THEWILSONBULLETINA QUARTERLYMAGAZINE

OF ORNITHOLOGY

Published by the Wilson Ornithological SocietyVOL. 104, No. 4

DECEMBER 1992

PAGES 571-816

Wilson Bull., 104(4), 1992, pp. 571-579

THE TAXONOMIC
STATUS OF THE
WHITE-TAILEDKITEWILLIAM S. CLARK’ AND RICHARD C. BANKS~
AssraAcr.-The
White-tailed Kite (Elanus leucurus)of the Americas has been mergedwith the Black-shouldered (or Black-winged) Kite (E. caeruleus)of the Old World and theAustralian Black-shouldered Kite (E. axillaris) by North American authorities (but notelsewhere), primarily because of similarity in plumage. However, American kites differ from
Old World kites in greater size and weight, in proportions (relatively longer tail and smaller
bill and feet), plumage pattern (particularly of juveniles), and in behavior. Here we argue
that these characters are sufficiently distinctive to warrant recognition of E. leucurusat the
species level. Received19 May 1992. acceptedI Sept. 1992.

The White-tailed Rite (Elanus leucurus)of the Americas was long considered a species distinct from the Black-shouldered (or Black-winged)
Rite (E. caeruleus)of the Old World, and the Australian Black-shouldered
Rite (E. axillaris). We follow Schodde and Mason (1980) and McAllan
and Bruce (1989) in the use of axillaris rather than notatus as the namefor the Australian species. However, Parkes (1958) and Husain (1959)
suggested that both leucurusand axillaris should be considered conspecificwith caeruleus,and that the complex forms a superspecies with the Letterwinged Kite, E. scriptus, of Australia. Arguments for this merger werebased primarily on the similarity of plumage. Parkes (1958) stated that,for any plumage character, forms of the combined species E. caeruleus
could be arranged in a progressively intergrading series. Husain (1959)
postulated a biogeographical scheme to account for the distribution of
the forms, considering only the amount of black on the underwing.
The American Ornithologists’ Union (AOU 1983), Palmer (1988), andAmerican field guides based on AOU (1983) follow Parkes (1958) andHusain (1959) and treat the American forms under the combined name’ 4554 Shetland Green Rd., Alexandria, Virginia 223 12.
2 U.S. Fish and Wildlife Service,National Museum of Natural History, Washington, DC. 20560.
571

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Vd. 104, No. 4, December 199.2

caeruleus. On the other hand, all authorities outside of and many from
North America have continued to recognize E. leucurusand E. axillaris
as species distinct from, but closely related to, E. caeruleus(Vaurie 1965,
Brown and Amadon 1968, Mayr and Short 1970, Glutz von Blotzheim
197 1, Condon 1975, Stresemann and Amadon 1979, Cramp and Simmons 1980, Brown et al. 1982, Amadon and Bull 1988, Short et al. 1990,
and Sibley and Monroe 1990). The three taxa are often treated as allospeties.
Here we discuss differences in size, proportions, plumage, and behavior
that lead us to the conclusion that Elanus leucurusof the Americas shouldbe treated as a species distinct from caeruleusand its races. We believethat axillaris should similarly be considered distinct, although our study
was directed less toward that form and we have no experience with it in
the field. Mees (1982) suggested that several other recognized subspeciesof caeruleusshould be combined with E. c. hypoleucusand recognized at
the same level as caeruleus,leucurus,and axillaris.
METHODS
The senior author has observed both E. 1eucuru.sand E. caeruleus in the field for many
hours, leucurus in California, Texas, Mexico, Guatemala, and Panama, and caeruleus in
Senegal, Kenya, Spain, and India. His notes on behavior and appearance are supplemented
by information in the literature.
Plumage characters were examined on and measurements of wing chord, tail length,
culmen from cere, and hallux length were taken from specimens in the American Museumof Natural History, the British Museum (Natural History), and the U.S. National Museumof Natural History. Additional sets of measurement data were obtained from the literature,
as was information on weight.
RESULTSWhite-tailed Kites (E. Eeucurus)
differ from Black-shouldered Kites (E.
caeruleus)in size and especially in proportions, in characteristics of theplumage, especially in young birds, and in hunting behavior and flight.
Australian Black-shouldered Kites (E. axillaris) are similar to caeruleus
in proportions but resemble one or the other of the Old and New World
forms in plumage characters and behavior.
Size and proportions.-White-tailed Kites are considerably larger thanBlack-shouldered Kites (Table 1; Fig. l), particularly in length of wing
and tail. In all forms of the genus other than E. scriptusthe sexes are
similar in size and samples have been combined by many authors; femalesof scriptusare larger than males (Table 1; Brown and Amadon 1968). We
attribute differences in means in samples of Zeucurusto differences inmethod of measurement, as several of the samples may have included
the same individual specimens (Palmer 1988: 135). As noted by Bangs

