--A cladistic analysis of the stiff-tailed ducks (Anatidae: Oxyurini) was conducted using 92 morphological characters. The analysis produced one minimum-length, completely dichotomous phylogenetic tree of high consistency (consistency index for informative characters, 0.74). Monophyly of the tribe was supported by 17 unambiguous synapomorphies. Within the tribe, Heteronetta (1 species) is the sister-group of other members; within the latter clade (supported by 21 unambiguous synapomorphies), Nomonyx (1 species) is the sister-group of Oxyura (6 species) + Biziura (1 species). The latter clade is supported by 10 unambiguous synapomorphies. Monophyly of Oxyura proper is supported by three unambiguous synapomorphies. All branches in the shortest tree except that uniting Oxyura, exclusive of jamaicensis, were conserved in a majority-rule consensus tree of 1000 boot-strapped replicates. Biziura and (to a lesser extent) Heteronetta were highly autapomorphic. Modest evolutionary patterns in body mass, reproductive parameters, and sexual dimorphism are evident, with the most marked, correlated changes occurring in Heteronetta and (especially) Biziura. The implications of these evolutionary trends for reproductive ecology and biogeographic patterns are discussed, and a phylogenetic classification of the tribe is presented. Received 27 April 1994, accepted 10 Nov. 1994.

The stiff-tailed ducks (Anatidae: Oxyurini) include some of the most distinctive species of waterfowl; among its members are the only obligate nest-parasite (Black-headed Duck; Heteronetta atricapilla) and the spe- cies showing the greatest sexual size dimorphism (Musk Duck; Biziura lobata) in the order Anseriformes (Delacour 1959; Johnsgard 1962, 1978; Weller 1968; Livezey 1986). Members of the Oxyurini are foot-propelled diving birds (Townsend 1909, Brooks 1945, Tome and Wrubleski 1988) and range in diving ability from the largely surface-feeding Black-headed Duck to the highly specialized Musk Duck (Raikow 1970, 1973; Livezey 1986). Stiff-tailed ducks generally inhabit freshwater lakes and marshes, typically construct over-water nests, and 'occur in most major land areas worldwide (Delacour 1959; Weller 1964a-d; Johnsgard 1978). Since the reclassification of the White-backed Duck (Dendrocygninae: Thalassornis leuconotus) on behavioral and morphological grounds (Johnsgard 1967; Raikow 1971; Livezey 1986, 1995), systematic contro- versies concerning the stiff-tailed ducks have focused on the tribal posi- tion of the Black-headed Duck, currently considered to be a comparatively primitive member of the Oxyurini (e.g., Salvadori 1895; Phillips 1925, 1926; Peters 1931; Boetticher 1942, 1952; Delacour and Mayr 1945; Ver- Section of Birds, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 5213-4080. 214 Livezey ß STIFF-TAILED DUCKS 215 heyen 1953; Delacour 1959; Johnsgard 1961a, 1965a, 1978; Rees and Hilgarth 1984; Livezey 1986; Sibley and Monroe 1993). Intergeneric re- lationships within the tribe were considered explicitly by Johnsgard (1961 a; figs. 8, 9), who depicted Heteronetta as the sister-group of Oxyura and Biziura. Species of Oxyura have been grouped, either by implication through the taxonomic sequences (Salvadori 1895; Phillips 1926; Peters 1931; .Boetticher 1942, 1952; Delacour 1959; Wolters 1976) or by explicit ar- gument or diagram (Delacour and Mayr 1945; Johnsgard 1961a, 1965a, 1967, 1968), as follows: (1) the unusual Masked Duck ("O." dominica) of the neotropics, segregated in the monotypic genus Nomonyx by some (Salvadori 1895, Phillips 1926, Peters 1931, Woolfenden 1961, Livezey 1986); (2) the Northern Ruddy Duck (O. jamaicensis; sometimes includ- ing the Peruvian Ruddy Duck, O. ferruginea) and White-headed Duck (0. leucocephala) of the northern hemisphere; and (3) the variously par- titioned and ranked southern-hemisphere forms (Argentine Ruddy Duck, O. vittata; Maccoa Duck, O. maccoa; Blue-billed Duck, O. australis; and Peruvian Ruddy Duck, O. [jamaicensis] ferruginea). Johnsgard (1966) described particular behavioral similarities between the Blue-billed Duck of Australia and the Argentine Ruddy Duck of southern South America. In two later works, Johnsgard (1978, 1979) proposed that the White- headed Duck was more closely related to the Maccoa Duck than to the Northern Ruddy Duck. The status and relationships of the Peruvian Rud- dy Duck and the Colombian Ruddy Duck remain controversial (Delacour and Mayr 1945; Delacour 1959; Johnsgard 1961a, 1965a, 1967, 1968, 1978, 1979; Siegfried 1976; Adams and Slavid 1984). An understanding of the marked evolutionary patterns in this unique group of waterfowl is contingent on a proposal of a robust, explicit phy- logenetic hypothesis. This paper presents a species-level phylogenetic (cladistic) analysis of the stiff-tailed ducks, using morphological charac- ters. This analysis is followed by an examination of evolutionary trends in selected life-history traits in the context of the phylogeny presented. Biogeographic issues also are considered and a phylogenetic classification of modern Oxyurini is proposed. MATERIALS AND METHODS Taxonomy.--As in my previous genus-level analysis of the Anseriformes (Livezey 1986), I recognize four genera in the tribe Oxyurini, of which three are monotypic: Heteronetta, Noraonyx, Oxyura, and Biziura. Opinions have varied regarding the number and composition of species to be recognized within Oxyura (exclusive of dominica); e.g., three species were recognized by Delacour and Mayr (1945), whereas Johnsgard (1961a, 1978, 1979) listed five. I adopted a conservative scheme, treating as a species any lineage diagnosable by the qualitative characters used in the phylogenetic analysis, an approach that admits a maximal 216 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 number of possible phylogenetic inferences and that is compatible with the phylogenetic species concept (Cracraft 1983, 1988). The following six species of Oxyura were recognized: jamaicensis (including rubida), ferruginea, vittata, australis, leucocephala, and maccoa. In addition, the andina population of northern South America (typically included in O. jamai- censis) was analyzed separately in some analyses in an attempt to assess the likelihood of its purportedly hybrid origin; the only defining character detected for andina is its variably black-marked, white cheeks, a character interpretable as evidence of introgression (Adams and Slavid 1984). Definition of characters.