ENV: On Birds and Molluscs Part I
This is part one of a three part post on interrelationships between birds and molluscs -- specifically raptors of the kite group and snails in the genera Pomacea and Rabdotus. I have been trying to collect some info and thoughts on this for a while, and am spurred to finally post because of the impending deadline of my favorite carnivals I and the Bird and Circus of the Spineless. Part one will focus on the kites; part two on the snails at issue; part three on the broader topic of birds and molluscs.
An article posted in Birdwatcher’s Digest recently points to a decline in numbers of the Snail Kite (also known as the Everglades Kite – a specific genetically isolated population; photo of a female of this race, Rostrhamus sociabilis plumbeus, by Pat Lamond can be seen here) in Florida:
The U.S. Army Corps of Engineers is keeping water levels in Florida's Lake Okeechobee too high, according to a suit filed last month by the National Wildlife Federation and its Florida affiliate. [The lawsuit can be accessed directly here.]
Corps policy is reportedly impacting the federally endangered snail kite by flooding many of the lake's marshes and reducing the number of apple snails available for the kites to eat. Conservationists argue that Lake Okeechobee should be kept between 12 and 15.5 feet above sea level, as recommended by state wildlife scientists, instead of the 13.5 to 17 feet, which is the current Corps policy.
The article say that Snail Kite numbers have declined by over half, from 3,500 (1999) to 1,610 ( 2003). The suit argues that the sugar can and other local agricultural industries require the additional water levels currently being maintained.
I want to visit the issue of Snail Kites, Rostrhamus sociabilis, and Hook-billed Kites, Chondrohierax uncinatus, and their differing food preferences across their ranges versus localized fringe populations in this first of a three-part series on the relationship between birds and molluscs.
The discussion will include notes on changes in the populations of snails that the two birds are dependent on in certain areas. That portion of the discussion will take place in a second post focusing on the snails.
[Further advance reading on some of this can be done in a previous post on milkriverblog that discussed possible declines in the south Texas snail Rabdotus alternatus, Roadrunner and Hook-billed Kite predation, and a now suspended search for answers.]
The Snail Kite, as it is familiar to American birders, is a denizen of the shallow waterways of central Florida through the Everglades. Lake Okeechobee is the location most educated birders would typically associate with the bird, although various other locations host “seeable” birds. Until the 1970s the bird was Americacentrically known as the Everglades Kite because of its distribution. The bird, however, is a well-known denizen of Cuba, and the Caribbean lowlands, through parts of South America into the Pampas. Indeed, it is a much more common and widespread bird than the name Everglades Kite would begin to suggest, and the name is now restricted to the Florida/Caribbean race of the bird.
From the U.S. Fish and Wildlife Service, which lists the bird as Endangered:
World species found primarily in lowland freshwater marshes in tropical and subtropical America. Four subspecies have been recognized: (1) R. s. plumbeus Ridgway, in Florida; (2) R. s. levis Friedmann, in Cuba; (3) R. s. major Nelson and Goldman, in Mexico, Belize, and Guatemala; and (4) R. s. sociabilis (Vieillot), in Central and South America. For descriptions of these see Friedmann. The plumages of the subspecies are the same, and bill, wing, and tarsus measurements overlap considerably. Recently Amadon reexamined the taxonomic status of the four subspecies and concluded that plumbeus and levis are not distinct. He synonymized the subspecies levis with the subspecies plumbeus, with which there is some agreement. As size is the only character distinguishing the subspecies and the measurements exhibit overlap, the separation of this species into subspecies is still open to question. The species was identified as Rosthramus sociabilis in the Western Hemisphere Convention Annex under Cuba's listing in 1941. The common name for the Everglade snail kite was officially changed to snail kite in 1982 by the American Ornithologist's Union. However, it is also known by the common names Everglade kite or Everglades kite (ISIS), Florida everglade kite (IUCN, 1979), snail hawk, and Gavilan caracolero (Spanish-Cuba).
