SCOTERS’ DIET WITH NOTES ON SOME WATERFOWL RANGE AND ABUNDANCE CHANGES.
PRELUDE. An offhand comment in one of my recent postings was a query as to what all those Surf Scoters off our shoreline are eating. This brought some detailed and fascinating replies from Larry Hindman (MD DNR), Matthew Perry (USGS), and Peter Osenton (USGS). I am extremely indebted to them for this information and will here attempt to summarize just some of it.
HANDBOOKS GET AN F. re APRÈS PREY APPREHENSION. I was hoping to find a detailed description somewhere of what happens to a scoter’s prey after it is caught. I already knew that some of the shells get ground up by the gizzard, but which way do the remains then go? Does a scoter crunch up a shellfish with its bill, too? Does a scoter cast pellets? In spite of, an admittedly quick, look in Bent, BNA, Delacour, Forbush, Johnsgard, Kortright, and Palmer I found nothing. But Leahy (p. 703) has this helpful note: “On their wintering grounds the scoters eat prodigious numbers of mussels and other shellfish, which are pulverized by the powerful muscles of their gizzards.”
RESEARCH BY MIDDLE ATLANTIC BIOLOGISTS:
Hindman sent useful pie charts that show for 236 Bay Surf Scoters these preferred foods: Hooked Mussel 37.16%, Dwarf Surf Clam24.41%, Amethyst Gem Clam 12.16%, Sout Tagelus 8.12%, Baltic Macoma 6.77%, False Angel Wing 2.96% and “other” 8.42%. Hooked Mussel also was the top choice of 10 Whitewings at 41.65% and 32 Blacks at 77.43%. In the Bay 32 Long-tailed Ducks were found to mostly favor Amethyst Gem Clam 53.09%, Dwarf Surf Clam 15.66%, and Hooked Mussel 5.44%. Hindman’s pie charts show the Blue Mussel reigns supreme as the pièce de résistance in the Maritimes; from samples of 12 White-winged it comprised 52.69%, 30 Black (50.97%) and 32 Surf (75%) scoters. I find it tasty, too, esp. with linguini.
Osenton “has completed gizzard and gut analysis on 1,877 birds [!], with 846 of them from the Maryland portion of the Chesapeake Bay,” 286 of the latter Surf Scoters. “Their diet differs some depending on where the birds were collected from,” and especially relative to the water salinity. Choptank River skunkheads prefer Hooked Mussel, Dwarf Surf Clam, and Amethyst Gem Clam with smaller amounts of Stout Tagelus, Baltic Clam, and Soft-shelled Clam. Those in the Smith Island, MD, area also take False Angel Wing and Atlantic Jacknife Clam.
Perry et al. have an extremely interesting “poster” where on one page there are pie charts of the food habits of 8 species of Chesapeake diving ducks. For Surf Scoter (sample size 84): Dwarf Surf Clam 18%, False Angel Wing 16%, Baltic Clam 14%, Gem Clam 6%, SAV 7%, and “other” 39%. Baltic Clams are the major food of Ruddy Duck (sample 21; 36%), Canvasback (sample 16; 89%), and Bufflehead (sample 14; 49%). Hooked Mussel maxes out at 67% for Black Scoter (sample size of 17). Long-tailed Duck, sample of 21, with Gem Clam 58%.
Interesting: the scaups differ: from 25 Lessers Atlantic Rangia is 60%, from 15 Greaters Hooked Mussel at 53%. The differing sizes of scaup species’ bills may account for some of this variance. Some of these are small samples that may be skewed by where the ducks came from, time of year, etc. For 6 species plus all scoters and all scaup Perry also shows changes in Chesapeake abundance from 1950-1999 with dramatic declines of Redhead, Canvasback, the scaups, Common Goldeneye, and Ruddy Duck but a big increase for Bufflehead, Long-tailed Duck being even, and a slight decline for scoters.
Telemetry. Using mist nets and night lighting Hindman et al. have captured in the Choptank River 8 Surf and 1 Black scoter and 10 longtails and fitted most of these with transmitters. They have plans to capture more. Those results ought to be interesting. Longtails nest as far north as Elsemere Island as well as along almost all of coastal Greenland. The scoters mostly breed farther south in the taiga. All are long istance travelers.
Prey Size. Many of these waterfowl seem to choose smaller individuals of each species to eat, presumably because those are easier to digest and grind up in their gizzards.
Geographical Bias. Historically much of the work on diving duck food habits has been done in New England, Canada, Alaska, and elsewhere on the West Coast, perhaps because some of the centers of learning were established in those areas before they were in the Middle Atlantic.
Seasonal Bias. Research elsewhere and during the breeding season indicates these ducks feed to a greater extent on non-shellfish invertebrates, including insects, as well as seeds.
