The Condor 90:16X-172 0 The Cooper Ornithological Society 1988 UNDERWATER FORAGING BEHAVIOR OF CANVASBACKS, LESSER SCAUPS, AND RUDDY DUCKS’ MICHAEL W. TOMES AND DALE A. WRUBLESKI~ Delta Waterfowl and WetlandsResearch Station, RR #I, Portage la Prairie, Manitoba RIN 3A1, Canada Abstract. We observedthe underwater behavior of captive Canvasbacks(Aythyu valisi- neria), LesserScaups (A. u&is), and Ruddy Ducks (Oxyurajamaicensis) while they foraged on a variety of food items in a large aquarium. Canvasbacksprobed into the substratewith the bill and body oriented perpendicular to the bottom. Lesser Scaupsand Ruddy Ducks strained food items from the substratesurface by moving their bills in short, lateral arcs while rapidly opening and closing their mandibles; their bills and bodies were oriented at a 3.5 to 45” angle to the substrate.Scaup also fed by grasping prey in the water column, where they appeared to locate prey visually. Ruddy Ducks did not appear to select prey visually. The speciesalso differed in their underwater locomotory behavior and postures; these differencesprobably are related to the prey and conditions typically encounteredby each. Key words: Canvasback;Aythya valisineria; LesserScaup; Aythya affinis; Ruddy Duck; Oxyura jamaicensis;foraging behavior;diving behavior. INTRODUCTION habitats and distributions of the three species Although the behavior of many speciesof diving overlap (Palmer 1976), but their diets differ ducks has been described,very little is known of markedly. During most of the year, Canvasbacks their underwater foraging and locomotory be- feed primarily on plant tubers or molluscs dis- havior. Diving behavior of some specieshas been tributed in wetland substrates,although prelay- observed in the wild from above the water sur- ing and laying females, and juveniles consume face (e.g., Brooks 1945; Humphrey 1957, 1958; large numbers of aquatic invertebrates (Bartonek Snell 1985) but turbid water hampered visibility and Hickey 1969). LesserScaups consume aquatic and the foraging actions of the birds were not invertebrates (primarily amphipods), both in the recorded.Only a few investigators have observed water column and on vegetation and substrate the underwater actions of captive waterfowl in surfaces(Bartonek and Hickey 1969, Hoppe et aquaria. Livezey and Humphrey (1984) ob- al. 1986). Ruddy Ducks feed almost exclusively served the underwater locomotory behavior of on benthic chironomid larvae during the breed- steamer-ducks(Tuchyeres spp.) and Suter (1982) ing season(Siegfried 1973, Tome 1981) and on briefly described some of the foraging behaviors oligochaetes(Stark 1978) or chironomid larvae of Common Goldeneyes (Bucephala clangula), (Hoppe et al. 1986) during the winter. Each of Common Pochards(Aythyaferina), Tufted Ducks these kinds of prey differs in mobility, antipred- (A. fi&z&z), and Eurasian Coots (IQ&u &-a). ator response,and the environmental conditions This paper describesthe underwater locomo- in which they are found; thus, we predicted that tory and foraging behavior of three species of the foraging behaviors of thesewaterfowl species diving ducks: the Canvasback (A. valisineriu), would vary accordingly. LesserScaup (A. a#%), and Ruddy Duck (Oxy- METHODS ura jamaicensis). The breeding and wintering During the summers of 1983 to 1985, we ob- served the foraging behavior of seven Ruddy Ducks (four males, three females), four Canvas- ’ ’ Received22 April 1987. Final acceptance8 August backs (two males, two females) and two male 1987. Lesser Scaup in a 5 x 2 x 2-m indoor concrete z Present address:Patuxent Wildlife ResearchCen- aquarium through four, 1 x 1-m plate-glasswin- ter, Laurel, MD 20708. 3Present address: Department of Entomology, Uni- dows. We photographed underwater behavior versity of Alberta, Edmonton, Alberta T6G 2E3, Can- using a 35-mm camera and both super-8 and 16- ada. mm movie cameras. f1681 DIVING DUCK FORAGING BEHAVIOR 169 The bottom of the aquarium consisted of a the bird. The birds used simultaneous strokesof 4 x 4arrayof 1.0 x 0.5 x O.l-mwoodentrays the legsto propel themselves towards the aquar- filled with 6 cm ofsand that provided a substrate ium substrate. At the end of each power stroke, for the benthic prey. In 1985, we anchored into the feet convergedmedially, below the tail. Dur- the substrateone end of several 50-cm long, 0.6- ing the “recovery stroke” (the portion ofthe stroke cm diameter polypropyleneropes which had been when the leg is brought forward), the toes and separatedinto fine strands. The free end of the web were folded posteriorly, reducing resistance rope floated in the water column; this simulated to the water. All speciesswam to the bottom with submerged vegetation and provided a refuge for necks stretchedforward. None of the speciesex- benthic and pelagic invertebrates. tended their wings for propulsion or stabilization We attempted to observe each speciesforage while underwater. on prey that they normally consumed in the wild; Upon reaching