98 The Wildfowl Trust

Observations on the behaviour and relationships of the White-backed and the Stiff-tailed PAUL A. JOHNSGARD1 Summary 1. A number of behavioural and anatomical sources of evidence indicate that the White- backed Duck (Thalassornis) should be placed in the tribe Dendrocygnini and be regarded as a rather than an aberrant Stiff-tailed Duck. 2. Behavioural evidence supports earlier views that the Black-headed Duck (Heteronetta) is the most generalized of the true stiff-tails, and it is suggested that the numerous unusual aspects of sexual behaviour found in this tribe can be traced back to a reduction of pair- forming and pair-maintaining mechanisms which can already be detected in Heteronetta. 3. The distinctive aspects of morphology and behaviour found in the () can be attributed to the predictable effects of intense sexual selection resulting from the breakdown of pair bonds and the establishment of a completely polygamous or promis­ cuous breeding system in Biziura. 4. There is still too litde behavioural information concerning the to advo­ cate its generic separation (Nomonyx), although it is suggested that this is probably the most isolated of the typical stiff-tails and presumably represents a less specialised evolu­ tionary line than do the other stiff-tails of the Oxyura. 5. Recent behavioural evidence tends to support earlier views that the remaining five species of Oxyura fall into two broad evolutionary groups; including leucocephala and jamaicensis on the one hand, and maccoa, vittata and australis on the other.

