Food Habits

Main Foods Taken

Bivalves, molluscs, and worms—almost exclusively shellfish and other marine invertebrates that inhabit intertidal areas. In contrast, European Oystercatcher feeds at inland locations when breeding (Heppleston 1972).

Microhabitat For Foraging

Restricted to intertidal sand or mud flats, oyster or mussel reefs or shoals; less commonly, rocky shoreline. In sand or mud flats, often feeds along edge of receding tide. Feed in shellfish beds while mussels or oysters are still submerged (see below). Distribution believed to be limited by availability of intertidal areas supporting shellfish beds (Tomkins 1954).

Food Capture And Consumption

Oystercatchers use directed movements (generally alternating left and right turns) while searching for food, regardless of its visibility (e.g., submerged into sand or emergent; EN). When feeding on mussels or oysters, they locate food visually, wading through slightly submerged shellfish beds (Fig. 2). When a bivalve with open valves is located, the oystercatcher employs a technique known as “stabbing,” whereby it quickly inserts its knife-like bill into the open valves, and with several quick thrusts severs the adductor chain that holds the two valves together. The bird then cleans out and consumes the soft parts.

American Oystercatchers also use a “hammering” technique, removing an individual mussel from a clump and moving it to a location above water. Here they orient it properly with their bill and begin hammering at the point where the adductor chain lies inside the shell. Once they have broken through the shell, they quickly sever the adductor chain, allowing the 2 halves of the bivalve to separate. The soft parts are then consumed completely. When feeding on soft-shell clams (Mya arenaria) and razor clams (Ensis directus, Solen viridis, Tagelus plebeius), oystercatchers appear to feed tactily by probing the substrate with their long bill. Once a clam is located, the oystercatcher uses its bill simultaneously as a shovel and lever to loosen the sand and push the clam upward. The oystercatcher then pulls the clam up to the surface, opens it, and eats it as it would a mussel or oyster. Accidents have been recorded where oystercatchers bill becomes caught by shellfish and held so that bird drowns with rising tide (Terres 1980). Oystercatchers also locate marine worms by probing in intertidal flats.

Major Food Items

Northern part of range (Massachusetts south to New York): blue mussels (Mytilus edulis), ribbed mussels (G. demissa, Modiolus plicatus), soft-shell clams (Mya arenaria), sandworms (Nereis pelagica), razor clams (Ensis directus), hard clams (Venus mercenaria; Post and Raynor 1964, RCH); Southern part of range (Virginia, N. Carolina, S. Carolina, Georgia, and Florida): oysters (Crassostrea virginica), soft-shell and razor clams, stout razor clams (Tagelus plebeius), ribbed mussels, mole crabs (Emerita talpoida), sandworms, limpets (Aemaeu sp.), jellyfish (Coelenterata), sea urchins (Strongylocentratus sp.), starfish (Asteria spp.), and crabs (Bent 1929, Tomkins 1947, Cadman 1979, Johnsgard 1981, Nol 1989).

In a study of breeding season time budgets (Nol 1985), birds spent 10%, 19%, 10.5%, and 23% of their time searching for food in the pre-laying, laying, incubation, and chick-rearing periods, respectively. No significant differences between the sexes. Energy expenditure (kj) is about 16% greater for females than males (17,985 vs. 15,569 kj) during laying; by the end of the season, however, only slightly higher for females than males (54,647 vs. 51,410 kj respectively; Nol 1985). In Virginia, daily intake of food (for breeders) appears to be about 343 g /d (Nol 1984). At fledging, both parents are expending an average of two times their cumulative daily basal metabolic rate to feed chicks (Nol 1985).

Never observed to stand over eggs during incubation; gular fluttering rare (EN).

In Virginia, food profitability at distant feeding areas 1.2–4.2 g wet weight/min (mean 2.4 g); range of flight time to distant feeding area 19.2–111.6 s (mean 46.7 s; Nol 1989). Average mass of the most common food item in Virginia (G. demissa) 6.17 g (Nol 1984). Handling time and foraging behavior (e.g., length of walk, etc.) vary depending on the food item, but are distinguished primarily by whether the prey is visible (e.g., oysters and mussels) or submerged in mud and not visible (e.g., razor clams); shorter walks and searching bouts, and less time pecking and fewer captures on submerged prey, compared to the large, visible oysters and horse mussels (Appendix 1).

Breeding birds spend 0.04%, 0%, 3%, and 0.4% of the daytime drinking during pre-laying, laying, incubation, and chick-rearing, respectively (Nol 1985), suggesting more drinking when the birds are inactive and sitting in the sun on the nest (EN). No significant differences between the sexes.