--Characters were defined on the basis of direct examination of study skins, skeletons, and tracheae of stiff-tailed ducks and other Anatinae, as well as a review of the relevant literature (Pycraft 1906; Phillips 1925, 1926; Delacour and Mayr 1945; Delacour 1959, 1964; Johnsgard 1961b, 1962, 1965b, 1967, 1978; Humphrey and Clark 1964; Lowe 1966; Weller 1968; Siegfried 1970; Palmer 1976; Madge 1984; Torres 1984; Torres and Ayala 1986; Madge and Burn 1988; King 1989; McLelland 1989; Mar- chant and Higgins 1990; del Hoyo et al. 1992; Nelson 1993). The natal pattern of O. (j.) andina was examined using photographs of three specimens. A unique set of tracheal, esophageal, and gular diverticula in the Oxyurini, which produce the inflation of the neck during courtship in males, are confusingly described in the literature and remain incom- pletely known (e.g., Johnsgard 1961b, 1965a, 1967; Clark 1964; Siegfried 1976); states for these characters were based on direct examination of Heteronetta atricapilla, Noraonyx dominicus, Oxyura jamaicensis, O. maccoa, O. australis, and Biziura lobata and the few adequately detailed anatomical descriptions (Forbes 1882; Wetmore 1917, 1918, 1926, 1965; McLelland 1989). Each character is a trait, judged to be homologous across taxa, that comprised a primitive (plesiomorphic) state and one or more derived (apomorphic) states. A total of 92 morpho- logical characters were defined--39 skeletal characters (most after Livezey 1986), three characters of the trachea or esophagus, 10 characters of natal plumage, and 40 characters of the plumage and soft parts of adults (available from the author on request). Species which a character state could not be ascertained were assigned a missing-datum code for that character. These character-states, together with a "hypothetical ancestor" vector (see below) compose a data matrix of dimension 10 x 92 (available from the author on request). Characters having more than one derived state were considered unordered unless a logical, consistent ordination by count, degree, or relative extent was evident (e.g., number of pairs of rectrices was analyzed as ordinal). Characters in which the derived state(s) were possessed by single species (autapomorphies) were included in the analysis because of the pervasive influence such differences have had on traditional classification and because this permits estimates of evolutionary divergence and phenetic differences among species. Outgroups and polarities.--Representatives of the three other tribes of the Anatinae-- Anatini, Mergini; Aythyini (sensu Livezey 1986, 1991)--were chosen as outgroups, in ac- cordance with the preponderance of anatomical and behavioral evidence (e.g., Woolfenden 1961; Johnsgard 1961a, 1965a, 1978) and a phylogenetic analysis of the order (Livezey 1986). Although the inclusion of Mergini and Oxyurini in the Anatinae is reasonably well supported (Livezey 1986), the relationships among the tribes of Anatinae remain poorly resolved (Livezey, unpubl. data). Polarities (directionalities of character change) were es- tablished by ascertaining the distributions of states of each character in the other tribes of Anatinae; these states were hypothesized to be the primitive conditions (plesiomorphous states) for the characters with respect to the ingroup (Oxyurini). These individual plesiorn- orphous states were combined into a single vector or "hypothetical ancestor" which in turn was used to root the tree(s). An early divergence of the Oxyurini from most other Anseriformes was proposed by Livezey ß STIFF-TAILED DUCKS 217 Madsen et al. (1988) and Sibley and Ahlquist (1990) and adopted by Sibley and Monroe (1990). This view (Sibley and Ahlquist 1990: fig. 357) implies that any member of the Anseriformes exclusive of the Anhimidae, Anseranas, and Dendrocygna is equally closely related to the Oxyurini, a diverse set of outgroups indeed. This proposal, however, is not considered credible in light of numerous substantive criticisms of DNA hybridization as a tool for phylogenetic inference (Cracraft 1987, Houde 1987, Sarich et al. 1989), additional irregularities in the application of the technique by Sibley and colleagues (Lewin 1988a, b; Krajewski 1991; Mindell 1992), the subjective placement of the Oxyurini within the An- seriformes and unfavorable metric properties associated with sparse data matrices (Lanyon 1992), inconsistent classificatory methodology (Siegel-Causey 1993), and the inadequate representation of the tribe in the analysis (the sole representative being one species of Oxyura). Other molecular studies including at least one representative of the Oxyurini, all of which also were phenetic, included too few genera or failed to resolve intergeneric relationships adequately for meaningful comparisons (Brush 1976, Patton and Avise 1985, Scherer and Sontag 1986). P. J. Fullagar (in Marchant and Higgins 1990) suggested that the unique and primitive Freckled Duck (Stictonetta naevosa) is closely related to the stiff-tailed ducks, primarily on the basis of the phenetic findings of Madsen et al. (1988) and Sibley and Monroe (1990), similarities in selected behavioral and anatomical characters, and a misinterpretation of a study by Faith (1989). Although polarities of most skeletal characters used herein were unchanged, use of Stictonetta as the outgroup necessitated equivocal, mutually contradictory changes in polarities in seven osteological characters (Stictonetta exclusively sharing three with Heteronetta, three others with Biziura, and one with Oxyurini exclusive of Hetero- netta). Also, postulation of Stictonetta as the sister-group of the Oxyurini required six ad- ditional changes in osteological characters shared by the Tadorninae and Anatinae (Livezey 1986), while saving only one step in the inferred polarity of the bulla syringealis. Natal characters of Oxyurini proved all but non-comparable with the unique natal pattern of Stic- tonetta. Characters of the definitive integument of Oxyurini also were difficult to compare with Stictonetta and require additional homoplasies in the wing speculum and tarsal scutel- lation in a global analysis. Pending documentation and phylogenetic analysis of the alleged similarities between Stictonetta and the Oxyurini mentioned by Fullagar (including the ques- tionably homologous coloration of the maxillary rhamphotheca and undescribed details of the hemipenes) or other corroborafing evidence, use of the Anatinae as the outgroup for the Oxyurini seems justified, and (for the characters analyzed here) probably of negligible to- pological effect within the tribe. Derivation of trees.