The Stiff-tailed Ducks have traditionally At the species level the classification of been a relatively undisputed and well- the stiff-tails has been hampered by the defined group of species in the family considerable similarities of most , most members of which may species of Oxyura, as well as the meagre be easily distinguished from all other information available on their ranges and waterfowl by their elongated and pointed comparative . The problem has rectrices with stiffened shafts and by been especially vexing in southern South their bills having recurved nails. When America, where the affinities of the forms Eyton (1838) produced the first compre­ vittata and ferruginea to one another hensive classification of the Anatidae he have been in doubt, and their possible separated the stiff-tails as a distinct sub­ relationship to the North American Ruddy family (Erismaturinae) containing three Duck O. j. jamaicensis has also been a genera, Erismatura (Oxyura), Biziura, and problematic matter. Delacour and Mayr Thalassornis, the last genus being erected (1945) initially regarded these two South for the newly discovered White-backed American forms and the African , which Eyton first formally des­ Duck O. maccoa as races of a single cribed. He regarded this species as a “con­ southern hemisphere species that also in­ necting link between the genera Clangula cluded the Australian Blue-billed Duck and Biziura, the structure of the tail being O. australis. Boetticher (1952) later took nearly like that of the former genus, while the more extreme view that all of the the bill is like that of the latter.” Later southern hemisphere populations were systematists accepted this classification subspecies of O. jamaicensis. However, almost without modification, the only Helmayr and Conover (1948) pointed out change being that the genus Nomonyx that both South American forms breed was erected by Ridgway (1880) to distin­ together in central Chile, thus two species guish the Masked Duck from the other must be recognised. Lehmann’s descrip­ typical stiff-tails, primarily on the basis tion, in 1946, of a Colombian race of of its less recurved bill nail. This arrange­ Ruddy Duck O. jamaicensis andina sup­ ment persisted until the classic revision ported the view that the North American of the family by Delacour and Mayr Ruddy Duck is thus geographically con­ (1945), who reduced the stiff-tails to tribal nected to the South American popula­ rank (), merged Nomonyx with tions, and that ferruginea rather than Oxyura, and transferred the Black-headed vittata represents the southernmost repre­ Duck Heteronetta atricapilla into the sentative of this geographic series. tribe from its former position in the dab­ In an earlier review of the behaviour bling duck group, largely on the basis of patterns of the Anatidae (Johnsgard, Wetmore’s (1926) anatomical observations. 1960), I deplored the lack of behavioural 1 Contribution (No. 380) from the Department of Zoology and Physiology, University of Nebraska, Lincoln, Nebraska. Illustrated by the author. Stiff-tail Behaviour 99 information on the stiff-tails, and sug­ rushes or reeds over water and contain no gested that such knowledge might serve down. However, they are sometimes loca­ to evaluate the validity of including such ted at the edges of ponds as well (D’Eath, aberrant genera as Heteronetta and Thal- 1965). The clutch size has been variously assomis in the tribe, as well as to help reported as ranging between two and 14 establish species limits and taxonomic , although the latter figure doubt­ groupings within the genus Oxyura. Since less represents “ dump-nests ” produced then it has been my good fortune to by more than one female. The eggs are observe all of the nine species of Oxyurini surprisingly large (Dr. Janet Kear in­ (Delacour, 1959) in life, and to observe formed me that the average weight of 21 sexual displays among six of them. eggs was 82.5 grams), and are of a dis­ Although it is still premature to believe tinctive pale rusty brown colouration. that an adequate knowlege of stiff-tail This colour and their very smooth surface behaviour is at hand, it is nonetheless distinguish them from typical stiff-tailed possible now to make some comparisons duck eggs. Each represents more than and conclusions that were impossible in ten per cent of the adult ’s weight, 1960. Some such observations have since which ranges from 680 to 790 grams been published (Johnsgard, 1965a), but (Phillips, 1926). This surprisingly large others were obtained too late to permit size is roughly equivalent to the situation inclusion in my general survey of Anati­ in stiff-tails, since unincubated eggs of dae behaviour. Thus, special attention North American Ruddy Ducks average will be paid here to the re-evaluation and 74.7 grams (Janet Kear, pers, com.), and probable relationships of the various adults of this species range from 560 to genera included in the tribe, and to a to 680 grams (Phillips, 1926). It may be review of the available behavioural infor­ hypothesized that in both species this mation on the more