilt would adopt an upright posture, or head-bob, until individual distance was restored. The pair at Vedor shared the site with a pair of Little Ringed Plovers Chamdrius dubhu. ESCAPE A few instances were recorded where stilts flew away from too close an approach by Common Sandpipers Aclitis hypoieucos, a little Egret Egretta garzetta and a Black Kite Milvus migrans. One female had to flee from an irate Magpie Pita pica by flying to belly-deep water and flattening herself parallel to the surface to avoid a swoop (fig. 2d).

Intraspecific behaviour The most common interaction between stilts, of either sex, was supplanting (Hamilton 1975), which was recorded even by moonlight, although the area then defended was much contracted. Only a metre was tolerated within a feeding area, and even then an individual distance of 1-2 m was maintained. One of a pair coming too close to the other uninvited elicited the upright posture and/or head-bobbing, just as for another species. The offending bird, however, often mimicked its partner's response, so that mutual upright posturing and/or head-bobbing took place. This may serve as an appeasement display, reinforcing the pair bond. Hamilton (1975) described a group mob display by several stilts; some­ thing similar was twice observed in Portugal, although the number of stilts taking part was only three and four respectively.

Sexual interactions The stilts appeared to form stable pairs, which were maintained throughout the period. Sexual behaviour was confined to the chosen partner. PAIRING That a pair bond existed was demonstrated by the sharing of a feeding area. Very little overt pair-bonding display was seen: only two short ceremonies. First, at the beginning of May, when pair bonds may have been strengthening, the male of a pair feeding about 5 m apart in shallow water walked to dry land and squatted down in a posture strongly resembling incubation (fig. 5a); the female immediately rushed up to him, and he stood up when she arrived; both then made pecking actions at the mud patch for a few seconds, as though turning over eggs; finally, they separated and walked off in opposite directions. Fifteen minutes later, they rejoined and copulated, but they never returned to that area, and no evidence of nesting was found. On a later date, this same pair mated and separated as normal (see below), but, about 20 minutes afterwards, they approached within 2 m of each other and both adopted upright postures, as usual; standing side by side, however, they leaned towards and then away from each other, by tilting the head and neck three or four times, before parting again to resume feeding (fig. 5c). COPULATION The pre- and post-copulatory displays, described here fully for the first time for the Black-winged Stilt, are exactly comparable to those given by Hamilton (1975) for the Avocet. All observed copulations took place in water, which seemed necessary for success (probably owing to the bill-dipping sequence by the male). Two attempts on land failed well before mounting would have occurred. On the four successful occasions, the female always initiated the ceremony, usually by adopting her characteristic inclined posture as the male walked past. Once, on land, a female pecked at the male's bill as though picking at a shared food item, and then led him into shallow water, where mating took place. 20 Behaviour of Black-winged Stilts

Fig. 5. Postures and displays of Black-winged Stilts Himantopus himanlopus, Portugal, 1979: a male in incubation posture during pairing, b prancing display, c mutual leaning during pair-bonding, rfpre-copulatory ceremony As soon as the female assumed the copulation posture (fig. 6a), which was held rigidly throughout, the male became very excited, puffing out his feathers to appear much larger than his mate, and striding in semicircles from one side of her to the other, always passing behind (fig. 5d). Each time that he came up to her shoulder, he paused to bill-dip and preen his breast or underwing (figs. 6b, 6c, 6d). This cycle was repeated two to five times, ending when the male adopted an erect posture prior to mounting (fig. 6e). To achieve cloacal contact, the male flexed his legs so that the whole length of the tarsi rested on the female's back; balance was Behaviour of Black-winged Stilts 21

