Proposal (201) to South American Classification Committee
Resurrect
the monotypic genus Helicolestes for the Slender-billed Kite
Effect on South American CL: Slender-billed Kite and Snail
Kite are currently treated as congeners in Rostrhamus. This proposal is
for splitting them into two monotypic genera, Helicolestes for
Slender-billed Kite and Rostrhamus for Snail Kite.
Background: Temminck first described The Slender-billed
Kite in 1821 as Falco hamatus. In one early treatment, Sharpe (1874)
included it in Rostrhamus, along with the Snail Kite R.
sociabilis, and a third species based on an immature plumage of
Slender-billed Kite. (Bangs & Penard 1918) cleared up the mess by
describing adult and immature plumages of the Slender-billed Kite and erecting
a new monotypic genus for the species (Helicolestes), a treatment
followed by many authors (e.g., (Peters 1931, Hellmayr & Conover 1949,
Meyer de Schauensee 1970). The merger of Helicolestes and Rostrhamus
(with the latter taking priority) was proposed just over 40 years ago by
(Amadon 1964), and followed by most subsequent authors (e.g., Stresemann &
Amadon 1979, Thiollay 1994, Ferguson-Lees & Christie 2001), although
usually with misgivings.
Analysis: Amadon's taxonomic revision (found here:
http://digitallibrary.amnh.org/dspace/handle/2246/3352) rests on three short
paragraphs containing a qualitative analysis of specimens. After describing a
few similarities, chiefly the bill design and the adaptation to the middle toe,
Amadon pronounced that "After careful consideration, my opinion is that
the clear-cut affinity of these two species is of more importance than their
differences." In the following sections I use points of similarity and
difference between hamatus and sociabilis to arrive at the
opposite conclusion.
Morphology
At a glance, the heads of the two species are very similar in
shape and colour pattern. The most impressive similarity is the bill, but even
this feature is not the same: in hamatus "the upper mandible is
even thinner and more decurved" (Amadon 1964). Aside from their similar
coloration, Amadon (1964) also noted that they overlapped even in such a minor
adaptation as "a pectinate flange on the inner edge of the middle
claw", which they presumably use to remove slime from plumage.
Given their overlapping taste for freshwater snails, it need
hardly be pointed out that bill and toe morphology may have resulted as much
from convergence as from shared evolutionary ancestry. The same pectinate
flange is found in unrelated fish-eating birds, e.g. Ardeidae.
Balanced against these overlaps, there are striking differences in
the proportions of wing and tail: "When seen in life, Helicolestes shows
no similarity to Rostrhamus, for its short tail and broad wings make it
look like a buzzard (Buteo), while the long tail and wings of Rostrhamus
suggest a harrier (Circus)" (Haverschmidt 1959). These differences
tally with distinct flight actions. In normal flight hamatus flaps
with a rapid series of stiff-winged beats between glides, quite like a Roadside
Hawk (Buteo magnirostris). This could scarcely be more different from
the familiar rangy flight of Snail Kites, with their loose, manoeuvrable wings,
and tilting tails. In addition, hamatus regularly soars on
wide-open wings, whereas sociabilis soars much less readily, and
then with wings bowed.
The distinctive, harrier-like plumage of female and juvenile Snail
Kites is well known. Unlike adult males they are mostly brown, with streaked or
blotched underparts. By contrast, female Slender-billed Kites are similar to
males, and juveniles are similar to adults, differing only in the slight buff
barring or tipping to scapulars and banding to the tail (Ferguson-Lees &
Christie 2001).
Voice
The main call of the Snail Kite is an odd clicking
"krikik-ik-ik, ik, ik, ik" (Ferguson-Lees & Christie 2001),
versions of which are used in social interactions and alarm. Males apparently
also give a squawked "koreea; ker-wuck ker-wuck" in a variety of
contexts: at roosts, in aerial displays, and during social interactions
(Ferguson-Lees & Christie 2001). The Slender-billed Kite seems to have one
main call, which is a very different, kazoo-like mewing (Beissinger et al.
1988, Ferguson-Lees & Christie 2001), much more readily overlooked as a
Roadside Hawk (B. magnirostris) than a Snail Kite.