typical stiff-tails large egg size is a functional (Oxyura), insofar as it may reflect and that permits the development of unusu­ further clarify their evolutionary relation­ ally precocial young that are able to forage ships. independently by diving shortly after Without question the White-backed hatching. Thus, Clark (1964) reports that Duck Thalassornis leuconotus is one of downy Maccoa Ducks may spend ten to the most inadequately studied species of fifteen seconds under water when forag­ waterfowl. The downy young were first ing. Interestingly, Johnson (1965) reports correctly described by Delacour and illus­ that unusually large eggs are also typical trated by Peter Scott in 1959, the of Torrent Ducks (Merganetta armata) tracheal anatomy remained undescribed which likewise have extremely precocial until 1961, and it also was not until then young that are able to dive and navigate that the reticulated tarsal surface condi­ swift currents with ease (Johnsgard, tion of the species was first noted (Johns­ 1966a). gard, 1961a). The unusual tracheal struc­ In 1965, the first nesting attempt by the ture, the associated whistling voices of White-backed Ducks at Slimbridge was both sexes, and the reticulated tarsal pat­ in a clump of willow herb Epilobium suggested to me that perhaps the and reeds within a foot or two of the White-backed Duck had been incorrectly water’s edge, perhaps because no emer­ placed in the Oxyurini, and that it might gent vegetation is present in their pond. actually be an aberrant whistling duck. Likewise, in 1966, a nest was built in a However, during my two years of study similar clump of willow herb and reeds at the Wildfowl Trust between 1959 and approximately 18 inches from the water, 1961, the species, although represented by but in this case it was also immediately nine individuals, failed to breed, and very adjacent to the gravelled public pathway few behavioural observations of taxono­ that is used by visitors to the Trust. In­ mic significance could be obtained (Johns­ deed, several large pebbles an inch or gard, 1965a). Then, during a return visit more in diameter were incorporated into to the Trust in July of 1966, I learned the nest scrape, which was lined with that, following a relatively unsuccessful grass but lacked any contour or breeding attempt in 1965 (Johnstone, down. The nest was approximately one 1966), a second nesting had begun shortly foot above the level of the water, but no before my arrival. ramp led to it. Rather, the climbed Accounts in the literature (e.g., Mack- up on shore a foot or two to the side of worth-Praed and Grant, 1952) indicate the nest and entered it indirectly. At the that under natural conditions the nests of time I arrived on 4th July, six eggs had White-backed Ducks are usually built in already been laid at approximately daily 100 The Wildfowl Trust intervals. These eggs had been initially by the male was most surprising, since replaced with white wooden dummy eggs male stiff-tails normally take no interest until it was found that the adults were in the nest. I was further astonished by ejecting these substitutes, and it was not the male’s remarkable threat posture, with until domestic fowl eggs that had been ruffled scapulars, wings raised and spread dyed brown were tried that they were somewhat in the manner of a threatening accepted. This unusual egg discrimination , the head low with neck outstretched suggests that possibly the unique brown and bill gaping (Fig. 1), and with inter­ colour of their own eggs enables White- mittent loud hissing as he paddled his backed Ducks to recognize and eject the feet so rapidly as to make the water fairly white eggs of Maccoa Ducks, which are boil. During this display the white back otherwise identical to their own. It is feathers, normally hidden below the known that Maccoa Ducks probably per­ wings, were readily visible and greatly form parasitic egg-laying under wild con­ ruffled. I have observed somewhat simi­ ditions (Siegfried, 1964). lar but less intense displays in various During my first visit to the nest I whistling ducks defending their nests or noticed that both birds were present in broods, although none of these has in­ the clump of vegetation that contained volved wing-spreading. At lower inten­ the nest. The female, which I learned to sities of threat display the male assumed recognize on the basis of her smaller head, a posture similar to the “ Head-back ” slightly more mottled cheeks and a more threat of whistling ducks. These displays uniformly blackish bill, had been sitting contrast with those of Oxyura females beside the nest but immediately entered when protecting their broods, which latter the water as I approached. The male involve stretching the neck forward, gap­ initially remained on the nest, hissing and ing, sometimes uttering repeated squeak­ raising his scapulars, then jumped into the ing notes, and repeatedly raising and water and made several vigorous but lowering the folded wings. Frequently an abortive attacks. This fierce nest defence attack or threat by the White-backed