the bottom, the birds main- however, in some instances we could not obtain tained their location with leg strokes directed numbers of natural prey necessaryto conduct our perpendicular to the water surface. The Lesser observations and, thus, we provided substitute Scaup and Ruddy Ducks always used simulta- prey. In some of the Ruddy Duck observations, neous leg strokes;the Canvasbacks,however, oc- we used wheat grains to simulate patchesof ben- casionally used alternate leg strokes. When for- thic prey. The grains were placed approximately aging,the birds positionedtheir bodies at an angle 1 cm below the surface of the sand in densities to the substratewhich varied among species(see of 500 to 1,000 grains/m2. For all Canvasback next section). When its body was angled away trials, we placed corn kernels in the sand in den- from perpendicular to the substrate, the power sities of 100 to 500 kernels/m2 to simulate plant stroke moved the bird forward and also pre- tubers distributed beneath the substrate surface. vented the bird from floating toward the water For all LesserScaup and some Ruddy Duck trials, surface.All speciesmoved forward along the bot- we releasedseveral thousand amphipods into the tom with the power stroke of the legs parallel to aquarium at least 4 hr before feeding trials began. the axis of the body. This provided time for the invertebrates to ac- To return to the water surface,the birds stopped climatize to the experimental habitat. The ma- moving their legs and briefly (< 1 set) floated jority of the amphipods attached to the substrate backwards, towards the water surface, until a or to the walls near the bottom of the aquarium. power stroke and an upward motion of the head At least 5 hr before each observation session, oriented the body towardsthe water surface.They we placed the birds in pens beside the aquarium floated to the water surfacewith little or no effort. and deprived them of food. We observed each The feet normally trailed behind the bird in a speciesseparately, but also observed the Lesser relaxed position with the webbing folded so that Scaupand Ruddy Ducks together. During all ob- the legsand feet provided little resistanceagainst servation sessionsat least two individuals of a the water. Occasionally, however, the birds specieswere in the aquarium. moved their feet to changedirection; e.g.,to swing to the right, the bird extended the right foot lat- RESULTS erally with its web spread open. As the birds GENERAL UNDERWATER LOCOMOTORY neared the water surface,they extended both legs BEHAVIOR laterally and spread the foot webbing, which de- All speciesinitiated dives similarly. Immediately creased their velocity. Occasionally, the birds before diving, the birds exhaled and brought both adopted this posture to slow their upward ve- feet forward, close to the body on each side of locity and maneuvered to grasp a food item. the mid-sternum region. Each dive began when The Ruddy Ducks floated to the water surface the bird swept both feet simultaneously in a ven- with the neck and bill slightly forward of the tral-posterior direction (“power stroke”) while posture adopted when sitting on the surface.The arching the neck forward into the water. The Lesser Scaup and Canvasback arched the neck head and neck entered the water when the feet so the distal end of the bill was near or against were approximately half-way through the power the upper chest or lower neck. These positions stroke. During the descent,the legs were rotated were maintained until the head broke the water laterally from the normal swimming posture to surface and the birds returned to the normal a position near or above the horizontal plane of swimming position. 170 MICHAEL W. TOME AND DALE A. WRUBLESIU FORAGING BEHAVIOR consumed prey distributed on the artificial vege- Ruddy Duck. When foraging on prey distributed tation; but unlike them, they often pursued and in the substrate,the Ruddy Ducks inserted their consumed amphipods that would drop to the bills at a 35 to 45” angle into the substrate to a substrate when the vegetation was disturbed. point slightly distal to the nares. The birds swam Canvasback. The body and bill of the Can- forward and rapidly opened and closedthe man- vasbackswere oriented perpendicular to the sub- dibles while also moving their heads in short, strate when they foraged on prey distributed in lateral arcs so that an area of about 1.5 times the the substrate. The Canvasback used the bill as a bill width was searchedfor food. This mandib- probe to locate and extract food items from the ular movement caused substrate and associated substrate. When searching for corn kernels dis- benthos to be drawn into the bird’s mouth when tributed beneath the substrate, Canvasbacks in- the mandibles were opened. Upon closing, sub- serted their bills into the substrate using slight, strate was forced out the sides of the bill while lateral head shaking motions. The bill remained the lamellae retained items that were consumed.