--Trees were derived according to the principles of phylogenetic anal- ysis under the criterion of global parsimony (Wiley 1981). Trees were constructed using the phylogenetic software PAUP 3.1 (Swofford 1993); supplementary topological analyses, a posteriori character mappings, and printing of trees were performed on MacClade 3.01 (Maddison and Maddison 1992). Both programs were implemented on a Macintosh Quadra 800. I used the deterministic "branch-and-bound" algorithm to find the shortest tree(s). The character-state optimization used was accelerated transformation (ACCTRAN); employment of delayed transformation (DELTRAN) did not affect the solution set. A bootstrapping procedure using the branch-and-bound algorithm, intended as an index to empirical support for branches and not rigorous statistical inference (Felsenstein 1985, Sanderson 1989), was employed to generate 1000 topological replications. Stability of branches within the final tree was summarized by a 50% majority-rule consensus tree of these replicate trees. Ecomorphological trends.--Selected functionally important but presumptively nonhom- ologous characteristics were compiled for assessment of evolutionary patterns; these attri- butes were mapped a posteriori onto the independently inferred phylogenetic tree, thereby 218 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 permitting phylogenetically based comparative analyses (Smith and Patterson 1988, Brooks and McLennan 1991, Harvey and Pagel 1991). States of these attributes-including body mass, clutch size, egg size, nesting habits, diet, and diving behavior--were taken from the literature (Delacour 1959, SchiSnwetter 1961, Frith 1967, Raikow 1973, Palmer 1976, Cramp and Simmons 1977, Todd 1979, Brown et al. 1982, Livezey 1986, Madge and Burn 1988, Rohwer 1988, Marchant and Higgins 1990, del Hoyo et al. 1992, Dunning 1993). Mean body masses of species were estimated by the unweighted mean of the mean masses for adults of the two sexes separately. "Sexual size dimorphism" was measured by the ratio of the mean body mass of males divided by the mean body mass of females. "Relative clutch mass" was defined by the product of mean clutch size and mean egg mass divided by the mean body mass of an adult female. Phylogenetic classification.The resultant phylogenetic tree(s) formed the basis for a Linnean classification that maximally reflects the relationships inferred for the ingroup. Unconventional taxonomic ranks (subtribes, supergenera, and subgenera) were based on senior taxa of appropriate rank, in part based on the classifications of Boetticher (1942, 1952) and the synonomies of Phillips (1925, 1926), Brodkorb (1964), and Wolters (1976). RESULTS Shortest tree.--A single, completely dichotomous, shortest tree was found for the Oxyurini, having a total length of 127, a consistency index (excluding autapomorphies) of 0.81, a retention index of 0.81, and a re- scaled consistency index of 0.71. Monophyly of the tribe is supported by 17 unambiguous synapomorphies (Fig. 1). Within the tree, Heteronetta is the sister-group of all other Oxyurini; the latter group (supported by 20 unambiguous synapomorphies) comprises two major subgroups--(1)No- monyx and (2) Oxyura + Biziura (Fig. 1). Monophyly of the clade com- prising Oxyura and Biziura is supported by 10 unambiguous synapomor- phies; three unambiguous synapomorphies support the monophyly of Oxyura. Within Oxyura, O. jamaicensis is resolved to be the sister-group of its congeners. The latter clade (supported by a single synapomorphy) comprises two basic subgroups, both polyspecific (Fig. 1): (1) O. ferru- ginea and its sister-group O. vittata + O. australis (each branch supported by one synapomorphy) and (2) the sister-species O. maccoa and O. leu- cocephala (supported by four synapomorphies). An analysis in which the Colombian Ruddy-Duck (O. "andina") was included as a separate operational unit placed the taxon in two equally parsimonious positions within Oxyura exclusive of O. jamaicensis: (1) as the sister-group of the clade comprising the remaining Oxyura or (2) as a monotypic branch in a trichotomy including two other clades,ferruginea + vittata + australis and maccoa + leucocephala. Apomorphic divergence.--The three monotypic genera each had sev- eral autapomorphies: Heteronetta (8), Nomonyx (5), and Biziura (28). The genus Biziura exceeds all other Anseriformes in autapomorphic diver- gence, both in this analysis (Fig. 1) and an earlier genus-level study (Liv- Livezey ß STIFF-TAILED DUCKS 219 FIG. 1. Most-parsimonious phylogenetic tree for the Oxyurini based on 92 morpholog- ical characters. Numbers of unambiguous character changes supporting each branch are indicated. ezey 1986). Within the genus Oxyura (excluding Oxyura "andina"), only O. ferruginea lacked a defining autapomorphy (Fig. 1); numbers of au- tapomorphies in the other five species ranged from four in O. jamaicensis to one each in O. vittata and O. maccoa (Fig. 1). Robustness of tree.--Bootstrapping revealed that all branches in the shortest tree (Fig. 1), with the exception of that uniting Oxyura exclusive ofjamaicensis, were conserved in a majority of replications with vary- ing frequency (Fig. 2). The branches defining the entire tribe and uniting 220 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 H. atricapilla N. dominicus lOO 96 8_.,..2_O 54 61 88 O. jamaicensis O. ferruginea o. v/tram O. australis O. frisco08 O. leucocephala B. Iobata Ancestor FG. 2. Fifty-percent majority-role consensus tree of 1000 bootstrapped replications of the shortest phylogenetic tree of the Oxyurini (Fig. 1). Percentages of replications in which each branch was conserved are indicated. Livezey ß STIFF-TAILED DUCKS 221 the Oxyurini exclusive of Heteronetta were retained in 100% of the bootstrapped replicates (Fig. 2). The branch supporting the clade com- prising Oxyura and Biziura was conserved in 96% of the bootstrapped replicates. Nodes within Oxyura were conserved in 54-88% of the rep- licates (Fig. 2). Ecomorphological patterns.