Fig. 6. Pre- and post-copulatory displays of Black-winged Stilts Himantopus himantopus, Portugal, 1979: a female adopting copulation posture, b male arriving at female's shoulder, c male bill-dipping, (/male underwing-preening, e pre-mount erect posture of male,/coition, g bills-crossed run, h male and female separating maintained by wing-flapping (fig. 6f). After dismounting, both male and female adopted upright postures and performed the bills-crossed ceremony: standing close to the female, the male crossed his bill over hers and extended his wing over her back; in this pose, both walked about a metre, either directly forward or in a shallow arc, before separating (fig. 6g, 6h). The male generally then resumed feeding immediately, while the female stood or preened for some seconds before commencing to feed herself. The whole procedure lasted about one minute. 22 Behaviour of Black-winged Stilts Nesting The stilts that I studied nested rather solitarily, over a prolonged period (table 1), in great contrast to the social colonies described by Hamilton (1975), or that which I saw in Tunisia (where about 100 pairs occupied some 100 ha of marshland: Hollis 1977). The only nest found at Vedor (plate 5) was within 200 m of a road and a cottage. A rather hemispherical scrape, about 15 cm across and generously lined with dried stalks of composites (Compositae), rather than grasses, it contained four eggs, deposited well below the lip of the nest-cup: they weighed 9 g, 10.3 g, 10.3 g and 10.5 g, respectively. This was the exact

5. Nest of Black-winged Stilt Himantopus himantopus, Vedor, Portugal, 1979 (P. D. Goriup)

opposite of what I found in Tunisia, where the substratum was deep, soft, silty clay, and the nest a platform of twigs built up above the flats (plate 6). These constructions may represent adaptations to the likelihood of flooding and accessibility to ground predators. On drier ground, there is little chance of immersion, but eggs are more vulnerable to mammalian predation and so are sunk out of view. In the wet, silty marsh, flooding may occur, so a platform is required to keep the eggs dry, while the soft substratum prevents approach by mammals.

6. Nest oi'Black-winged Stilt Himantopus himantopus, Ichkei.il, Tunisia, 1977 {P. D. Goriup) Behaviour of Black-winged Stilts 23

I was not able to study incubation behaviour at Vedor owing to the site's very public nature. Chicks were hatched (I later (bund eggshell fragments near the nest), and the continued mob displays of the adults indicated their presence, but I did not locate them. I found chicks, however, at Caia. During an evening watch, pair 3 was giving prolonged mob displays to anglers and shepherds near the water; as the disturbance decreased towards dusk, the male walked to a hay-field about 25 m from the shore, giving single-note calls; on his arrival at the edge, two one-third-grown chicks appeared, which he led down to the water. The male and chicks fed as a loose party, within 10 m of each other, while the female fed alone about 60 m off. Next morning, only the parents were to be seen in the backwater, so I lost my last chance of studying chick behaviour. When a Black Kite flew over, however, the male performed the curious prancing display, also recorded by Staton (1945) when nearby chicks were threatened: calling all the time, the stilt adopted an upright posture, flapping his wings and prancing from one foot to the other (fig. 5b) for about ten seconds, until the kite was no longer overhead.

Table 1. Activity pattern (excluding feeding and minor actions) of Black-winged Stilts Himantopus himanlopus at two sites in Portugal, May-June 1979