WATERFOWL: some spatiotemporal changes in the past 60 years.
Waterfowl seem more adept at shifting their winter ranges than other bird groups. See how Tundra Swans shifted south from Chesapeake Bay to NE North Carolina after most of the SAV died out c. 1970. Around that time the swans began showing up in big numbers in winter grain fields for the first time in my experience. Off of our shoreline in Talbot County, MD, the big numbers of Long-tailed Ducks I have seen, often carefully estimated from my skiff, include 9125 on March 17, 1995, 6700 on April 1, 1988, 6480 on March 28, 1987, and 4210 on March 25, 1989. Counts of as many as even a few hundred are rare now and involve birds farther offshore than previously.
Taking their place are Surf Scoters, which have become as dramatically abundant now as longtails have become somewhat scarce. Our highs of Surf Scoters include several dozen totals of 1000 or more with the highest a few estimates of 3000-5000. Prior to 1990 the high was 81 on March 25, 1989. It is typical for the scoters’ peak numbers to be 1-2 weeks later than the longtails. The longtails clear out earlier, perhaps because they are more cold-tolerant and breed much farther north than the scoters.
Long-tailed Duck populations vary wildly at their stronghold in the Nantucket area. This past December the Nantucket Christmas count recorded 7172 and marked that as a low count. It certainly was compared with many other years, such as 178,958 in 1987, 251,754 in 1995, 254,302 in 2001, and 525,505 in 2002, or, even compared with dramatically lower but still impressive totals of 26,772 in 1975, 65,080 in 1979, and 81,525 in 1982. Some estimates state c. 30% of the total North American population winter in the Nantucket area (White et al.).
Other examples of waterfowl shifting the ranges include many recent reports of Black-bellied Whistling Ducks well north of their usual locales, Mallards spreading east historically as agriculture opened up land east of the Mississippi and Ohio river drainages, Mottled Ducks recently being seen farther north, the dramatic increase in Common Eiders in the past 2 winters in the Middle Atlantic, and, of course, Snow and Ross’s geese breeding farther east in the Arctic, not to mention the explosion of Mute Swan numbers in the Chesapeake starting in the 1960s, although the majority have now been “removed.”
Canada Geese reached epic numbers in the Chesapeake area in the late 1960s on into the 1980s while concurrently their numbers declined south of here, such as in the Florida panhandle where 1000s used to winter. Now they are scarcer, the population having shifted to the north even as “resident geese” have become more abundant in our local urban and semi-urban areas.
In the panhandle at St. Marks N.W.R. the Christmas count routinely tallied 1000-4800 CGs from 1950 until 1967 with as many as 9710 once. This tapered off to 53 in 1980. None have been seen since 1981 save for 1 in 2005 and 2 in 2009. With the geese the presence of waste grain in agricultural fields has been a boon to their winter population survival farther north.
Locally in the areas of the central Chesapeake that I frequent there have been far fewer Lesser Scaup and Red-breasted Mergansers, especially in the early spring. Common Goldeneyes are much less common now in coastal areas in the winter. Buffleheads are much more abundant off of our shoreline beginning c. 1980 (and especially in the 1990s) but their burgeoning presence here is much less so in the last few winters.
SHELLFISH: key species (illus. numbers refer to plate [Morris] or illus. numbers [Rehder], not page numbers, all photographs). Measurements are as given in Rehder.:
Amethyst Gem Clam (Gemma gemma). a.k.a. Gem Venus Clam, Amethyst Gem Shell, or Gem Shell. p. 801, illus. 536, Rehder. p. 67, illus. 24, Morris. 1/8”. “Early settlers in Massachusetts sent boxes of them back to England as curiosities. There was a time when it was believed that they were the young of the Quahog.” Morris, p. 67.
Atlantic Dogwinkle. (Nucella lapillus). Sometimes called Thais lapillus, or, just plain Dogwinkle. p. 526, illus. 153 & 697, Rehder. p. 199, illus. 1 & 53 Morris. Unlike most scoter food it is not a bivalve but a gastropod. Feeds of mollusks and barnacles and its color can vary with its diet. “Those that feed on the Blue Mussel tend to be darker...” (Rehder, p. 527). 7/8 – 2”. Wide variation in color and shape.
Atlantic Jacknife Clam (Ensis directus). a.k.a. Common Razor Clam. p. 762, illus. 613, Rehder. p. 86, illus. 30, Morris. Good to eat, for scoter or human. Shaped as a straightedge razor. 3-8”. Can burrow rapidly and swim in fast, spring-like motion.
Baltic Clam (Macoma balthica) a.k.a. Baltic Macoma. p. 774, illus. 578, Rehder. P. 77, illus. 29, Morris. Also occurs, as the name implies, in northern Europe as well as the West Coast. 0.75 – 1.5”.