Behaviour
Courtship display of the Slender-billed Kite consists of "a
series of repeated swooping dives in which the bird would fold its wings and
plummet about 5 m before opening them again and rising" (Beissinger et
al. 1988). This buteonine display differs from the display of the
Snail Kite, which involves butterfly-flight elements (Ferguson-Lees &
Christie 2001).
The nest of the Snail Kite is made of twigs, reeds etc., and
usually placed less than 2 m up, non-arboreally, and always close to water
(Ferguson-Lees & Christie 2001). That of the Slender-billed Kite is
arboreal, usually placed in the canopy of swamp forest or forest near wetlands
(Beissinger et al. 1988, Ferguson-Lees & Christie 2001, Greeney et
al. 2004). Haverschmidt (1959) found one in the crown of a large Ceiba
pentandra, describing a bulky Buteo-like nest made of dead sticks.
Unlike sociabilis, nests of hamatus are "well
dispersed" (Beissinger et al. 1988).
Although both species eat large freshwater snails, their hunting
methods are different, as would be expected from their morphology. Rostrhamus
quarters like a harrier, catching snails in flight, whereas Helicolestes
sits motionless and pounces on snails from low branches.
The Snail Kite often travels, feeds and roosts in groups,
sometimes as large as 1,000 individuals, and it regularly nests in scattered colonies
of up to 100 pairs; the Slender-billed Kite is essentially solitary or
pair-living, especially when nesting (Ferguson-Lees & Christie 2001).
In conclusion, it appears that (1) the differences between the two
species are many and varied, and (2) their main similarities are associated
with specialist prey. Given the fair chance that the bill and middle toe are
homoplasies, an overall similarity in plumage and bare parts colours is not
sufficient evidence for Amadon's merger. It is not impossible that the rarer
Slender-billed Kite is mimetic, achieving some advantage via its apparent
similarity to the Snail Kite. Moreover, I can think of no other raptor genus,
worldwide, that embraces the scale of differentiation found in Rostrhamus as
currently composed. There are so many differences, including in that powerful
evolutionary clue, the voice, it would not be surprising if genetic studies
revealed that they were not each other's closest relative.
Recommendation: The key point is that the original merger was
poorly substantiated. I advocate a return to an earlier, long-established
treatment that better reflects the unique characters of these two species. I
recommend a YES vote to this proposal, which would return the Slender-billed
Kite to the monotypic genus Helicolestes.
References
Amadon, D. (1964) Taxonomic notes on birds of prey. American
Museum Novitates 2166: 1-24.
Bangs, O. & Penard, T.E. (1918) Notes on a collection of
Surinam birds. Bull. Mus. Comp. Zool. 62: 38-39.
Beissinger, S.R., Thomas, B.T. & Strahl, S.D. (1988)
Vocalisations, food habits, and nesting biology of the Slender-billed Kite with
comparisons to the Snail Kite. Wilson Bulletin 100:
604-616.
Ferguson-Lees, J. & Christie, D.A. (2001) Raptors of
the world. Christopher Helm, London.
Greeney, H.F., Gelis, R.A. & White, R. (2004) Notes on
breeding birds from an Ecuadorian lowland forest. Bull. Brit. Orn. Club 124:
28-37.
Haverschmidt, F. (1959) Notes on Helicolestes hamatus in
Surinam. Auk 76: 32-36.
Hellmayr, C.E. & Conover, B. (1949) Catalogue of birds of the
Americas and Adjacent Islands. Field Mus. Nat. Hist. Publ., Zool.
Ser., 13(pt. 1, no. 4).
Meyer de Schauensee, R. (1970) A guide to the birds of South
America. Livingston Publishing Co., Wynnewood, Pennsylvania.
Peters, J.L. (1931) Check-list of birds of the world. Harvard University
Press, Cambridge, Mass.
Stresemann, E. & Amadon, D. (1979) Order Falconiformes. Pp. in
E. Mayr and G.W. Cottrell editors), Check-list of birds of the World. Museum of
Comparative Zoology, Cambridge, Massachusetts.
Thiollay, J. (1994) Family Accipitridae (hawks and eagles). Pp.