The kite hunts mainly over extensive, shallow (water depth 0.2 - 1.3 m) sloughs of white waterlily (Nymphaea odorata) and wet prairies or flats of spikerush (Eleocharis elongata and E. cellulosa) that retain some surface water through the dry season in most years. Such areas occur in extensive stands of sawgrass (Cladium jamaicensis) or cattails (Typha domingensis and T. angustifolia) or on flat river courses and margins of large shallow lakes. Approximately 97% of foraging bouts are over aquatic sloughs and wet prairies. Other flooded freshwater marsh habitats used for hunting include: Shallow littoral zones of lakes and ponds, old alligator holes, shallow river margins with aquatic vegetation, canals, and ditches. Kites in Florida, with a few rare exceptions, are completely dependent upon the apple snail (Pomacea paludosa) for food. Snails are available to kites only when the marsh is flooded. Snails are captured while they are near the water surface feeding, traversing, or while resting on aquatic vegetation below the surface. Kites hunt over relatively open water areas, containing minimal emergent aquatic vegetation. Kites hunt by two methods; still-hunting (initiated from a convenient perch over water) and course-hunting (on the wing).
And, somewhat redundantly:
The snail kite is a specialist whose diet consists of freshwater Pomacea snails. With a few rare exceptions, kites in Florida are completely dependent upon the apple snail (Pomacea paludosa) for food . The birds hunt in a number of flooded freshwater marsh habitats: Aquatic sloughs, wet prairies, shallow littoral zones of lakes and ponds, old alligator holes, shallow river margins with aquatic vegetation, canals, and ditches. Approximately 97% of the foraging bouts are over aquatic sloughs and wet prairies. Snails are captured while they are near the water surface feeding, traversing, or while resting on aquatic vegetation below the surface. Kites hunt by two methods; still-hunting (initiated from a convenient perch over water) and course-hunting (on the wing). In both methods snails are taken by the bird while hovering or flying very slowly above the water surface. Snails are captured with the talons, while seldom getting the feathers of the wings, tail, or body wet in the process (kites do not plunge to capture prey as do ospreys and eagles). Kites hunt over relatively open water areas, containing minimal emergent aquatic vegetation. The captured snail is taken to a perch where the operculum is removed and discarded, the soft tissue of the snail extracted, the shell dropped, and the soft parts then eaten. Kites always avoid eating the albumen glands of female snails and often discard all or part of the viscera. Adults feed their young extracted snails. Kites hunt throughout the day with intermittent periods of individual inactivity (particularly between 10 am and 3 pm).
Outside of Florida, the bird has occurred twice to our knowledge in the United States – both times in Texas. A young bird showed up near Alice, Texas from 22-26 July 1977 and was seen by hundreds of birders in its almost-a-week of presence. The finder of that bird, Dr. Richard O. Albert reported it again in 1978, but it did not linger and was not documented that time around. The bird had caused a huge stir in 1977, and apparently there was minimal interest when it returned.
A second bird, an adult female, showed up 17-29 May 1998 near Bentsen-Rio Grande Valley State Park when it was captured on film by photographers Bob Rozinski and Wendy Shattil (pictures of that bird can be seen here). That well-photographed bird is Texas’ second documented occurrence.
While these two might seem insignificant records considering the couple thousand that make their home in Florida, and the thousands more present in the marshes of Veracruz around the gulf coast of the Yucatan Peninsula, they provided an important glimpse into the livelihood of the overall population.
The key is that neither of these birds had apple snails available to them as a food source. Florida Apple Snails, Pomacea paludosa, long thought to be essentially the exclusive food source for Everglades Kites (and taking the role of the actual culprit for the declines above, tied to the maintenance of water levels), are limited in range to the Everglades and Florida (into Georgia) coastal swamps themselves (except in areas where they recently may have been introduced and gone feral; here is a picture by Erik Breden of an “Everglades” Snail Kite eating a Florida Apple Snail on the Tamiami Trail, November 1979). Several other species of Pomacea occur throughout the range of the Snail Kite, but only certain species, or certain size groups of species are usable by the kites.