--Body mass showed only moderate phy- logenetic conservatism in the Oxyurini (CI = 0.60), evidently having undergone one autapomorphic decrease in Noraonyx and independent in- creases in O. ferruginea, O. australis, and (especially) Biziura (Fig. 3). Clutch size was comparable to body mass in consistency (CI = 0.67); this parameter appears to have increased in O. jamaicensis and to have decreased in Biziura and is undetermined in the strictly nest-parasitic Heteronetta (Fig. 4). Egg mass (CI = 0.67) evidently increased homo- logously in Oxyura exclusive of O. jamaicensis and independently to a greater degree in Biziura (Fig. 5). Taken together, these patterns in egg mass and clutch size (relative to female body mass) indicate modest, independent increases in relative clutch mass in Noraonyx and in two lineages of Oxyura (0. jamaicensis and in O. maccoa + O. leucocephala), and a decrease in this parameter in Biziura (CI = 0.60). Sexual size dimorphism showed a moderately complex evolutionary pattern in the Oxyurini, evidently having (Fig. 6; CI = 0.60) (1) shifted to female-larger dimorphism in Heteronetta, (2) increased paraphyleti- cally in Nomonyx, O. jamaicensis, and (especially) Biziura, and (3) un- dergone a reversal in other Oxyura (especially O. australis). Sexual di- chromatism of plumage, however, showed a very different pattern, with a reduction of sexual differences in Heteronetta and an independent re- versal to monochromatism in Biziura (CI = 0.60). DISCUSSION Phylogenetic inferences and classification.--The trees presented by Johnsgard (1961a: figs. 8, 9) are the only species-level hypotheses of the phylogeny of the Oxyurini available for comparison. The relationships depicted by Johnsgard (1961a) differ from those proposed here in three major ways: (1) the positions of "N." dominicus and B. lobata were reversed, (2) Oxyura was partitioned into two major groups--leucoceph- ala + jamaicensis and vittata + maccoa + australis, and (3) O. ferru- ginea was merged with O. jamaicensis. Based on the data matrix com- plied here, the topology proposed by Johnsgard (1961a) is 13 steps (10%) longer (less parsimonious) than the shortest tree inferred here (Fig. 1). Six of these additional steps are attributable to the inclusion by Johnsgard (1961a) of Nomonyx as the sister-group of Oxyura (instead of Biziura), a topology implied also by all classifications in which dominicus is listed 222 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 [] ß Body Mass ::.":' b Id  equivocal FIG. 3. Map of mean body mass (g) on phylogeny of the Oxyurini (primitive state for tribe in boldface): a--<500, b--500-750, c--751-1000, d> 1000. Character descriptions and data matrix available from author on request. first within Oxyura (e.g., Delacour and Mayr 1945; Delacour 1959; Johns- gard 1978, 1979). If Biziura is to be generically distinguished from Ox- yura (its sister group), the present analysis requires (at least) that No- monyx dominicus also be generically separated. Another classificatory recommendation deriving from the present analysis is that O. ferruginea not be merged with O. jamaicensis but instead be grouped separately with two other southern-hemisphere species (see classification in Appendix I). Livezey ß STIFF-TAILED DUCKS .,. .u. O.X,,,- 0.x ß [] ß [] 223 0  0  0 xø b xø Clutch Size b lc [==::=l equivocal FIG. 4. Map of mean clutch size on phylogeny of the Oxyurini (primitive state lbr tribe in boldface): a--<3, b--3-6, c-->6. Character descriptions and data matrix available from author on request. The taxonomic status of the variable O. (j.) andina remains unresolved, and its purported hybrid status deserves intensive study; it is merged provisionally with O. jamaicensis. Moreover, if O. (j.) andina were to be confirmed to be a hybrid swarm between O. jamaicensis and O. ferru- ginea, no reliable inferences concerning the phylogenetic relationship be- tween the two "parent" forms could be made. Although propensity for interspecific hybridization traditionally has been considered to be pre- 224 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 .o o:  o? o? ø'  o? o? Egg Mass lb /c [1 equivocal FIG. 5. Map of mean egg mass (g) on phylogeny of the Oxyurini (primitive state for tribe in boldface): a--50-75, b--76-100, c-->100. Character descriptions and data matrix available from author on request. vented by adaptive "isolating mechanisms" and to indicate closeness of phylogenetic relationship (Sibley 1957; Johnsgard 1960b, 1963), inter- specific hybridization instead reflects the retention of primitive interfer- tility subsequent to speciation, and therefore this symplesiomorphy pro- vides no direct evidence of relatedness (Livezey 1991). Furthermore, the great diversity of recorded interspecific, intergeneric, and intertribal hy- brids among Anseriformes further undermines any evolutionary insights Livezey ß STIFF-TAILED DUCKS 225 Size Dimorphism a /d FIG. 6. Map of sexual size dimorphism (ratio of means [g] for sexes) on phylogeny of the Oxyurini (primitive states for tribe in boldface): a-- 1.30. Character descriptions and data matrix available from author on request.. to be gained from patterns of hybridization (Johnsgard 1960b, Scherer and Hilsberg 1982). Evolutionary trends.