Clark and Bunks TAXONOMIC

STATUSl

TABLE

OF WHITE-TAILED

KITE

573

1

MESWJREMENTS(MM) FROM POPULATIONSIN THE GENUS ELANUS
Wing
N sex=

Range

Tail
*

Culmen

N

Range

R

N

Range

*

SOUICeb

1025
14

170-181170-188174-186

177
179
181

18.0-19.1
16.5-2 1.O
18-19

184
183

9
25
14
19
15
9
7

18.7
19.2
18.7
19.1
19.5
18.5
19.2

1
2
3
4
4
5
5

157-169
149-l 77

162
162

10
14

16.6-17.9
16.5-19.5

17.4
17.7

1
2

E. 1. majusculus,North America
10
25
14
20 m
15f
9m
7f

300-324300-325302-328

309
316
314
316
318
309
307

9
7

E. 1.leucurus,South America
1014288-304290-310

295
301

10
14

E. c. caeruleus,Europe and Africa
13 m
13 f

249-292262-297

271
273

18
13

108-136108-127

118
118

17
15

15.8-17.9
15.9-18.2

17.1
17.2

6
6

260

7

116-127120

715.2-17.0

16.2

1

9

113-130123915.2-17.9

16.7

1

294
294

12
13

132-146130-146

137
138

12
13

17.8-19.0
18.5-20.3

7
7

298
302

18
18

132-150131-151

140
142

18
18

18.8-19.8
19.0-20.8

77

E. c. caeruleus,Africa
7248-272

E. c. vociferus,Southern Asia
9248-278264

E. c. sumatranus,Sumatra
11 m
13 f

290-303285-305

E. c. hypoleucus,Java
16 m
16 f

292-310294-310

E. c. wahgiensis,New Guinea
8

297

-

E. axillaris, Australia
6291-2982941;

280-310280-302

298
294

-

136-142
142-153
142-154-

146-150
156-162

6

139

1
8
8

E. scriptus,Australia-m-f292-296302-3 138
8

* Sexes combined unless indicated (m for male, f for female).
b Sources: 1, this study; 2, Bangs and Pennard (I 920); 3, Friedman(1950); 4, Snyder and Wiley (1976); 5, Hwbecker(1942); 6, Cramp and Simmons (1980); 7, Mess (1982); 8, Brown and Amadon (1968).

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lVol. 104, No. 4. DecemberI992_--

FIG. I. Two specimens of E. caeruleus (left) and two ofE. leucurus(right)showing dorsal(above) and ventral (below) surfaces. Note differences in size, relative lengths of tails, andback color.