Blue Mussel. (Mytilus edulis). p. 676, illus. 638, Rehder. p. 17, illus. 12, Morris. The shellfish often served with linguini and also favored by Raccoons and American Oystercatchers, the birds devouring them “straight up,” hold the linguini. Of greater commercial importance and use in Europe. 1.25 – 4”.
Channeled Barrel Bubble (Acteocina canaliculata). other binomials Tornatina canaliculata, Retusa canaliculata. p. 635, illus. 353 Rehder. p. 266, illus. 73, Morris. 1/8 – 1/4“. A gastropod.
Dwarf Surf Clam (Mulinia lateralis). a.k.a. Coot Clam [? Sea Coot is a colloquial name for scoter], Duck Clam, Little Surf Clam. p. 754, illus. 545, Rehder. p. 70, illus. 27, Morris. ½ - ¾”. Favored food of Black Drum and some other fishes..
False Angel Wing (Petricola pholadiformis). p. 810, illus. 516, Rehder. p. 68, illus. 25, Morris. 1.5 – 2.25”. Sometimes bores into waterlogged wood.
Hooked Mussel (Ischadium recurvum). p. 680, illus. 643, Rehder. p. 18, illus. 12, Morris, where termed Bent Mussel (Brachidontes recurvus). I don’t know which is preferred. 1 – 2 3/8”. Maryland is its north limit. Prefers brackish water. Formerly known as Mytilus hamatus.
Little Cockle. (Cerastoderma pinnulatum). Apparently not in Rehder. p. 55, illus. 23, Morris. 0.5”. “ … said to scamper over the gravelly bottom with surprising ability by making expert use of its recurved, extensible foot.” p. 56 Morris.
Soft-shell Clam (Mya arenaria). Native Americans in the Northeast refer to it as Manninose; still somewhat known as that locally, or as just Mannose. Has commercial importance. Found also in Europe where known as Sand-gaper. Introduced to California and has spread from there to B.C. 1-.5”. p. 811, illus. 590 Rehder. p. 90, illus. 31, Morris. Also known as Long-necked Clam, Long Clam, Soft-shelled Clam, and Steamer Clam. Can be 6”. I can remember it occasionally being offered in fast food places in Cambridge.
Stout Tagelus (Tagelus plebeius) p. 789 Rehder. p. 83, illus., 31 Morris. Up to 3.75”. Formerly Tagelus gibbus.
BIOLOGISTS AND BIRDERS: A DIVIDE. I believe there is something of a gulf between professional biologists and birders. The rich trove of information on the food habits of diving ducks is something I had no idea even existed. Thanks to Larry Hindman, Peter Osenton, and Mathew Perry I’ve been inundated with so much good information I can’t even do justice to it in this brief summary here. I think most of us birders have no understanding of the impressive work done by government biologists.
REFERENCES:
Erskine, A. J. Buffleheads. Canadian Wildlife Service (monograph series 4). 1971. 240p. Has detailed food information, and I mean detailed, on pp. 152-161.
Hindman, Larry J. pers. comm. (e-mails of March 2011).
Leahy, Christopher W. The birdwatcher’s companion to North American birdlife. Princeton U. Pr. 2004. 1039p.
Morris, Percy A. A field guide to shells of the Atlantic and Gulf coasts and the West Indies. 3/e. Houghton Mifflin (Peterson Field Guide 3), 1973. 330p. plates 1-8 in color, 9-76 in black-and-white.
Osenton, Peter C. pers. comm. e-mail of March 28, 2011.
Perry, Matthew C., Alicia Wells-Berlin, David M. Kidwell & Peter C. Osenton “Temporal changes of populations and trophic relationships of wintering diving ducks in Chesapeake Bay,” Waterbirds 30 (special publication 1), 2007, pp. 4-16.
Perry, Matthew C., Peter C. Osenton & Edward J. R. Lohnes. “Diving duck distribution, abundance, and food habits in Chesapeake Bay.” I don’t know how to cite this. It is a “poster,” and a marvel of concentrated information.
Rehder, Harald A. The Audubon Society field guide to North American seashells. Alfred A. Knopf, 1981. 894p. photographs 1-705 in color.
Robertson, Gregory J. & Jean-Pierre L. Savard. 2002. Long-tailed Duck. Birds of North America 651 (AOU, ANSP). 28p.
Savard, Jean-Pierre L., Daniel Bordage & Austin Reed. 1998. Surf Scoter. Birds of North America 363 (AOU & ANSP). 28p.
White, Thomas P., Richard R. Veit & Matthew C. Perry, “Feeding ecology of Long-tailed Ducks Clangula hyemalis wintering on the Nantucket shoals,” Waterbirds 32(2), 2009, pp. 293-299.
Best to all. – Harry Armistead, Philadelphia.
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