Fîgïire I. Posteres of White-backed Ducks. (Upper right) Extreme threat posture assumed during nest defence. (Upper left) Less intense threat posture of male. (Lower left) Chin-lifting threat of female. (Lower right) Resting posture sometimes assumed by male. Drawings by P. A. Johnsgard. See also Photograph Section p. VIII upper. Stiff-tail Behaviour 101

Ducks was followed by a general body ducks ! The male also raised his wing on shake, which I have also observed in the side away from the female in the usual various whistling ducks but not in stiff- manner of whistling ducks, but the female tails. raised hers little if at all. I regard this Following a threat or attack the male observation as providing the strongest would quickly return to the vicinity of the possible behavioural evidence favouring nest and female, and occasionally both whistling duck affinities of T halas s omis. birds would then utter their whistling Only one other copulation attempt was notes. I was unable to separate the sexes observed. In this case a male swam by their vocalizations, since both seemed toward two females foraging near shore, to use the same calls. The loudest of approached one of them closely, and these, an apparent alarm note, is a clear dipped its bill in the water two or three double whistle, rather similar to that of a times. The female did not reciprocate, but Killdeer Charadrius vociferus but not almost immediately flattened out on the repeated. Another common vocalization is water. The male then mounted but this a “ conversational ” or contact call con­ mating attempt was also apparently un­ sisting of from three to five notes uttered successful. In these three observations, as a rising series of soft whistles. and in another made by S. T. Johnstone After either member of the pair had (pers, com.), the copulations all occurred within a foot or two of the shore, whereas been forced to leave the nest, the male typical stiff-tails do so in deeper water. was invariably the first to return to it. In Furthermore, the precopulatory bill-dip­ instances when the male was foraging during a disturbance that forced the ping observed in the third instance was female from the nest he quickly returned identically the same as that of various whistling ducks. to the nest site to defend it. Other than its mate, the male paid little heed to On 9th July, I observed a female ex­ female ducks when they approached the amining clumps of willow herb on the nest, but actively chased other males as shore opposite the nest under study and well as any individuals of other species near the place where I had observed a that came too near, even including such copulation on 7th July. It was therefore large birds as male Comb Ducks Sarki- no surprise when a second nest with one diomis melanotos. The female rarely egg was located on 11th July in that area. chased any birds, but would vigorously Like the others, it was built in willow threaten those other than her mate with herbs and rushes only a foot or two from strong chin-lifting movements associated water. This second breeding pair, whose with a trilled whistle (Fig. 1). nest was built about 40 yards from the Although incubation had certainly not first, may have been responsible for the yet begun by my first visit on 4th July, last two copulations I observed. Strangely, the nest was never thereafter left un­ at about this same time five additional guarded so far as I could determine. On the eggs were deposited in and near the first morning of 5th July I noted that the male nest, so that ultimately eleven eggs were was on the nest when I arrived at 9.30 associated with that nest. In the second a.m., and later that morning he was re­ nest eggs were deposited daily after placed by the female. However, the male 11th July, but on several days two eggs stayed near, and at 12.20 p.m. a second rather than one were collected. This indi­ female approached the nest. The male did cated that one or more additional females not threaten her, but instead the two were depositing eggs in both the nests, birds swam about in a tight circle im­ and thus it is impossible to judge how mediately in front of the nest. Suddenly many were produced by a single female. the female went prone, and the male Altogether, 22 eggs were deposited in the mounted her. The mating attempt was two nests by three or more females. This probably unsuccessful since no post- sudden “ explosion ” of breeding activity copulatory display by either bird by the White-backed Ducks is difficult occurred. That afternoon I observed an­ to explain since these birds have been in other copulation which involved birds the collection since 1960 and had pre­ other than the nesting pair. In this case viously made only a single attempt to the preliminaries were not seen, but the breed. Possibly the presence of the initial treading was evidently successfully com­ nesting pair stimulated the remaining pleted for, on its termination, the male birds in some manner; at least the other whistled loudly once and both birds per­ White-backed Ducks seemed to take un­ formed a “ step-dance ” parallel to one usual interest in the nest and frequently another in the exact manner of whistling gathered around it. 102 The Wildfowl Trust

When not guarding the nest or incu­ noticed that the male was still sitting on bating, the White-backed Ducks some­ the nest. It is well known that in several times rested near the middle of the pond. and possibly all of the species of Dendro­ Although on a few occasions I observed cygna (Phillips, 1926, lists viduata, bicolor the birds thus sleeping with their bills and autumnalis) the male performs much buried in the scapular feathers (a posture or all of the incubation. In Dendrocygna I had not observed before), they usually the incubation period ranges between 27 adopted a curious resting position in and 30 days (Delacour, 1954), whereas in which one or both feet would be lifted the North American Ruddy Duck it is out of the water and placed anteriorly on only 20—21 days (Delacour, 1959). Clark the back so that the webs of the feet (1964) reports an incubation period of looked like miniature sails (Fig. 1). All 25—27 days for the Maccoa Duck. The of the foraging was apparently done by majority of the White-backed Duck eggs diving, and it is evident that in their were incubated under broody hens, and abilities to remain submerged the birds hatched in periods from 29 to 33 days are at least the equals of stiff-tails. In (Janet Kear, pers. com.). However, none Table I some observed periods of sub­ survived beyond 15 days. The only suc­ mersion and intervening surfacing periods cess in hatching and rearing this species of White-backs can be compared with in captivity was that of Ezra (1934), who North American Ruddy Ducks and two noted the unusual precocity of the young representative whistling ducks that regu­ in its diving abilities, and that both larly forage in this manner. These obser­ parents cared for the young. Dr. J. M. vations were all made on the relatively Winterbottom and Mr. W. R. Siegfried shallow ponds of the Wildfowl Trust at (pers, com.) have informed me that they Slimbridge. have observed both parents attending Table I. Duration of dives and intervening panses of Wlslte-backed, Rtaddy aædS Whistling Ducks. Diving_ Periods (in seconds) Intervening Periods (in seconds) Observations Average Range Observations Average Range White-backed Duck 14 20.5 13-30 13 14.5 9-28 N. American Ruddy Duck 19 14.2 8-29 17 10.1 8-15 Fulvous Whistling Duck 7 13.6 9-15 6 11.0 10-18 Javan Whistling Duck 7 11.4 8-15 6 9.0 5-13