--Of the several quantitative parameters mapped onto the phylogenetic tree for the Oxyurini, probably the most distinct patterns emerged for egg mass (Fig. 5). The general increase in absolute egg mass evident in the tribe (Fig. 5), coincident with a general decrease in clutch size (Fig. 4), conforms with a weak, inverse relationship between 226 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 sizes of eggs and clutches in Anseriformes (Lack 1967, Rohwer 1988). Egg masses predicted by the equation of Rahn et al. (1975) relating egg mass to female body mass for the Anseriformes confirm that the egg masses of Oxyurini are large relative to the corresponding mean body masses of females (Lack 1967, Briggs 1988). The combination of small clutch size and large (absolute) egg size in the Oxyurini resembles that of many insular species of dabbling ducks (Anatini; Livezey 1990, 1993), but clearly the selection regimes of the latter differ significantly from those shared by the Oxyurini (Weller 1964a-c). Moreover, two compar- atively massive species of Oxyurini--Oxyura australis and Biziura lob- ata--have relative egg masses typical of most Anatini, whereas egg size in most insular endemics is especially high relative to body mass (Livezey 1993). Unfortunately, the proportion of egg mass composed of yolk, a relationship with important implications for growth rates, is not known for the Oxyurini (Lack 1968). In addition to the complex changes in body mass, general decreases in clutch size, and changes in egg mass evident in the Oxyurini (Fig. 5), evolutionary patterns in several other aspects of the life-history of the stiff-tailed ducks can be inferred in the context of the phylogenetic hy- pothesis proposed here (Fig. 1). Sexual dichromatism underwent reduc- tion or loss in two very different lineages but manifests little correlation with sexual size dimorphism (Fig. 6). Comparatively inconspicuous sex- ual differences color of plumage and soft parts characterize Heteronetta, an obligate nest parasite showing modest female-larger size dimorphism (Weller 1967, 1968; Maynard Smith 1977), and the virtually monochro- matic Biziura, a polygynous species showing extreme male-larger size dimorphism (Frith 1967, Livezey and Humphrey 1984). Infrequent nest parasitism evidently characterizes all other Oxyurini and is probably ple- siomorphous in the Anatinae (Attiwill et al. 1981, Eadie et al. 1988, Rohwer and Freeman 1989). The Oxyurini exclusive of Heteronetta also share a moderate tendency toward (at least infrequent) crepuscular or nocturnal feeding, a habit perhaps most developed in Nornonyx (Weller 1968, McNeil et al. 1992). The courtship displays of the Oxyurini include several distinctive be- haviors, although the behaviors of several species remain inadequately known (Delacour 1959; Johnsgard 1960a, 1961a, 1962, 1965a, 1978). Males of at least Nornonyx and Oxyura perform unique displays involving the rapid thumping of the bill on the inflated throat sac(s); other displays that unite males of most Oxyurini include "tail cocking," "bill dipping," various surface rushes, and "sousing" (Johnsgard 1961a, 1962, 1965a, b, 1966, 1967, 1968, 1978; Fullagar and Carbonell 1986). Male Musk Ducks perform bizarre courtship displays, which typically involve a number of Livezey ß STIFF-TAILED DUCKS 227 acoustically and visually conspicuous components, including "paddling kicks," "whistle kicks," "plonk kicks," and exhibition of the fleshy gular lobe or palear (Johnsgard 1965a, 1966, 1967; Frith 1967; Fuilagar and Carbonell 1986). Females of the Oxyurini, including Heteronetta, lack the "decrescendo" and "inciting" vocalizations that characterize female Anatini. Biogeographic patterns.--Modern members of the Oxyurini are limited in distribution to the Neotropics, southern Nearctic, southern Africa, the Mediterranean, central Palearctic, and southern Australia (Weller 1964d). Fossils of the tribe are limited to a few weakly differentiated forms from these same regions that were assignable to modern genera (Brodkorb 1964, Howard 1964). Subfossil remains merely extend the former ranges of the two Australian members--O. australis and B. lobata--to New Zealand (Brodkorb 1964, Horn 1983). The phylogenetic hypothesis proposed here strongly indicates a south- ern-hemispheric origin for the tribe, in that the two deepest branches (Heteronetta and Nomonyx) are Neotropical in distribution, and the next earliest branch (Biziura) is Australian (Fig. 1). Within Oxyura, the first branch corresponds to a northern-hemisphere species (jamaicensis), with the remaining members grouped into Palearctic-African (maccoa and leu- cocephala) and Neotropical-Australian clades (ferruginea, vittata, and australis). A strict vicariance model for the Oxyurini, in which a nested subdivision of regions corresponds with phylogenetic patterns, is not pos- sible for extant species (Fig. 1). The complete absence of insular popu- lations of Oxyurini suggests that members of the tribe, although most are migratory, are among the poorest dispersers in the Anseriformes (Weller 1964a). Field observations substantiate the poor flying abilities of most Oxyurini (Johnsgard 1978). Musk Ducks have especially high wing-load- ings (Frith 1967), and large males not in wing molt may be at least temporarily flightless (Goodwin 1974, Marchant and Higgins 1990). The lack of insular isolates in the Oxyurini, however, may be related as much to the freshwater habitats of the member species (Weller 1964b, c), a preference that would deter trans-oceanic migration and insular coloni- zation. Evolutionary anomalies of the Musk Duck.--The Musk Duck, in ad- dition to its unusual courtship displays, is unique among Oxyurini in its comparatively huge body size, highly autapomorphic skeletal anatomy, predominantly animal diet, frequency of maternal feeding of young, de- layed sexual maturation (two years of age), lack of pair-bonds and as- sociated "promiscuous" or polybrachygynous mating system, absence of sexual dichromatism of plumage, extreme sexual size dimorphism, dark natal plumage (Fig. 7), and small clutch size (Frith 1967; Johnsgard 228 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 ! I D FIG. 7. Plumage patterns of natal Oxyurini, lateral views: A--Heteronetta atricapilla (USNM 337995); B--Nomonyx dominicus (AMNH 2240); C--Oxyura jamaicensis (USNM 259366); and DBiziura lobata (AMNH 734131). 