ENV: On Birds and Molluscs Redux
Previous articles on the question of Hook-billed Kites and their prey snails in the genus Rabdotus:
On The Conversations of Science
On Birds and Molluscs Part I
Some questions were being asked on TexBirds this weekend about the apparent decline of Rabdotus snails in South Texas, and i posted my two articles about this very subject to the list. John Arvin responded to the list with this nice account with some new info:
Tony and Texbirders, It is interesting to me that you have not seen Roadrunner predation on Rabdotus. I personally have watched them. They carry snails to a hard object (rock, which the Valley is very short of, or old beer bottle, which are plentiful) and whack the shells into fragments and eat the snail. You see a halo of shell fragments around just about any hard object on the ground in mesquite-nopal brush. The kites' prey can be told by the pattern of damage to the shell. They insert the tip of the beak just to the rear of the aperture and bite away a hole that permits the bill to snip the adductor muscle and out comes the body.
I found the first Hook-billed Kite's nest I ever saw by being drawn by curiosity to a 3 ft. diameter pile of empty shells. The parents soon made an appearance and solved the mystery. I was standing right under the nest which was directly over a trail at Santa Ana NWR. They fly in with snails, perch nearby on the same limb every time, de-shell the snails letting the shell drop, and then move to the nest to feed the young. The only other species of bird that I have observed prey on the snails is Brown Jay.
As for the invasive Pomacea, I hear that it has been found in a number of counties in SE Texas. I don't know what species of Pomacea is involved, but quite recently an Apple Snail of one species or another has invaded Gatun Lake in Panama, which is part of the Panama Canal system. As of as recently as 1989 when Ridgely's Birds of Panama, 2nd ed., was published there were only a few records of Snail Kite for the country. Last March I rode the Panama Railroad from Colon to Panama City. It cuts across the shallow upper end of Gatun Lake. We counted over 100 Snail Kites from the train. If you know the species of Pomacea that has invaded Gatun Lake I can guarantee that Snail Kites prey on it. If we have the same species I think it is just a matter of time before Snail Kites and Limpkins appear on the scene. -- John C. Arvin, Research Coordinator, Gulf Coast Bird Observatory
After the discussion over the weekend on TexBirds, re: Hook-billed and Snail Kites, and the decline of Rabdotus in the Lower Rio Grande Valley, there was another spate of discussion. Here it is:
I doubt that last winter's brief cold spell had any effect on Rabdotus alternatus populations. They went through the severe freezes of '83 and ‘89 without noticeable damage and kite populations did not fluctuate to any notable degree. I suspect prolonged and chronic drought has more to do with fluctuations in the snail populations. -- John C. Arvin
I agree with you wholeheartedly, John--I was out in the field this morning, and you beat me to the punch. Snail populations have been low for several years, at least at the major parks/refuges along the lower Rio Grande. To some extent the kites are using alternative foraging sites, many of which are inaccessible to the general public. -- Tim Brush
This dang drought has been around so long, that I have grown to forget about it! Few seeds, buds, fruit, bugs OR snails for the birds to feed upon. -- David T. Dauphin
On Hook-billed Kites … one should note that the hook-billed has been documented to feed on a number of items other than snails. According to the Global Raptor Information Network GRIN site (a great website for information on raptors developed by the Peregrine Fund) found at http://www.globalraptors.org. Chondrohierax uncinatus feeds mostly on snails, including large land snails (Strophocheilus) and apple snails (Ampullaria), and tree snails (Polymita), but also frogs, lizards, birds, large insects, and spiders. Several articles document it feeding on turtles, fish and crawfish as well. If snail numbers are low the Valley birds are likely feeding on something else to fill the energy balance. I have always found it interesting that the raptor migration in Veracruz has documented several hundred southbound hook-bills. While previous populations discovered by Tom Bates, Noel Snyder and Stan Temple no longer exist, kite populations obviously exist somewhere between Veracruz and the Rio Grande. ps. Unfortunately the species was not included in the Birds of North America series. -- Jack Eitniear
Coincidentally, I just saw something on TV about Open-billed Storks in Thailand that eat the same Pomacea snail, extracting it in the same way as described for the Hook-billed Kites. Completely different bill shape, same can opener technique. -- Judy Kestner
And my return commentary: i get TexBirds in the digest form so i don't get to read posts until the next day. so i'm just now catching up on yesterday's snails replies, and wanted to add a handful more notes, especially for those who did not read my long articles.
first, with regard to John's comment about the species of Pomacea, i wrote him a note separately, but for anyone else interested the invasive species in SE Texas is Pomacea canaliculata, the Channeled Apple Snail. it is considered a potential large-scale pest of rice, though this hasn't yet been a problem in Texas. because of the potential however, possession and transport of live snails of this species (and Pomacea paludicola, the Florida Apple Snail) has been banned in Texas. the common aquarium snail is also a Pomacea -- P. bridgesi -- but Robert Howells has shown that it is not a potential pest. because of that it was not included in the ban. nevertheless we occasionally find the other two species in pet stores masquerading as P. bridgesi. there is some research that shows Snail Kites are not able to work with just any species of Pomacea, and thus the impetus for (i think) John's comments about the Pomacea sp. in Gatun Lake. by the way, i googled up an article about Pomacea and Snail Kites from Gatun Lake via SORA, but the pdf wouldn't open for me. someone else may want to try. this is the URL.