Another aspect of general behaviour broods under wild conditions. I was un­ that has not yet been previously reported able to stay long enough to observe the is the occurrence of pre-flight movements ducklings, but Dr. Kear informed me in White-backed Ducks. Presumably this that, judging from sound spectrographic species very little, and an absence of analysis, the distress calls of downy such signals might not be surprising. White-backed Ducks are very similar to However, I did once observe a pair that those of whistling ducks but are totally was oriented side by side, facing open different from those of the North Ameri­ water, and appearing very alert. Both per­ can Ruddy Duck (Kear, 1967). Although formed rapid lateral head-shaking move­ the plumage of the young is admittedly ments with their necks vertically distinct from that of downy whistling stretched, and they also occasionally ducks, it is also different from that of rubbed their cheeks on the scapulars. The stiff-tails. Additionally, although the head male then uttered a few short whistles pattern is somewhat obscured by a tawny and the pair attempted to take off. This suffusion, it does in some respects resem­ sequence corresponds closely to the pre­ ble that of whistling ducks. It has long flight behaviour of whistling ducks. been recognized that the whistling ducks Because of the continuous guarding of are unique in the Anatidae in that their the nest by one or both adults, it was dif­ skulls have extensions of the lachrymal ficult to determine when incubation actu­ bones that project backward to enclose ally began. However, incubation was the orbits completely (Phillips, 1923). The probably begun at the first nest sometime during the period 13th to 17th July, dur­ White-backed Duck, although lacking ing which interval no more eggs appeared, this trait, has a skull configuration more and I observed only the male attending closely resembling a whistling duck than the nest. I was not able to visit the Wild­ a stiff-tailed duck (Janet Kear, pers, fowl Trust again until 27th July, when I com.). Stiff-tail Behaviour 103