There are now several anecdotal reports of Everglades Kites consuming other prey in Florida, but the fact that they are publishable accounts is indicative of the rarity of this occurrence.
Among the other predators of Florida Apple Snails are the Limpkin, Aramus guarauna, a large marsh bird almost exclusively limited to Florida in the U.S. and whose range is somewhat delimited by the presence of the snails as well.
Bruce Miller and Ronald Tilson (1985) report that Snail Kites were found to steal snails from Limpkins in Belize, perhaps in response to diminishing populations of the molluscs. Without an extended resource the birds are left to compete for what’s available, in this case a primary prey species, and conjecturally resort to theft rather than displace to a secondary food source.
Notes from the Hawk Conservancy Trust:
Members of the genus Rostrhamus are medium-sized kites with broad wings and a short tail that is only very slightly forked. The bill is slender and the upper mandible is elongated and sharply curved to aid extraction of snails. The talons are long and sharply pointed, but have little curvature. This is a very specialised genus, and has no obvious close relations. It comprises two species in the tropical and sub-tropical areas of the Americas.
Snail Kite or Everglade Kite (Rostrhamus sociabilis) is found in freshwater marshes of the lowlands of southern Florida and from eastern Mexico south throughout Central and South America to the pampas of Uruguay and Argentina. There are also populations on Cuba and on the Isle of Pines.
Food Habits: Entirely freshwater snails of genus Pomacea. Two hundred or more snail shells may accumulate under a feeding-post.
The Texas birds, without recourse to Florida Apple Snails, and in those particular locations without recourse to any sizable snails at all, still had to find food and chose crayfish. Both birds were documented several times feeding on crayfish, and on nothing else (meaning that no one saw them using anything else – perhaps they did but weren’t seen; a picture of the 1998 bird eating a crayfish can be seen here).
All the native freshwater snails of south Texas are tiny – less than an eighth-inch in length – and would probably not even get stuck on a kite’s hooked billtip much less provide any food (at least as far as the tradeoff between energy expended in finding and extracting the snail versus its food value). Florida kites under drought stress have been known to utilize the smaller snail Viviparus georgianus until conditions revert to normal -- a period of weeks, but even Viviparus is much larger than the native Texas Physa and Lymnaea.
A non-native snail, Melanoides tuberculata, is known from the valley, although not documented near Alice, and is larger at up to an inch and a quarter, but is an elongate mitre-shaped shell which holds a tiny animal, hardly a meal for a kite. The known valley location is about 30 miles from the location where the Snail Kite showed up.
In a flipside situation Hook-billed Kites are invertebrate specialists throughout the bulk of their range (in which, to be straightforward, there may actually be more than one species involved). However, a small population of these birds has been in the Lower Rio Grande Valley since at least the mid-1970s (although the first record was a nest discovered in 1964; see Fleetwood and Hamilton 1967), and they are apparently limited in food choice to snails of the genus Rabdotus.
The group ebbs in size now and then, but a small population remains in the vicinities of Santa Ana National Wildlife Refuge, Bentsen-Rio Grande Valley State Park, and perhaps other non-public sections of the Lower Rio Grande Valley National Wildlife Refuge System, and through the thorn woodlands into Starr County. (see photos of the only dark morph ever seen in the U.S. in photos by Jim Culberson from Bentsen-RGV State Park on 10 December 1998 – here; and photos of the more usual phases from Bentsen-RGV by John Arvin are here; and by Mike Danzenbaker here; and from Chapeño by Steve Metz here).
In the valley they likely limit themselves to the snail species Rabdotus alternatus, though Rabdotus dealbatus is also present. The only other terrestrial snail that approaches the size of these (up to one and three-quarter inches) in the valley is the Texas Wolfsnail, Euglandina texasiana, itself a predator of land snails, and consequently limited in numbers such that it is not a feasible prey item, though opportunistic use of them may occur.