Date Site Time period (GMT]) Activities

5 Caia 10.00-10.45 Pairing behaviour and copulation by pair 1 8 Caia 08.00-15.30 Group mob display (pairs 1 and 2); copulation and pair behaviour by pair 1; copulation attempt by pair 2; female 1 bathed and preened for nine minutes 11 Caia 10.00-16.00 Group mob display (pair 1 and female 2) 1 'panic" be­ haviour by wader flock Vedor 20.00-20.30 Mobbing of Raven Corvus corax; mob display near to future nest site 16 Vedor 19.00-20.00 Mob display; male incubating four eggs 17 Caia 09.00-11.15 Female 1 slept for ten minutes 18 Vedor 21.00-21.30 Mobbing of Buzzard Buteo buteo; mob display; eggs weighed 25 Caia 20.00-22.00 Feeding watched until darkness (sunset 21.15) 27 Caia 19.30-21.00 No notable behaviour 28 Vedor 11.00-12.00 Intermittent mob display; egg-shell fragments near nest 30 Caia 12.30-13.00 No notable behaviour 1 Caia 13.00-14.00 Male preening on one leg 3 Caia 21.00-21.30 Copulation by pair 2 7 Caia 19.15-20.00 Mob display by pair 3 9 Caia 06.00-07.00 Mob display by male 3, in half-light (dawn at 07.15) 10 Caia 00.15-01.00 Full moon: birds preening and defending feeding areas; no mob display on close approach 21 Caia 21.00-22.00 Pair 3 with two one-third-grown chicks 22 Caia 09.00-11.00 Prancing display by male 3 24 Behaviour of Black-winged Stilts Vocalisations The stilts' vocabulary was not large: a few notes, either singly or strung together more or less continuously, according to circumstances. CONTACT A sharp 'krek' or 'kek' recalling a Coot Fulica atra, much softer and quieter when the partner or chicks were close. ALARM During mob displays, 'kraak-kraak .... -kraak' or 'keyack-keyack .... -keyack'. The pitch varied with frequency: a higher pitch was associated with a more intense, rapid calling. A single 'kee-arr' resembling a Sterna tern was given as a flight warning when chicks were present, but the parents not sufficiently worried to produce a full mob display.

Discussion The observations above confirmed that the behaviour of Black-winged Stilts of the nominate race in Portugal is essentially the same as that recorded by Hamilton (1975) for the race mexicanus in California. Some feeding techniques previously recorded only for the Avocet were used by the stilts, and a full account of stilt copulation is given. These new descriptions provide further evidence for the close taxonomic affinities between the genera Recurvirostra and Himantopus (see Hamilton 1975).

Summary Observations were carried out in May and June 1979, in the Alto Alentejo region of Portugal, on the feeding techniques, individual activities, social interactions, copulation, nesting and voice of Black-winged Stilts Himantopus himantopus of the nominate race. Comparisons are made with similar work carried out on the race H. h. mexicanus in California, USA (Hamilton 1975).

Acknowledgments I am very grateful to David and Primrose Ridley-Thomas for their generous hospitality during my stay in Portugal. Dr Nigel Collar gave valuable advice and suggestions, and undertook a critical reading of the manuscript, while Susan House endured patiently to produce her excellent illustrations from scraps of field notes and photographs. References BENSON, G. B. G. 1950. Black-winged Stilts in Suffolk and Devon. Brit. Birds 43: 132-133. BRUUN, B., & SINGER, A. 1970. Birds of Britain and Europe. Feltham. GLUTZ VON BLOTZHEIM, U. N., BAUER, K. M., & BEZZEL, E. 1977. Handbuch der Vogel Mitteleuropas. vol. 7. Wiesbaden. HAMILTON, R. B. 1975. Comparative behavior of the American Avocet and the Black-necked Stilt (Recurvirostridae). Ornithological Monographs 17. Amer. Orn. Union. HEINZEL, H., FITTER, R., & PARSLOW,J. 1972. The Birds of Britain and Europe with North Africa and the Middle East. London. HOLLIS, E. (ed.) 1977. A Management Plan for the Proposed National Park of Gareat L'Ichkeul, Tunisia. Conservation Course, University College London. Report No. 10. PETERSON, R., MOUNTFORT, G., & HOLLOM, P. A. D. 1974. A Field Guide to the Birds of Britain and Europe. 3rd edn. London. PRATER, A. J., MARCHANT, J. H., & VUORINEN, J. 1977. Guide to the Identification and Ageing of Holarctic Waders. I ring. SMITH, F. R., & THE RARITIES COMMITTEE. 1970. Report on rare birds in Great Britain in 1969. Brit. Birds 63: 267-293. STATON, J. 1945. The breeding of Black-winged Stilts in Nottinghamshire in 1945. Brit. Birds 38:322-328.

Paul D. Goriup, Lyndisfarne, Lower Road, Salisbury SP29BB