1965a, b, 1966, 1978; Kear 1970; Livezey 1986). The association of exaggerated sexual size dimorphism with low parental investment by males in Biziura conforms with broad patterns documented for birds (Se- lander 1972, Trivers 1972, Payne 1984; but see H6glund 1989), but this association is comparatively weak among waterfowl generally (Sigur- j6nsd6ttir 1981). Similarly, the coincidence of the loss of sexual dichro- matism with polygyny in Biziura opposes general avian patterns (Butcher and Rohwer 1989) but conforms with the comparatively poor correspon- dence between plumage dimorphism and paternal investment in Anseri- formes (Sigurj6nsd6ttir 1981, Scott and Clutton-Brock 1989). The unique gular lobe or palear in male Biziura, and its use in courtship, performs some of the advertising functions of bright plumage (Frith 1967; Johns- gard 1965a, b, 1966). The possible role of the seasonally pungent uro- pygial secretions of male Biziura, if any, remains obscure (Frith 1967). Magnitude of sexual size dimorphism is weakly associated with body size in some avian groups (Reiss 1986), but the sexual size differences of Biziura exceeds that attributable to the comparatively large body size of the species. The importance of body size to thermodynamics, metab- Livezey ß STIFF-TAILED DUCKS 229 olism, energy requirements, survivorship, and reproductive investment is profound, therefore, the evolutionary trade-offs implicit in the increased body size of male Musk Ducks (and other ecological peculiarities) de- serve detailed study, especially within a phylogenetic context (Fig. 1) and with special attention to comparisons involving the partly sympatric O. australis. ACKNOWLEDGMENTS This research was supported by National Science Foundation grants BSR-8515523, BSR- 9129545, and BSR-9396249, as well as collection visitation grants from the National Mu- seum of National History (USNM) and the American Museum of Natural History (AMNH). The generous hospitality of G. Mack and R. L. Zusi made several prolonged visits to New York and Washington productive and enjoyable. I am grateful for loans and/or access to collections provided by the curatorial personnel of the following institutions: Division of Birds, National Museum of Natural History; Department of Ornithology, American Museum of Natural History; Division of Birds, Museum of Zoology, University of Michigan; Division of Birds, Field Museum of Natural History; Sub-department of Ornithology, British Museum of Natural History; Wildfowl and Wetlands Trust; and Division of Ornithology, Museum of Natural History, University of Kansas. M. Brown (Wildfowl and Wetlands Trust) examined tracheae and esophagi of several Oxyurini, and F. G. Stiles (Institut de Ciencias Naturales, Universidad de Colombia) kindly provided photographs of three natal Oxyura (j.) andina. I also thank P.S. Humphrey, R. F. Johnston, D. Siegel-Causey, L. Trueb, and E. O. Wiley for a variety of helpful inputs, and P. A. Johnsgard and R. O. Prum made helpful comments on the manuscript. LITERATURE CITED ADAMS, J. AND E. R. SLAVID. 1984. Cheek plumage pattern in Colombian Ruddy Duck Oxyura jamaicensis. Ibis 126:405-407. ATTIWm., A. R., J. M. BOURNE, ^NO S. A. P^RKER. 1981. Possible nest-parasitism in the Australian stiff-tailed ducks (Anatidae: Oxyurini). Emu 81:41-42. BOETTICHER, H., VON. 1942. lber die Einteilung der Familie der Entenv6gel (Anatidae) in Unterfamilien und Sektionen. Zool. Anz. 140:37-48. 1952. Ginse- und Entenv6gel ams aller Welt. Geest and Portig K.-G., Leipzig, Germany. BRIGGS, S. V. 1988. Weight changes and reproduction in female Blue-billed and Musk ducks, compared with North American Ruddy Ducks. Wildfowl 39:98-101. BRODKORB, P. 1964. Catalogue of fossil birds: part 2 (Anseriformes through Galliformes). Bull. Florida State Mus. (Biol. Sci.) 8:195-335. BROOKS, A. 1945. The under-water actions of diving ducks. Auk 62:517-523. BROOKS, D. R. ^NO D. A. MCLENN^N. 1991. Phylogeny, ecology, and behavior: a research program in comparative biology. Univ. of Chicago Press, Chicago, Illinois. BROWN, L. H., E. K. URBAN, AND K. NEWMAN. 1982. The birds of Africa. Vol. 1. Academic Press, London, England. BRUSH, A. H. 1976. Waterfowl feather proteins: analysis of use in taxonomic studies. J. Zool. London 179:467-498. BUTCHER, G. S. AND S. ROHWER. 1989. The evolution of conspicuous and distinctive col- oration for communication in birds. Pp. 51-108 in Current ornithology, Vol. 6 (D. M. Power, ed.). Plenum Press, New York, New York. 230 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 CLARK, A. 1964. The Maccoa Duck (Oxyura maccoa (Eyton)). Ostrich 35:264-276. CRACRAFT, J. 1983. Species concepts and speciation analysis. Pp. 159-187 in Current or- nithology. Vol. 1 (R. F. Johnston, ed.). Plenum Press, New York, New York. 1987. DNA hybridization and avian phylogenetics. Evol. Biol. 21:47-96. 1988. Speciation and its ontology: the empirical consequences of alternative spe- cies concepts for understanding patterns and processes of differentiation. Pp. 28-59 in Speciation and its consequences (D. Otte and J. A. Endler, eds.). Sinauer Assoc., Sun- derland, Massachusetts. CRAMP, S. AND K. E. L. SIMMONS. 1977. Handbook of the birds of Europe and the Middle East and North Africa. Vol. 1. Oxford Univ. Press, Oxford, England. DELACOUR, J. 1959. The waterfowl of the world. Vol. 3. Country Life, London, England. 1964. Corrections and additions. Pp. 327-354 in The waterfowl of the world. Vol. 4 (J. Delacour, ed.). Country Life, London, England. --AND E. MAYR. 1945. The family Anatidae. Wilson Bull. 57:3-55. DUNNING, J. B., JR. (ED.). 1993. CRC handbook of avian body masses. CRC Press, Boca Raton, Florida. EADIE, J. McA., E P. KEHOE, AND T. D. NUDDS. 1988. Pre-hatch and post-hatch brood amalgamation in North American Anatidae: a review of hypotheses. Can. J. Zool. 66: 1709-1721. FAITH, D. P. 1989. Homoplasy as pattern: multivariate analysis of morphological conver- gence in anseriforms. Cladistics 5:235-258. ['ELSENSTEIN, J. 1985. Confidence limits on phylogenies: an approach using the bootstrap. Evolution 39:783-791. FORBES, W. A. 1882. Notes on some points in the anatomy of an Australian duck (Biziura lobata). Proc. Zool. Soc. London 1882:455458. FRITH, H. J. 1967. Waterfowl in Australia. East-West Center Press, Honolulu, Hawaii. FULLAGAR, P. J. AND m. CARBONELL. 1986. The display postures of the male Musk Duck. Wildfowl 37:142-150. GOODWIN, D. 1974. Anseriformes--swans, geese & ducks. Pp. 47-51 in Birds of the Harold Hall Australian expeditions 1962-70 (B. P. Hall, ed.). British Museum (Natural History), London, England. HARVEY, P. H., AND m.D. PAGEL. 1991. The comparative method in evolutionary biology. Oxford University Press, Oxford, England. HAYS, H. AND H. m. HABERMANN. 1969. Note on bill color of the Ruddy Duck, Oxyura jamaicensis rubida. Auk 86:765-766. HOGLUND, J. 1989. Size and plumage dimorphism in lek-breeding birds: a comparative analysis. Am. Natur. 