Clark and Banksl

TAXONOMICSTATUSTABLE
WING/TAIL

OF WHITE-TAILED

KITE

575

2

RATIOS OF SEVERALPOPULATIONS OF ELANW (BASED ON DATA IN TABLE 1)
Sue&s

Elanus leucurus
E. IeucurusE. caeruleus
E. caeruleus
E. “hypoleucus”
E. axillaris
E. scriptus

PODUlZltiOll

North America
South America
AfricaAsia
Sumatra, Java
Australia
Australia

Ran@ of ratio

1.70-1.80
1.75-1.86
2.05-2.28
2.05-2.28
2.14a
2.06-2.14
1.93-1.99

a From data in Mees (1982).

and Pennard (1920), North American White-tailed Kites, E. 1.mujusculus,
are somewhat larger than South American birds, E. 1. Zeucurus.
The latterare nevertheless notably larger than Old World caeruleusand Australianuxillaris. The populations constituting hypoleucus of Mees (1982) are close
to the size of American birds in wing chord measurements but have much
shorter tails (Table 1).
The difference in tail length between leucurus and caeruleusis greaterthan the difference in wing length (Table 1). This difference in proportions
may be best appreciated from calculation of the wing/tail ratio. In all
American birds, this ratio is less than 2.0; in Old World birds and in
those of Australia it is invariably more than 2.0 (Table 2). The ratio isintermediate in E. scriptus. This proportional difference is particularly
noticeable on perched birds of both forms. On perched leucurusthe wingtips do not extend to the tail tip, whereas on perched caeruleusthey project
far beyond the tail tip (Frontispiece).
Lengths of the culmen (Table 1) and hallux (data not shown; sample
means 15.7-l 6.5 mm) are similar throughout the complex and cannot be
used to separate Ieucurusand caeruleus.However, as caeruleusis smaller,
the bill and feet are proportionally larger; in particular, caeruleusappears
larger headed than does Zeucurus(Frontispiece).
Mendelsohn and Jaksic (1989) summarized weight data, and showedthat American leucurusaverages 50-65 g heavier than African caeruleus;
Australian axilluris is intermediate, but is nearer to South American leu-

curus.
Data summarized by Mendelsohn and Jaksic (1989) indicate that thelength of the secondaries in Californian E. 1. majusculusand African E.
cueruleusis similar (158 vs 146 mm, respectively), but as leucurusis larger

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VoZ. 104, No. 4, December 1992

and longer winged, the wing shapes of the two are different. This and thedifferences in relative tail length result in quite different shapes of flying
White-tailed Rites and Black-shouldered Rites. According to Mendelsohn
and Jaksic (1989), Zeucurus has both a higher aspect ratio and a muchgreater wing area than does caeruleus (100 1 vs 843 cm*).
Plumage. -E. leucurusand E. axillaris have a patch of black underwing
coverts at the base of the primaries. This patch is lacking in E. caeruleus,
although some individuals of that species have a few coverts with dusky
tips (Husain 1959). The lower surface of the primaries is dark in leucurus,
axilluris, and most caeruleus.The primaries are nearly white below, witha variable amount of gray on the wing tip, in E. c. hypoleucusand E. c.
sumatranus(Salomonsen 1953) and apparently in E. c. wahgiensis(Mayrand Gilliard 1954).
The outer five pairs of rectrices of young leucurus are white, with avariable amount of pale gray on the distal portion of the outer webs. On
all but the outer pair the gray extends onto the inner web forming acontinuous subterminal tail band. The central rectrices are entirely grayexcept for a narrow white tip. The gray on the rectrices is darker andseldom, if ever, forms a band across the tip of the tail in young caeruleus,
usually being restricted to the outer webs. In E. c. hypoleucusand in E.
axilluris, the gray on the tail is much reduced and except for the central
rectrices the tail is essentially white.
Behavior.-White-tailed Rites hunt almost exclusively by hovering;
Jaksic et al. (1987) reported seeing only four instances of hunting fromperches during observations of 80 hover-hunts in Chile. Australian axilluris, like leucurus,hunts mostly by hovering (R. Schodde, pers. comm.).
After reviewing the literature, Mendelsohn and Jaksic (1989) could reportno observations ofperch-hunting from North America. On the other hand,
Old World Black-shouldered Rites hunt regularly from perches (Mendelsohn and Jaksic 1989), hunting from the hover only about 30% of thetime (Tarboton 1978), although the proportional number of hunting events(as opposed to time spent) is not given. Clark and P. Bloom were able tocapture perched Black-shouldered Rites in India easily using bal-chatri
traps with mice. Bloom (pers. comm.) was able to capture only one Whitetailed Rite in California using the same trap with mice out of more than
200 attempts. The one caught was undernourished and was captured
outside of the breeding range of the kite.
Differences in hunting methods may be related to differences in preyselection. The White-tail is a rodent specialist, whereas the Black-shoulder, at least in some areas, takes a wider selection of prey including birds
and lizards (Cramp and Simmons 1980, Palmer 1988: 147). White-tails
catch prey twice as quickly as do Black-shoulders from hovers (Mendelsohn and Jaksic 1989).