The beaks of both these species of raptors are adapted tightly to feeding on these snails. While they must break off portions of the shell in order to reach the “foot” the long hooked extension of the upper beaks is notably adept at prying the meat from the remaining shell, and indeed the length of the bill allows its use as a fulcrum in the tightly adapted feeding behavior of the Hook-billed Kite.
This apparently does not keep the birds from feeding on other invertebrates – as evidenced by Snail Kites eating crayfish, and Hook-billed Kites feeding on the occasional arthropod and small slimy vertebrate – but it may limit their choice of molluscs to those species found within their traditional range, and that limited choice of mollusc in turn may prevent the birds’ spread beyond the fringes of their currently known ranges.
In fact, Hook-billed Kites, which are polymorphic for bill size, have only small billed morphs in the northern fringe, including Texas, where only Rabdotus is available. In areas of their range where there is a diversity of snails, the species may have a range of bill sizes which, in addition to expanding the food source, may also limit its impact on a given species -- a subject we'll return to in Part II.
A compendium of the races known, from the Hawk Conservancy Trust site:
Chondrohierax is less specialised than the Snail Kites of the genus Rostrhamus, as it also eats frogs, salamanders, insects, and caterpillars, as well as snails.
Chondrohierax uncinatus uncinatus occurs in of all the range except Mexico and West Indies. In addition to phases and sexual differences, there is much individual and age variation.
Chondrohierax uncinatus aquilonis of Mexico. In its normal phase it is slightly darker above, and more broadly barred below.
Chondrohierax uncinatus mirus of Grenada Island is rare and little known. It is a small race, with some slight colour differences also, the male lacking the greyish edges or tawny ventral barring; the female having tawny edgings above, and being more tawny below.
Chondrohierax uncinatus wilsonii. This race occurs locally in eastern Cuba, wherever its food, a land-snail, Polymita picta, is available. It is a small variant, with an entirely yellowish bill.
. . . a separate species, Chondrohierax megarhynchus, was once recognised for . . . big-billed birds. It is now believed that they are only individual variants. Although most frequent in Amazonian Peru and Ecuador, this type occurs elsewhere as well and intermediates are not uncommon.
The Cuban race is especially interesting for its dependence on another species of snail, Polymita picta, also known as the Painted Snail or the Coffee Snail. This mollusc is globular as opposed to the conically shaped Rabdotus.
Russell Thorstrom of The Peregrine Fund adds this note:
The Grenada subspecies feeds on small arboreal snails like the rest of the subspecies and not on large terrestrial snails. I recorded nearly 200 prey items at one nest with a nestling 3 weeks of age. Both male and female fed with the female having a higher prey delivery rate.Hook-billed Kites have been proposed as the culprit in a perceived decline of Rabdotus sp. in south Texas (see my previous correspondence and conjecture here), and Snail Kites may be experiencing trouble beyond the maintenance of water levels because of the spread of introduced snails in Florida and the twin calamities of genetic swamping and ecological displacement.
Those stories coming up in Part II when i detail some natural and unnatural history about the snails involved.
Selected Bibliogrpahy and References Cited
Amadon, D. 1960. Notes on the genus Chondrohierax. Novedades Colombianas, 1: 237-238.
Amadon, D. 1975. Variation in the Everglade kite. Auk 92:380-382.
Beissinger, S. R. 1983. Hunting behavior, prey selection, and energetics of snail kite in Guyana: consumer choice by a specialist. Auk 100:84-92.
Beissinger, S. R. 1986. Demography, environmental uncertainty, and the evolution of mate desertion in the snail kite. Ecology 67:1445-1459.
Beissinger, S. R. 1988. Snail kite. Pages 148-165 in R. S. Palmer, eds. Handbook of North American birds, vol. 4, Yale University Press, New Haven, Connecticut.
Beissinger, S. R., A. Sprunt IV, and R. Chandler. 1983. Notes on the snail (Everglade) Kite in Cuba. American Birds 37:262-265.