In summary, I believe that there is now published MS.), but female Musk Ducks sufficient evidence to propose that the Biziura lobata evidently do exhibit pro­ genus Thalassomis be transferred to the longed care of their young which usually Dendrocygnini and regarded as a whist­ number only one or two (Lowe, 1966). ling duck rather than an aberrant stiff­ Since the publication of my observa­ tail. tions (1965b, 1966b) on the Musk Duck, Air. V. T. Lowe (1966) has produced a Black-headed Duck and Musk Duck similar survey of its behaviour. He is in agreement with most of my observa­ I reported earlier (1965a) on certain male tions regarding the display forms and the displays of the Black-headed Duck, based origins of the various sounds produced on limited observations of a single bird by male Musk Ducks. However, he ques­ at the Wildfowl Trust. I have since seen tions the importance of sexual selection these same displays performed by males in promoting the evolution of the male’s at the Philadelphia and Bronx zoos, and remarkable behaviour patterns and the ex­ have also observed females at the latter treme sexual dimorphism of the species, location. Although the female Black­ and instead suggests that the male’s dis­ headed Duck’s plumage is similar to that plays perform a function of recreation and of various Anas species, its behaviour pat­ ego-boosting as a substitute for a strong appear to be Oxyimz-like. Thus, I sex interest. Mr. Lowe does support my heard no Decrescendo Calls nor have I belief that no pair bond exists at any sea­ detected any Inciting calls and postures son in this species. It is significant that in the three females I observed. Instead, Clark’s (1964) study of the Maccoa Duck, they remained silent, even when one of the largest species of Oxyura, in­ approached by a displaying male. Their dicates a situation closely approaching most frequent response was a silent gap­ typical Musk Duck behaviour. Thus, ing threat directed toward the males in a during breeding periods individual male manner reminiscent of female Ruddy Maccoa Ducks patrol small reed-bordered Ducks. Thus, the females appeared to stretches of water where they display and lack signals that might facilitate the from which they forcibly expel other development of Anas-like social courtship males, whereas females apparently drift groups; indeed, the males appeared to dis­ aimlessly from one such territory to an­ play independently and indiscriminately other until each chooses “ an exception­ to any female that came near. The ally vigorous ” male to associate with and apparent absence of a recognizable Incit­ in whose territory she may build her nest ing display in this species and in the more without interference from other males. In typical stiff-tails suggests that distinct pair­ this way more energetic and dominant bonds may be weak or lacking in most males might accumulate two or more members of this tribe, although Milton females simultaneously or consecutively Weller (pers, com.) reports the presence during the prolonged breeding season. It of seasonal pair-bonds in Black-headed may thus readily be visualized how the Ducks. Instead, male stiff-tails may similar behaviour of male Musk Ducks, simply defend individual territories and whose overt territorial battles have evi­ mate with any females that are attracted dently been largely replaced with visual by the male’s display activities. This and auditory displays that serve equally behaviour is almost certainly true of the well to delineate territories and attract genus Biziura (Johnsgard, 1965) and there females, represents a culmination of the is suggestive evidence for at least one trends toward the breakdown of strong species of Oxyura (Clark, 1964). The fre­ pair bonds that can be traced through quently quoted statement that male North American Ruddy Ducks “ assist” almost the entire stiff-tail tribe. Therefore, (Kortright, 1942) the female in rearing the the male Musk Duck’s apparently para­ brood is misleading; little if any attention doxical subordination of interest in is paid to the young by the male, which females in favour of a preoccupation with appears to be merely sexually attracted seemingly senseless displays which Mr. toward the female (Helen Hays, pers, Lowe has stressed is no refutation of the com.). In the North American Ruddy influence of sexual selection, for it is Duck and also the Maccoa Duck the only by the complete breakdown of pair female herself frequently abandons (or is abandoned by) the brood only a few bonds and a reduction of the male’s weeks after their hatching. Parental care attachment to individual females that an has been completely lost in the socially effective polygamous or promiscuous mat­ parasitic Black-headed Duck (Weller, un­ ing system can be established. 104 The Wildfowl Trust