Clark and Bunksl

TAXONOMICSTATUSOF WHITE-TAILED

KITE

577

Black-shoulders often cock the tail up over their backs and then down
(tail-bobbing of Palmer 1988: 145); Tarboton (1978) reported tail-cocking
during 14% of perch-hunting time, when intensely excited just before orafter a strike. Although Steyn (1963) had previously linked tail-cocking
to excitement, Mendelsohn and Jaksic (1989) suggested that it served towarn other birds away from the hunting area. Goriup (198 1) reported
tail-cocking during preening in Portugal. This behavior is seldom reported
for White-tails (Brooks 1943). Despite many hours of observation, WSC
has observed only one instance of tail-bobbing in American kites, by an
adult female in California being harassed by her begging young.
Black-shouldered Rites often perch with their folded wings drooped
below the tail whereas White-tailed Rites usually perch with the wing tips
folded over the tail (Frontispiece), but both can hold their wings either
way. A perched Australian Black-shouldered Kite is illustrated with thewings folded over the tail (Macdonald 1973) the usual posture for thatspecies (R. Schodde, pers. comm.).
White-tailed Rites are somewhat communal and nest in fairly close
proximity to one another and defend territories weakly. Watson (1940)
reported some fighting by White-tails in the establishment of territories
but noted that pairs often breed communally when prey are abundant.
Peyton (19 15) found two nests within 200 yards (184 m), and Pickwell(1930) reported three nests at points of a triangle measuring 320, 200,
and 175 yards (294, 184, and 161 m). Hawbecker (1942:269) reported
two pairs nesting within 400 yards (368 m) of one another, with only one
aggressive encounter. Dixon et al. (1957) reported three nests within aradius of 500 feet (153 m), with two others in adjoining groves, and saw
no territorial fighting. On the other hand, Black-shouldered Rites arestrong in their territorial defense; Mendelsohn (1983) reported that South
African birds defended their territories vigorously and persistently, where
average territory size ranged from about 2.4 to about 4.5 km*. However,
Madden (1977) found two nests in South Africa that were approximately
200 m apart.
DISCUSSION

The inclusion of leucuruswith caeruleusby the AOU was based mainly
on Parkes (1958), and to a lesser extent on Husain (1959). Parkes (1958)
made two points to support this merger. First, that one could arrange
specimens of both forms progressively in an intergrading series and second, that these forms resemble each other more than do subspecies ofsome other raptors, specifically Accipiter gentilis, A. striatus, and Falco

peregrinus.
We have found that E. leucuruscan always be separated from E. caeruleus by the presence of black carpal patches and by a wing/tail ratio less

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lVol. 104, No. 4, December1992

than 2.0; caeruleuslacks the black carpal patch and has a wing/tail ratio
greater than 2.0.
Clark has observed in the field on different continents two or moreraces of 12 species of diurnal raptors, including Accipitergentilisand Falco
peregrinus.Although plumage can vary greatly between races of some ofthese species, none show the differences in behavior and overall proportions exhibited between leucurusand caeruleusas presented above.
We believe that the characters of the American birds are sufficiently
distinctive to warrant recognition of the White-tailed Kite E. leucurusat
the species level.
ACKNOWLEDGMENTS
We thank the curatorial staffs of the American Museum of Natural History and the British
Museum (Natural History) for allowing access to those collections. We thank M. R. Browning, F. Gill, F. M. Jaksic, J. M. Mendelsohn, S. L. Olson, K. C. Parkes, and R. Schodde
for comments on the developing manuscript. Clark’s field work in India was sponsored by
the Office of International Affairs of the U.S. Fish and Wildlife Service and the Bombay
Natural History Society.
LITERATURE