i wanted to say also that some of the weather talk re: Rabdotus may have resulted from my early speculation back when we first were trying to come up with reasons for a decline (see the banter among us via my article “On the Conversations of Science”). at the time i speculated some on the possibility of the freeze playing a part last year, largely because of the potential circumstances. Rabdotus aestivate and obviously do well in cold weather, existing well north of Texas. and of course there are plenty of snail species in Canada. cold by itself is not a factor for aestivating snails. but there is a geographical band where tropical weather/consistently warm winters can be spiked with sudden, and suddenly deep freezes. these occasionally can be temporarily devastating to invertebrate and plant populations -- we probably know this best from Monarch wintering populations. when i was throwing out suggestions i hinted at that freeze, not because of the freeze per se, but because a period of warm, and wet, weather preceding the freeze. Rabdotus aestivates even in warm weather, coming out to feed, mate and lay eggs only when the humidity/temperature ratio hits some threshhold, and can best be seen moving en masse during rainy spells. i proposed that snails out and about in the warm, wet weather might have been caught unawares by a sudden, hard freeze, and that that might knock a bunch out. but i wasn't down there to know, and it was all pure speculation on my part.
and, in fact, my stronger suspicion is that there was an element of misinterpretation by some folks new to South Texas, who found large numbers of dead shells. most folks only know Rabdotus by the hundreds of dead shells that are strewn across patches of brush country. it takes a moment for folks to realize that they may never have even seen a live one. and that, i think, may have resulted in a mass die-off idea. the fact is, those big white shells are abundantly obvious anywhere you go in brush country, and are extremely persistent. some that you can dig out of bluffs along the Rio Grande are fossils. this persistence means they accumulate over generations and that leads to an overly optimistic estimation of how many live snails should be around to begin with. the live critters are hardly ever abundant, it takes some searching to find them even in places where predators may be scarce. i might mention too that the life span of these snails is not longer than two years, and probably is more like the span of two spring seasons or about 15 months.
having said all that, it’s obvious that the folks who are most focused on the living elements of the LRGV landscape, and who have a temporal track record there – i.e. Arvin, the Dauphins, Brush – are the ones best able to judge a recent decline in the living critters, and i absolutely believe their judgments are valid.
Jack Eitniear pointed out the fact that the Hook-bills will feed on other critters. i have had some problems with the Perergine Fund site he had pointed out, in that it sometimes uses secondary literature (which means the material is interpreted from the research, and is not firsthand – this can occasionally lead to errors of interpretation and then exacerbation of the problem when these secondary sources are quoted as authoritative) and i have been unable to track down some of the info they promote for Snail Kites. i don't know if the same problems exist for the Hook-bill account but i am wary.
[actually (and see more explication below this paragraph) i checked this out myself just now and found a likely error of similar genesis; they list the Cuban Hook-billed Kite as a separate species from Hook-billed Kite, and rightly list the Cuban’s food as Polymita snails; however, they also list Polymita as a prey item of the Hook-bills; Polymita is endemic to Cuba, indeed it is endemic to Oriente Province, and there is no sympatry between the kites, meaning Polymita cannot be a prey source for the Hook-billed Kite if the two kites are considered separate species; likely this error of continuation comes from not being able to suss differences in the secondary literature accounts that have not split the kites, or else they simply didn’t change the account when they separated the taxa; either way it is erroneous]
beyond that, i want to address the fact that of the snails listed, two do not occur in the US, though one, Strophocheilus (depending on how you define the genus) gets into the Mante area of Tamaulipas; the other, Polymita, is the Cuban endemic coffee snail, and the Hook-bill that feeds on them has developed a different bill structure/size than ours to do so, and indeed may be a different species of kite. Ampullaria is an alternate generic name for Pomacea. i am trying to get my head around the idea that Hook-bills may feed on them, in the way that Cuban Hook-bills have become adapted to Polymita (Polymita is a large snail, roughly midway in size between Rabdotus and Pomacea; Polymita is globular like Pomacea also). perhaps the predator in this case is the South American race known as the Large-billed Kite, Chondrohierax (uncinatus) megarhynchus, which is also sometimes split from the Hook-bill. i may post again later on this.
Update 4 April: Bill Tarbox and David Bryant were both able to open the Gatun Lake pdf and kindly sent me copies. Interesting reading. There is a native Pomacea in the lake, P. cumingi, which was not enough perhaps to draw Snail Kites. Pomacea latrrei was introduced to control Hydrilla verticillata (at least somewhat successfully), followed rapidly by Snail Kite expansion. The authors speculate the Snail Kites spread from the Colombian population. I have neither of these Pomacea in my collection and am unfamiliar with the taxa, but neither of these species are ones that have been found in Texas. Thanks again to Bill and David. – tony gallucci
More 5 April: I was told that the bazillion Willets in the area of Yacht Basin Rd. (Bolivar Peninsula) feed on the snails on the adjacent marsh grass. True? And, if so, how do they handle them without the bills that the kites have? And, do the White-tailed Kites eat snails as well? Interesting thread. -- Sue Levy
The snail Willets are most likely finding are Littorina angulifera, the Salt Marsh Snail, or Mangrove Periwinkle. This is an interesting critter that feeds on marsh grass, Spartina, in areas where low tides leave the grass largely exposed part of the day (Bolivar, Anahuac are good examples). The snails simply close up and rest when the water is out. I don't know how the Willets are dealing with them, BUT these are operculate snails -- they have a "trapdoor" that slams the opening of the shell closed when they are disturbed. Since this is actually physically attached to the foot of the living snail however, it means that the fleshy animal cannot withdraw deeply into the shell like most land snails can. I might speculate that it makes it possible for the birds to grab the operculum and yank the critter out. You can see pictures here: http://www.gastropods.com/3/Shell_1803.html. P.s. i forgot to mention in my TexBirds post that i don’t know of, and could not find any mention of, White-tailed Kites eating snails. I think they are almost exclusively insect eaters, maybe with a small rodent or bird thrown in on occasion. – tony gallucci
Tags: Links, Blogs, Culture, Science, Politics, Molluscs, Environment, Snails, Kites, Birds, milkriverblog