134:72-87. HORN, P. L. 1983. Subfossil avian deposits from Poukawa, Hawkes Bay, and the first record of Oxyura australis (Blue-billed Duck) from New Zealand. J. Royal Soc. New Zealand 13:67-78. HOUDE, P. 1987. Critical evaluation of DNA hybridization studies in avian systematics. Auk 104:17-32. HOWARD, H. 1964. Fossil Anseriformes. Pp. 233-326 in The waterfowl of the world. Vol. 4 (J. Delacour, ed.). Country Life, London, England. HoYo, J. DEL, A. ELLIOTT, AND J. SARGATAL (EDs.). 1992. Handbook of the birds of the world. Vol. 1. Lynx Editions, Barcelona, Spain. HUMPHREY, P.S. AND G. A. CLARK, JR. 1964. The anatomy of waterfowl. Pp. 167-232 in The waterfowl of the world. Vol. 4 (J. Delacour, ed.). Country Life, London, England. JOHNSCARD, P. A. 1960a. Comparative behavior of the Anatidae and its evolutionary im- plications. Wildfowl 11:3145. Livezey ß STIFF-TAILED DUCKS 231 ß 1960b. Hybridization in the Anatidae and its taxonomic implicationsß Condor 62: 25-33. ß 1961a. The taxonomy of the Anatidae--a behavioural analysis. Ibis 103a:71-85. ß 1961b. Tracheal anatomy of the Anatidae and its taxonomic significance. Wildfowl 12:58-69. ß 1962. Evolutionary trends in the behaviour and morphology of the Anatidae. Wild- fowl 13:130-148. ß 1963. Behavioral isolating mechanisms in the family Anatidae. Pp. 531-543 in Proceedings XIII International Ornithological Congress. Vol. 1 (C. G. Sibley, ed.). American Ornithologists' Union, Washington, D.C. ß 1965a. Handbook of waterfowl behavior. Constable, London, England. ß 1965b. Observations on some aberrant Australian Anatidae. Wildfowl 16:73-83. ß 1966. Behavior of Australian Musk Duck and Blue-billed Duck. Auk 83:98-110. ß 1967. Observations on the behaviour and relationships of the White-backed Duck and the stiff-tailed ducks. Wildfowl 18:98-107. ß 1968. Observations on Maccoa Duck behaviour. Ostrich 39:219-222. ß 1978. Ducks, geese, and swans of the world. Univ. Nebraska Press, Lincoln, Ne- braska. ß 1979. Order Anseriformes. Pp. 425-506 in Checklist of birds of the world. Vol. 1, 2rid ed. (E. Mayr and C. W. Cottrell, eds.). Mus. Comp. Zool., Cambridge, Massa- chusetts. KEAR, J. 1970. The adaptive radiation of parental care in waterfowl. Pp. 357-392 in Social behaviour in birds and mammals (J. H. Crook, ed.). Academic Press, New York, New York. KNG, A. S. 1989. Functional anatomy of the syrinx. Pp. 105-192 in Form and function in birds. Vol. 4 (A. S. King and J. McLelland, eds.). Academic Press, London, England. IRAJEWSKI, C. 1991ß [Review of] "Phylogeny and classification of birds: a study in mo- lecular evolution." Auk 108:987-990. L^CK, D. 1967. The significance of clutch-size in waterfowl. Wildfowl 18:125-128. ß 1968. The proportion of yolk in the eggs of waterfowl. Wildfowl 19:67-69. LANYON, S. M. 1992. [Review of] "Phylogeny and classification of birds: a study in mo- lecular evolution." Condor 94:304-307. LEWIN, R. 1988a. Conflict over DNA clock results. Science 241:1598-1600. ß 1988b. DNA clock conflict continues. Science 241:1756-1759. LIVEZEY, B.C. 1986. A phylogenetic analysis of Recent anseriform genera using morpho- logical characters. Auk 103:737-754. ß 1990. Evolutionary morphology of flightlessness in the Auckland Islands Teal. Condor 92:639-673. ß 1991. A phylogenetic analysis and classification of Recent dabbling ducks (Tribe Anatini) based on comparative morphology. Auk 108:471-507. ß 1993. Comparative morphometrics of Arias ducks, with particular reference to the Hawaiian Duck Anas wyvilliana, Laysan Duck A. laysanensis, and Eaton's Pintail A. eatoni. Wildfowl 44:75-100. ß 1995. A phylogenetic analysis of the whistling and white-backed ducks (Anatidae, Dendrocygninae) using morphological characters. Ann. Carnegie Musß 64:65-97. --AND P.S. HUMPHREY. 1984. Sexual dimorphism in continental steamer-ducks. Con- dor 86:368-377. LOWE, V. T 1966. Notes on the Musk-duck Biziura lobataß Emu 65:279-290. MADDISON, W. P. AND D. R. MADDISON. 1992. MacClade: version 3. Sinauer Assoc., Sun- derland, Massachusetts. 232 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 MADGE, S. Cß 1984. White-headed Duck with black headß Brit. Birds 77:154. --AND H. BURNß 1988. Waterfowl: an identification guide to the ducks, geese and swans of the world. Houghton Mifflin, Boston, Massachusetts. MADSEN, C. S., K. P. MCHUGH, AND S. R. DE KLOET. 1988. A partial classification of waterfowl (Anatidae) based on single-copy DNA. Auk 105:452-459. MARCHANT, S. AND P. J. HIGGINS (CooRDS.). 1990. Handbook of Australian, New Zealand and Antarctic birds. Vol. 1, part Bß Oxford Univ. Press, Melbourne, Australia. MAYNARD SMITH, J. 1977. Parental investment: a prospective analysis. Anim. Behav. 25:1-9. McLELLAND, J. 1989. Larynx and trachea. Pp. 69-103 in Form and function in birds. Vol. 4 (A. S. King and J. McLelland, eds.). Academic Press, London, England. MCNEIL, R., P. DRAPEAU, AND J. D. GOSS-CUSTARD. 1992. The occurrence and adaptive significance of nocturnal habits in waterfowl. Biol. Rev. 67:381-419. MINDELL, D. P. 1992. [Review of] "Phylogeny and classification of birds: a study in mo- lecular evolution." Systß Biol. 41:126-134. NELSON, C.J. 1993. The downy waterfowl of North America. Delta Station Press, Deerfield, Illinois. PALMER, R. S. 1976. Handbook of North American birds. Vol. 3. Yale Univ. Press, New Haven, Connecticut. PATTON, J. C. AND J. C. AVISE. 1985. Evolutionary genetics of birds IV: rates of protein divergence in waterfowl (Anatidae). Genetica 68:129-143. PAYNE, R. B. 1984. Sexual selection, lek and arena behavior, and sexual size dimorphism in birds. Ornithol. Monogrß 33:1-52. PETERS, J. L. 1931. Check-list of birds of the world. Vol. I. Harvard Univ. Press, Cam- bridge, Massachusetts. PHILLIPS, J. C. 1925. A natural history of the ducks. Vol. 3. Houghton Mifflin, Boston, Massachusetts. ß 1926. A natural history of the ducks. Vol. 4. Houghton Mifflin, Boston, Massa- chusetts. PYCRAI:rr, W. P. 1906. Notes on a skeleton of the Musk-Duck, Biziura lobata, with special reference to skeletal characters evolved in relation to the diving habits of this bird. J. Linnean Soc. London 29:396-407. RAHN, H., C. V. PAGANELLI, AND A. hR. 1975. Relation of avian egg weight to body weight. Auk 92:750-765. RAIKOW, R. J. 1970. Evolution of diving adaptations in the stifftail ducks. Univ. Calif. Publ. Zool. 94:1-52. ß 1971. The osteology and taxonomic position of the White-backed Duck, Thalas- sornis leuconotus. Wilson Bull. 83:270-277. ß 1973. Locomotor mechanisms in North American ducks. Wilson Bull. 85:295-307. RUES, E. C. AND Nß HILLGARTH. 