It would thus appear that the Black­ unsuccessful, I am able here to add little headed Duck might in many ways be information regarding the behaviour of regarded, both behaviourally (Johnsgard, this most interesting of the Oxyura 1965 ; Weller, MS.) and anatomically (Wool­ species. The only person I have located fenden, 1961), as the most generalized of who has observed Masked Duck display the stiff-tailed ducks and as one derived is L. Irby Davis, who informed me (pers, rather directly from the presumed dab­ com.) that his observations date from bling duck-like ancestor of the tribe, more than 20 years ago and his notes whereas the Musk Duck can be considered have been lost or mislaid. However, he the predictable evolutionary end-product recalls that the male definitely cocked its of the behavioural trends that may be tail during display, that the throat or first detected in the Black-headed Duck neck was enlarged, and that a sound was and may be clearly observed in the genus produced by the male as it performed Oxyura. movements similar to those made when preening the neck feathers. The occur­ Typical sttiff-tails Oxyura rence of frequent quick rushes over the water surface by the male was another of For some time it has been my hope to the display features which he clearly obtain a detailed account of the displays remembers. These observations would of Masked Ducks Oxyura dominica, in suggest a great deal of similarity between several respects apparently the least Masked Duck displays and those of the specialized species of Oxyura. This lack other typical stiff-tails, but it may be of specialization is certainly indicated by hoped that more concrete information on its bill structure (“ Nomonyx ” refers to the species’ visual and auditory displays the ordinary appearance of the bill’s nail), will eventually become available for pur­ and by its skeletal anatomy (Woolfenden, poses of comparison. 1961). Thus, perhaps the species might be regarded as closest to an ancestral Oxyura The remaining five species of Oxyura type (Johnsgard, 1965b) and which in also pose considerable problems in under­ turn may have been derived from a standing their behaviour and relation­ Heteronetta-like ancester. Milton Weller ships. The only male displays which have (pers, com.) agrees with this view and been reliably reported in all of them in­ believes, like Woolfenden, that generic volve neck inflation and tail-cocking. In distinction is warranted. Bond’s (1961) all but the White-headed Duck O. leuco­ description of the downy young indicates cephala a display involving repeated bill- that Scott’s illustration (in Delacour, 1959) dipping followed by lateral head-shaking is misleading in various details, e.g., the (“ bill-flicking ”) or head-rolling on the pale areas are actually yellowish buff scapulars has been reported. This rather than white, and the light super­ sequence serves as a precopulatory dis­ ciliary stripe extends over the lores to the play in the North American Ruddy Duck, bill. These two mentioned features would probably also in the Maccoa Duck (Clark, seemingly bring the Masked Duck’s 1964), and possibly in the Argentine downy plumage closer in appearance to Ruddy Duck, but in the Australian Blue­ that of Heteronetta. Furthermore, Wet- billed Duck these same movements occur more (1965) has stated that the male at the end of a display sequence called Masked Duck’s oesophagus has an elon­ Sousing (Johnsgard, 1966). This display, gated and apparently inflatable middle first described as such for the Australiasi portion, and that the trachea has two air species, is clearly identical to one of the sacs, including a smaller one leading from displays described by Clark (1964), for the anterior ventral surface and a larger the Maccoa Duck, even to the preliminary one opening from the dorsal surface. So head-pumping and the following bill- far as is known, the male of no other dipping and head-shaking. Furthermore, Oxyura species has both an inflatable Dr. Martin Moynihan has recently shown oesophagus and well developed tracheal me sketches he made of display in the air sacs. The male Masked Duck in Argentine Ruddy Duck O. viitata and breeding condition does exhibit black which indicate the occurrence of Sousing spiny papillae around the base of the in­ in that species too. Considering the com­ tromittant organ (Wetmore, 1965), a plexity and frequency of this display feature apparently characteristic of many sequence in the Australian Blue-billed or all species of Oxyura (Helen Hays, Duck it may well be regarded as the pers. com.). species’ primary “ courtship” display, in Since my own attempts to locate and the same manner that the Bubbling study Masked Ducks have thus far been sequence may be so regarded for the Stiff-tail Behaviour 105

North American Ruddy Duck (Johnsgard, call by males is evidently much more 1965a). If this is the case, one might con­ restricted in occurrence. It is apparently clude that two species of Oxyura (jamai­ best developed in the Maccoa Duck, censis and leucocephala) probably have which produces a “ purring ” or “ vibrat­ as their primary male displays the Bub­ ing trumpet call ” with its tail erect, head bling sequence and that the three strictly forward and open, and with the southern hemisphere species (maccoa, crown feathers raised except for a central vittata and australis) presumably utilize groove running from front to back (Clark, Sousing as a characteristic display. This 1964). Except for the call, this description dichotomy would fit in well with the pos­ exactly fits the posture assumed by the tulated taxonomic relationships which I North American Ruddy Duck as it utters earlier (1961b) proposed on the basis of its weak belching note at the termination the more limited information available. It of the Bubbling sequence. A similar pos­ would further seem probable that each ture and presumably an associated call is species using Sousing as a display has an assumed at the end of the corresponding inflatable oesophagus whereas those which displays in White-headed Ducks (Mount- perform Bubbling may possess tracheal fort, 1958). Apparently the male Masked air sacs. Unfortunately, the male tracheal Duck also produces one or more vocal and oesophageal structures of the White- sounds, variously rendered as “ Kuri- headed Duck and the Maccoa Duck are kirro ” or like “ a short note from a motor still undescribed, although Clark (1964) horn,” although possibly this latter note suggests that a tracheal air sac may be is produced by the female. Lord William present in the Maccoa Duck. Percy (in Barber’s Birds of Cuba) stated Sound production through splashing that “ the male has a curious habit of movements is evidently a basic part of responding like a cock to such Oxyura displays. Thus, a rapid forward noises as the banging of a punt pole on Rush through the water by the male the water or an explosion in the distance,” (called Motor-boating in O. australis) has suggesting the significance of male vocal­ been reported for nearly all the Oxyura izations in territorial establishment and species, and a Ringing Rush (or Display maintenance. It would seem, however, Flight) just over the water surface accom­ that male vocalizations in most stiff-tails panied by a rattling sound produced by have been subordinated to non-vocally the wings and/or feet striking the water produced noises that may serve these has been noted in all of the species same functions. except the poorly studied White-headed As a means of summarizing our still Duck, Masked Duck, and Argentine relatively primitive state of knowledge Ruddy Duck. However, an actual display about stiff-tail behaviour, the accompany- Table II. Summary of some male displays and display structures in the species of Oxyura. An “ X ” indicates presence and a dash indicates apparent absences of the indicated feature.