52-105 in J. del Hoyo, A. Elliott, and J. Sargatal editors), Handbook of the
Birds of the World. Vol. 2. New World Vultures to Guineafowl. Lynx Edicions,
Barcelona.
Joe Tobias,
January 2006
___________________________________________________________________________________________
Comments from Remsen: "YES. Although no new data
per se have been published, the Amadon rationale would fall far short of sufficient
for a merger of two genera today. Regardless of whether these two birds are
really sister taxa, I would rather return to the previous classification than
follow Amadon's weakly supported opinion. After becoming familiar with Helicolestes
near Leticia, I was mystified by the post-Meyer de Schauensee merger into Rostrhamus.
If genetic data corroborate their sister relationship, I would be tempted to
vote to retain both in monotypic genera for the reasons outlined by Tobias
unless the genetic distance between them is short."
Comments from Stiles: "YES. Birds as a group are
replete with examples of impressive convergences related to similar feeding
habits - and the more socialized the habits, often the more impressive the
convergences. Hence, I am not surprised that a careful analysis of other,
non-feeding adaptations of the species point to separating them (again).
Certainly my own experience with both species convinces me that they are very
different critters!"
Comments from Zimmer: "YES. Despite the lack of
any new published analysis, I vote YES because it overturns a poorly supported
revision by Amadon. Structural, vocal and ecological differences (along with
lack of sexual dimorphism in plumage characters in Slender-billed Kite) all
support the conclusion that these are two very different beasts, and that
convergence of plumage characters and bill morphology are exactly that."
Comments from Robbins: "YES. Although no new
published data are presented in this proposal the original rationale for generically
merging these two is weak and the reinterpretation is enough to persuade me to
vote "yes". Rob Fleischer (pers. comm.) has a preliminary molecular
data set that has both of these represented. Hopefully, we see this in print
soon."
Comments from Silva: "YES, let us be back to the
original taxonomic situation before Amadon. However, we should tale in
consideration Van's viewpoint about molecular data. If these species are
monophyletic, I will vote for combining them into a single genus."
Comments from Pacheco: "YES. Em minha experiência, com ambos os táxons, também
considero bastante plausível a sugestão de convergência morfológica em lugar de
genuíno parentesco."
Comments from Nores: "NO. Aunque los fundamentos dados por Tobias son bien fundamentados, pienso
que no son suficientes cuando las diferencias son a nivel de género. Existen
muchos géneros en los cuales las especies tienen diferentes comportamientos,
canto, silueta, dimorfismo sexual, etc. Así, por ejemplo, Crotophaga, Caprimulgus,
Melanerpes, Upucerthia, Thamnophilus, Saltator tienen diferentes tipos de
cantos entre las especies. También hay muchos géneros de aves con diferente
proporción de alas y cola entre las especies. Por ejemplo, Buteo
(compare leucorrhous con brachyurus), Zenaida, Tyrannus, etc.
Yo no veo que Rostrhamus se parezca mucho a Circus; compare
en lo guía de aves argentinas de Narosky e Yzurieta la silueta de Rostrhamus
con la de Circus buffoni (ambas son tomadas de fotografías). También
en muchos géneros hay diferencias entre las especies en relación al color de
macho y hembra. En Molothrus bonariensis por ejemplo el macho es
negro violáceo y la hembra gris y el joven es similar a la hembra, mientras que
en M. rufoaxillaris ambos sexos son negros y el joven es similar a M.
(Agelaioides) badius. Otros ejemplos serían Anas, Knipolegus,
Progne, etc. Con más razón en comportamiento, tipo de nido y forma de
alimentarse. Por citar sólo un ejemplo de cada cosa, menciono el caso de Mimus
saturninus que durante el display hace círculos completos en el aire, cosa
que no hacen las otras especies; Anas flavirostris nidifica en huecos de
rocas o en nidos de cotorra (Myiopsitta monachus) veces a gran altura
mientras que la mayoría de los otros patos lo hacen en el suelo y Falco
peregrinus captura aves en vuelo, mientras que Falco sparverius captura
grandes insectos, roedores y lagartos en el suelo."
Comments from Jaramillo: "YES - If genetic data
clarifies that these are sister taxa, I would still feel comfortable retaining
two genera for these very different species."