Food Habits of a Small Sample of Long-tailed Ducks from Nantucket Sound
Matthew C. Perry
The gullet (esophagus and proventriculus) were analyzed to determine food eaten by ducks using techniques commonly used in other studies (Perry and Uhler 1988). Three of the five ducks from the December collection had food in their gullets that ranged from 0.1 to 34.59 ml with a mean 14.17 ml (Table 1). All five ducks had food in their gizzards and volumes ranged from 0.8 to 2.08 ml with a mean of 1.39 ml. The predominant food in the gullet and in the gizzard of these ducks was the pelagic gammarid amphipod, Gammarus annulatus. This amphipod was found in three of the ducks and comprised 67% of the gullet food and 55% of the gizzard food (Table 1).
Other soft food organisms found in lesser amount included the skeleton shrimp (Caprella sp.) and the isopod, Edotea triloba that were both found in the gullet of only one duck. Hydroids (Hydrozoa) were found in the gullet of one duck, and one duck had a small amount of an unidentified marine alga in its gizzard. The skeleton shrimp (an amphipod) and the isopod are typically benthic organisms associated with hydroids and algae, which were recorded from the same ducks (Table 1).
Two ducks that had not fed on the amphipod had very small amounts of food in their gizzards, which were remnants of chestnut astarte (Astarte castanea). Other hard food found in this small sample of ducks included sand dollars (Echinarchnius parma), the transverse ark (Anadara transversa), the New England dog whelk (Nassarius trivittatus), and an unknown gastropod (Gastropoda).
Grit was found in the gullet of three ducks and in the gizzard of all five ducks. The ducks that had grit in the gullet were the ones that were feeding on gammarid amphipods and other soft-bodied organisms. Ducks need grit to help grind food in the gizzard, but typically have little or no grit in their gizzard when they are feeding on hard-bodied food such as mollusks. They apparently use the hard surface of the mollusks to crush other mollusks and other food organisms. Mean grit volume was 0.67 ml for the gullet and 1.36 ml for the gizzard of this sample of ducks.
Although it can not be ascertained when and where the food found in the long-tailed ducks was obtained, it is believed that most of the food items (especially the amphipods) were obtained in the ocean before the ducks returned to the Sound. Earlier in the same day that the ducks were obtained, there were numerous amphipods observed in the ocean water over Nantucket Shoals from a boat in several areas 5-6 miles south of Nantucket Island. Unfortunately, these amphipods were not collected and identified. Surveys of pelagic amphipods have confirmed that Gammarus annulatus is occasionally abundant in the water column on Nantucket Shoals (Avery et al. 1996). Pelagic gammarid amphipods were found in the gullets in 6 long-tailed ducks collected during the winter of 1997-98 (R. Veit and T. White, pers. comm.).
Biologists studying the long-tailed ducks in the Nantucket Island area have questioned why the ducks make the long commute to the ocean each morning and return to the Sound at night. This phenomenon has received increased interest due to the potential construction of the large Cape Wind project to construct wind turbines over the shoals of Nantucket Sound to generate electricity. The commute varies in length depending mostly on wind conditions, but normally is approximately 10 miles one way and could be as much as 30 miles, based on recent observations of flight path of ducks made from land and boat. This type of long-distance daily movements is not typical for seaducks or other known wintering ducks. Geese and swans are known to make long daily flights on wintering areas, but this is mainly because they feed in fields during the day and rest on water at night.
Three major reasons have been proposed on why the ducks would make this long commute, which has to be a drain on their energy reserves during a stressful time of year. The first reason deals with the possibility that the ducks are feeding in the ocean area during day and the Sound area at night on different food items and can obtain enough food to make the trip profitable from an energetic perspective. Although there is a possibility that different organisms occur in the two areas, one during day and one during night, there is no evidence that this occurs in the Nantucket Island area. There is a belief that in Chesapeake Bay some ducks such as the canvasback (Aythya valisineria) might feed at night because this is when clam worms (Neries spp.) are more active and also when clams are closer to the surface of the sediment making both species potentially more vulnerable as prey. This idea is merely speculation in Chesapeake Bay and would have the same status in the Nantucket area.