CITED

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AMERICAN ORNITHOLOGISTS’UNION. 1983. Check-list of birds of North America. 6th ed.
Amer. Omith. Union, Washington, D.C.
BANGS, 0. AND T. E. PENNARD. 1920. Two new American hawks. Proc. New England
Zool. Club 7:45-47.
BROOKS,A. 1943. An unusual attitude in a hawk. Condor 45: 119.
BROWN, L. AND D. AMAD~N. 1968. Eagles, hawks and falcons of the world. Vol. 1. McGraw
Hill, New York, New York.
BROWN, L. H., E. K. URBAN, AND K. NEWIVIAN. 1982. The birds of Africa. Vol. 1. Academic
Press, London, England.
CONDON, H. T. 1975. Checklist of the birds of Australia, Part 1. Non-passerines. Royal
Australian Ornithologists Union, Melbourne, Australia.
CRAMP, S. AND K. E. L. SIMMONS (EDs.). 1980. Birds of the Western Palearctic. Vol. 2.
Oxford Univ. Press, Oxford, England.
DIXON, J. B., R. E. DIXON, AND J. E. DIXON. 1957. Natural history of the White-tailed
Kite in San Diego County, California. Condor 59: 156-l 65.
FRIEDMANN, H. 1950. The birds of North and Middle America. Part 11. U.S. Natl. Mus.
Bull. 50, Washington, D.C.
GLUTZ VON BLOTZHEIM, U. N. (ED.). 1971. Handbuch der Vogel Mitteleuropas. Band 4.
Academissche Verlag., Frankfurt, Germany.
GORIUP, P. D. 198 1. Observations on a pair of Black-winged Kites (Elanus cueruleus) ineastern Portugal. Arq. Mus. Bocage, ser. B, 1:65-79.
HAWBECKER, A. C. 1942. A life history study of the White-tailed Kite. Condor 44:267276.
HUSAIN, K. Z. 1959. Notes on the taxonomy and zoogeography of the genus Elanus.
Condor 61:153-154.

Clark and Bunks TAXONOMICl

STATUSOF WHITE-TAILED

KITE

579

JAKSIC, F. M., R. Rozzt, A. LABRA, AND J. E. JIMENEZ. 1987. The hunting behavior ofBlack-shouldered Kites (Elanus caeruleus1eucuru.s)
in central Chile. Condor 89:907-g 11.
MACDONALD, J. D. 1973. Birds of Australia. H. F. and G. Witherby, London, England.
MADDEN, S. T. 1977. Notes on two nests of the Blackshouldered Kite. Ostrich 48:115116.
MAYR, E. AND E. T. GILL-.
1954. Birds of central New Guinea. Bull. Amer. Mus. Nat.
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Zool. Verh. 191.
MENDELSOHN, J. M. 1983. Social behavior and dispersion of the Blackshouldered Kite.
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SNYDER, N. F. R. AND J. W. WILEY. 1976. Sexual size dimorphism in hawks and owls ofNorth America. Om. Monogr. 20.
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STRESEMANN,E. AND D. AMADON. 1979. Order Falconiformes. Pp. 271-425 in Check-listof birds of the world, Vol. 1, 2nd ed. (E. Mayr and G. W. Cottrell, eds.) Mus. Comp.
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WATSON, F. G. 1940. A behavior study of the White-tailed Kite. Condor 42:295-304.

COLOR PLATEThe frontispiece photographs by W. S. Clark and B. K. Wheeler have been made possible
by an endowment established by George Miksch Sutton.