S S O iS Q £ Q al t3 K1 -S “ f 3 ä» _.Q 0 S Q e I T3 C 2 -3 ? « ¿ S G *9 M < ■« 8 a « . i p «5 SP a § z ^ s a « Tail cocking XX XXXX Bill-dipping and water-flicking ■> X? XX? X Forward Rush in water XXX X ? X Ringing Rush over water 5 X ? X J X Sousing sequence ? - •I? XX X Bubbling sequence ? X X - -- Calling during tail-cocking X?XX X - Inflated Neck Display X X XXX X By oesophagus X - ? ? XX By air sacs XX??- - 106 The Wildfowl Trust

4 8 1

47

4 6 4

45 BLACK-HEADED W H IT E- HEADED DUCK N=6 DUCK N= II 44

43 BLUE- BILLED MACCOA DUCK 42 DUCK N = 8 N= !1 O N. A. RUDDY DUCK ^ 40 N = 13 ARGENTINE MUSK DUCK m 3 9 RUDDY DUCK N * 6 -J N * I® O Î 8 - « a ui 37 to o Q- X 36 H

35-

34.

MASKED DUCK 33- N = 23 32-

3 I i =T== —t = r = 17 18 19 20 21 22 23 24 25 32 33 34 35 MAXIMUM BILL WIDTH

Figure 2. Measurements (in millimetres) of bills of adult male stiff-tailed ducks. The lines intersect at the mean of each sample. ing Table II lists the apparent distribu­ appear to be among the most useful for tion of these various male behavioural characterizing the various species. It may and structural features in the Oxyura be seen that in the genus Oxyura a fairly species. Additionally, a diagram (Fig. 2) constant ratio between culmen width and showing interspecies variation in exposed length is maintained throughout the culmen lengths and maximum culmen genus, with the Masked Duck differing widths of adult male stiff-tail species has from the others only by virtue of its con­ been prepared, since these measurements siderably smaller size. On the other hand, Stiff-tail Behaviour 107 the Black-headed Duck’s bill is relatively Foundation research grant (G.B. 1030). : I long and narrow, much like that of an also wish to thank the Wildfowl Trust Anas species, whereas the massive bill of for fJle opportunity of again utilizing the Ae Musk Duck is unusually short and coIlection at Slimbridge. Additionally, un- broad, and is evidently adapted to crush- ,...... 5 . , ing rather than filtering foods. published observations were provided by Dr. Janet Kear, Mr. S. T. Johnstone, ÂcfaüowledgemeiBts Miss Helen Hays, Dr. Milton Weller, Mr. These observations were made with the L. I. Davis, and Mr. W. R. Siegfried, to financial support of a National Science all of whom I am very grateful.

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