Beissinger, S. R., and J. E. Takekawa. 1983. Habitat use and dispersal by snail kites in Florida during drought conditions. Florida Field Naturalist 11:89-106.
Beissinger, S. R., B. T. Thomas, and S. D. Strahl. 1988. Vocalizations, food habits, and nesting biology of the slender-billed kite with comparisons to the snail kite. Wilson Bulletin 100:604-616
Bennetts, R. E., M. W. Collopy, and J. A. Rodgers, Jr. 1994. The snail kite in the Florida Everglades: a food specialist in a changing environment. Pages 507-532 in J. Ogden and S. Davis, eds. Everglades: the ecosystem and its restoration, St. Lucie Press; Delray Beach, Florida
Bennetts, R. E., and W. M. Kitchens. 1992. Estimation and environmental correlates of survival and dispersal of snail kites in Florida. First annual report, prepared for the U.S. Fish and Wildlife Service and U.S. National Park Service, Florida Cooperative Fisheries and Wildlife Research Unit, University of Florida; Gainesville, Florida.
Bennetts, R.E., and W.M. Kitchens. 1997a. The demography and movements of snail kites in Florida. Final report. Florida Cooperative Fish and Wildlife Research Unit, National Biological Service, U.S. Department of the Interior; Gainesville, Florida.
Bennetts R.E., and W.M. Kitchens. 1997b. Population dynamics and conservation of snail kites in Florida: The importance of spatial and temporal scale. Colonial Waterbirds 20:324-329
Brown, L., & D. Amadon. 1968. Hawks, eagles and falcons of the world, vol. 1. New York, cGraw-Hill Book Company.
Brush, Tim. 1999. The Hook-billed Kite: A reclusive, snail- eating raptor of the Lower Rio Grande Valley. Texas Birds 1(2):26-32.
Cheng, T.C. 1980. Final report on the possible effects of commercial herbicides on the apple snail (Pomacea paludosa) in Florida. January 31, 1980. Unpubl. Rept. on file at: Jacksonville Field Office, USFWS, Jacksonville, FL. 13 pp + figs.
Comfort, A. 1957. The duration of life in molluscs. Proc. Malacol. Soc. (London) 32: 219-241.
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Darby, P.C., P. Valentine-Darby, and H.F. Percival. 1997. Assessing the impact of the Lake Kissimmee restoration on apple snails. 1997 annual report. Florida Game and Fresh Water Fish Commission; Tallahassee, Florida
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Haverschmidt, F. 1970. Notes on the Snail Kites in Surinam. Auk 87: 580-584.
Hellmayr, C.E. and B. Conover. 1949. Catalogue of birds of the Americas. Field Mus. Nat. Hist., Chicago. Zool. Ser. 13, Part 1, No. 4.
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Lockwood, Mark W. and Brush Freeman. 2004. The Texas Ornithological Society Handbook of Texas Birds. Texas A&M University Press, College Station, Texas.
Miller, Bruce W., and Ronald L. Tilson. 1985. Snail Kite Kleptoparistism of Limpkins. Auk 102: 170-171.
Paulson, Dennis R. 1973. Predator polymorphism and apostatic selection. Evolution 27: 269-277.
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Rodgers, J.A., Jr., and H. T. Smith. 1995. Set-back distances to protect nesting bird colonies from human disturbance in Florida. Conservation Biology 9:89-99.
Rodgers, J. A., Jr. 1996. Endangered Florida snail kite. Pages 42-51 in J. A. Rodgers, Jr., ed. Rare and endangered biota of Florida, Second Edition, University Presses
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Rodgers, J. A., Jr., S. T. Schwikert, and A. S. Wenner. 1988. Status of the snail kite in Florida: 1981-1985. American Birds 42:30-35.
Rodgers, J.A., Jr. and P.W. Stangel. 1996. Genetic variation and population structure of the endangered snail kite in south Florida. Journal of Raptor Research 30(3):111-
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Smith, Thomas Bates. 1982. Nests and young of two rare raptors from Mexico. Biotropica 14: 79-80.
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