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.
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).
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.
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.
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.
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."