The second potential reason is that the Sound area offers a less stressful environment due to smaller waves or other environmental factors affecting the ducks microhabitat. This might be initially plausible when one considers the prevailing wind during the winter is from the northwest and therefore the mainland might provide more attenuation of the wind. However, the breadth of the Sound (29 miles from Nantucket Island to Hyannis) and the fact that there is no evidence that the ducks seek shelter near shore (mainland or island) in the lee of the wind would discount this hypothesis. The depth of the water in the Sound where the ducks spend the night is equivalent to that where they are during the day in the ocean, so wave action, temperature, and other environmental factors would appear to be similar.
The third possible reason is that nocturnal predators (fish and mammals) exist in the ocean area and ducks have evolved a strategy to avoid predation (Straneck et al. 1983). A waterman in the Nantucket area has reported ( W. Blount, pers.comm.) that monkfish (Lophius americanus), collected in the area, had fed on surf scoters (Melanitta perspicillata). It is unknown in the Nantucket area if the monkfish is preying on ducks while the ducks are resting on the water surface, or if the monkfish is a “sit-and-wait predator” (Gordoa and Macpherson 1990) obtaining ducks as the ducks feed on the bottom. Monkfish, also known as the goosefish, are a bottom-dwelling shark and have swallowed scoters (Glegg 1945) and larger avian prey than ducks ( Davenport 1979). If the monkfish is a major predator of seaducks in the Nantucket Shoals area, it is possible that the seaducks have evolved a strategy to fly to the potentially more protected area of the Nantucket Sound to rest during nighttime when feeding activity is generally less for diving ducks. Other predators, including pinnepeds (Tallman and Sullivan 2004) and cetaceans (Cobb 1927; Hamilton 1946; Straneck 1983), which are common over Nantucket Shoals, may also prey on ducks in this area and be a cause of this unusual daily flight pattern of long-tailed ducks.
There is a great need to continue food habits analyses of long-tailed ducks in the Nantucket area to determine the value of the habitats of the ocean and the Sound from a food perspective. There also is a need to collect more data on the movements of the ducks in this area to determine how these daily flights could be impacted by the wind turbine project. Movement data should be obtained with the aid of telemetry and ground observers. Hopefully, research on the food habits and movements of this species will help to better understand the timing and reason of long-tailed duck movements in the Nantucket Island area so their habitats can receive optimum management and protection.
Acknowledgements: Taber Alison and Simon Perkins of Massachusetts Audubon Society provided travel funds as part of a project financed by Mineral Management Services. Simon Perkins also provided personal knowledge and encouragement for this study. Peter Osenton (USGS) conducted the analyses of the ducks in this sample at Patuxent Wildlife Research Center and has analyzed over 1600 waterfowl food habits samples during the last 6 years. This large data base offers a good comparison to this small sample of ducks, but also to the numerous historic food habits records that date back to 1885. Sheldon Pratt of the University of Rhode Island graciously identified the amphipods. William Blount of Nantucket Island and Debbie Palka of Woods Hole gave information about the predatory nature of monkfish. Rick Kotalac provided boat and waterman expertise for seaduck boat surveys. Field assistance was obtained from Melanie Kotalac, Glenn Olsen, Blair Perkins, and Edith Ray. Technical and library assistance was provided by Alicia Berlin and Lynda Garrett. Richard Veit, Robert Kennedy, and Timothy White provided expert information of the bird movements and food habits in the Nantucket area.
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Table 1. Food contents (gullet and gizzard) of five long-tailed ducks from Nantucket Sound, December 17, 2006.
1All organisms are maintained in reference collection at Patuxent Wildlife Research Center food habits laboratory.