1984. The breeding biology of captive Black-headed Ducks and the behaviour of their young. Condor 86:242-250. REISS, M. J. 1986. Sexual dimorphism in body size: are larger species more dimorphic? Jß Theor. Biol. 121:163-172. ROHWER, F. Cß 1988. Inter- and intraspecific relationships between egg size and clutch size in waterfowl. Auk 105:161-176. --AND S. FREEMAN. 1989. The distribution of conspecific nest parasitism in birds. Can. J. Zool. 67:239-253. SALVADORI, r. 1895. Catalogue of the Chenomorphae (Palamedeae, Phoenicopteri, Anseres), Crypturi and Ratitae in the collection of the British Museum. Longmans, London, England. Livezey ß STIFF-TAILED DUCKS 233 SANDERSON, M. J. 1989. Confidence limits on phylogenies: the bootstrap revisited. Cladis- tics 5:113-129. SAR1CH, V. M., C. W. SCHMID, AND J. MARKS. 1989. DNA hybridization as a guide to phylogenies: a critical analysis. Cladistics 5:3-32. SCHERER, S. AND T. H1LSBERG. 1982. Hybridisierung und Verwandtschaftsgrade innerhalb der Anatidae--eine systemafische und evolutionstheoretische Betrachtung. J. Ornithol. 123:357-380. --AND C. SONTAG. 1986. Zur molekularen Taxonomie und Evolution der Anatidae. Zeit. Zool. Syst. Evol.-forsch. 24:1-19. SCHONWETTER, M. 1961. Handbuch der Oologie. Part 3. Akademie Verlag, Berlin, Germany. SCOTT, D. m. AND T. H. CLUTTON-BRocK. 1989. Mating systems, parasites and plumage dimorphism in waterfowl. Behav. Ecol. Sociobiol. 26:261-273. SELANDER, R. m. 1972. Sexual selection and dimorphism in birds. Pp. 180-230 in Sexual selection and the descent of man: 1871-1971 (B.G. Campbell, ed.). Aldine Publ., Chicago, Illinois. SI}mEY, C. G. 1957. The evolutionary and taxonomic significance of sexual dimorphism and hybridization in birds. Condor 59:166-191. --AND J. E. AHLQUIST. 1990. Phylogeny and classification of birds: a study in molec- ular evolution. Yale Univ. Press, New Haven, Connecticut. --AND B. L. MONROE, JR. 1990. Distribution and taxonomy of birds of the world. Yale Univ. Press, New Haven, Connecticut. -- AND . 1993. A supplement to distribution and taxonomy of the birds of the world. Yale Univ. Press, New Haven, Connecticut. SIEGEL-CAUSEY, D. 1993. [Review of] "Distribution and taxonomy of birds of the world." Auk 109:939-944. SIEGFRIED, W. R. 1970. Double wing-moult in the Maccoa Duck. Wildfowl 21:122. 1976. Social organization in Ruddy and Maccoa ducks. Auk 93:560-570. SIGURJ(SNSD(STTIR, H. 1981. The evolution of sexual size dimorphism in gamebirds, water- fowl and raptors. Ornis Scand. 12:249-260. SMITH, m. B. AND C. PATTERSON. 1988. The influence of taxonomic method on the percep- tion of patterns of evolution. Pp. 127-216 in Evolutionary biology. Vol. 23 (M. K. Hecht and B. Wallace, eds.). Plenum Press, New York, New York. SWOFFORD, D. L. 1993. PAUP: phylogenetic analysis using parsimony. Version 3.1. Illinois Natur. Hist. Surv., Champaign, Illinois. TODD, E S. 1979. Waterfowl: ducks, geese, and swans of the world. Harcourt-Brace Jova- novich, New York, New York. TOME, M. W. AND D. m. WRUBLESKI. 1988. Underwater foraging behavior of Canvasbacks, Lesser Scaups, and Ruddy Ducks. Condor 90:168-172. TORRES E., J. A. 1984. Caractres distinctifs de deux femelles d' Oxyura leucocephala d'Espagne. Alauda 52:232-234. -- AND J. M. AYALA M. 1986. Variation du dessin cphalique des males de l'erismature fi tte blanche (Oxyura leucocephala). Alauda 54:197-206. TOWNSEND, C. W. 1909. The use of the wings and feet by diving birds. Auk 26:234-248. Trawls, R. L. 1972. Parental investment and sexual selection. Pp. 136-179 in Sexual selection and the descent of man: 1871-1971 (B.G. Campbell, ed.). Aldine Publ., Chicago, Illinois. VERHEYEN, R. 1953. Dijdrage tot de osteologie en de systematiek der Anseriformes. Gerfaut 43:373456. WELLE, M. W. 1964a. General habits. Pp. 15-34 in The waterfowl of the world. Vol. 4 (J. Delacour, ed.). Country Life, London, England. 234 THE WILSON BULLETIN ß Vol. 107, No. 2, June 1995 ß 1964b. Ecologyß Pp. 80-107 in The waterfowl of the world. Vol. 4 (J. Delacour, ed.). Country Life, London, England. 1964c. The reproductive cycle. Pp. 35-79 in The waterfowl of the world. Vol. 4 (J. Detacour, ed.). Country Life, London, England. ß 1964d. Distribution and species relationships. Pp. 108-120 in The waterfowl of the world. Vol. 4 (J. Delacour, ed.). Country Life, London, England. ß 1967. Notes on plumages and weights of the Black-headed Duck, Heteronetta atricapilla. Condor 69:133-145. ß 1968. The breeding biology of the parasitic Black-headed Duck. Living Bird 7: 169-207. WETMORE, A. 1917. On certain secondary sexual characters in the mate Ruddy Duck, Erismatura jamaicensis (Gmelin). Proc. U.S. Nat. Mus. 52:479-482. ß 1918. A note on the tracheal air-sac in the Ruddy Duck. Condor 20:19-20. ß 1926. Observations on the birds of Argentina, Paraguay, Uruguay, and Chile. U. S. Nat. Mus. Bull. 133:1-448. ß 1965. The birds of the Republic of Panama. Part 1. Smithsonian Misc. Coll. 150: 1-483. WILEY, E. O. 1981. Phylogenetics: the theory and practice of phylogenetic systematics. J. Wiley & Sons, New York, New York. WOLTERS, H. E. 1976. Die Vogelarten der Erde. Part 2. Paul Prey, Hamburg and Berlin, Germany. WOOLFENDEN, O. E. 1961. Postcranial osteology of the waterfowl. Florida State Mus. Butt. (Biol. Sci.) 6:1-129. APPENDIX CLASSIFICATION OF OXYURINI Tribe Oxyurini Delacour & Mayr, 1945.--Stiff-tailed ducks. Subtribe Heteronetteae Boetticher, 1952. Genus Heteronetta Salvadori, 1865. Heteronetta atricapilla (Metrere, 1841).Black-headed Duck. Subtribe Oxyureae Boetticher, 1942.--True stiff-tailed ducks. Supergenus Noraonyx Ridgway, 1880. Genus Noraonyx RJdgway, 1880. Noraonyx dominicus (Linnaeus, 1766).Masked Duck. Supergenus Oxyura Bonaparte, 1828.--Typical stiff-tailed ducks. Genus Oxyura Bonaparie, 1828. Subgenus Oxyura Bonaparte, 1828 (>Erismatura Bonaparte, 1832). Oxyurajamaicensis (Gmelin, 1789).--Northern Ruddy Duck. Subgenus Cerconectes Waglet, 1832. Species group "A." O)vura ferruginea (Eyton, 1838).--Peruvian Ruddy Duck. O.Evura vittata (Philippi, 1860).--Argentine Ruddy Duck. Oxvura australis (Gould, 1836).--Blue-billed Duck. Species-group "B." Ocqvura maccoa (Eyton, 1836).Maccoa Duck. Oxyura leucocephala (Scopoli, 1796).--White-headed Duck. Genus Biziura Stephens, 1824. Biziura lobata (Shaw, 1796).--Musk Duck.