Author: Jon Altman (JA), National Park Service, Cape Lookout National Seashore, Harkers Island, NC.
American oystercatchers feed almost exclusively on bivalves, mollusks, crustaceans, worms and other marine invertebrates that inhabit intertidal areas. In contrast, European Oystercatcher feeds at inland locations as well as intertidal locations when breeding (Heppleston 1972).
Foraging is restricted to intertidal sand or mud flats, oyster or mussel reefs and less commonly along rocky shorelines. Oystercatchers often feed along the edge of receding tide on sand or mud flats but in shellfish beds they forage while mussels or oysters are still submerged (see below). Distribution is thought to be limited by availability of intertidal areas supporting shellfish beds (Tomkins 1954). Foraging habitat may differ seasonally. In Virginia oystercatchers foraged on sand flats more in the autumn than in the winter (Tuckwell and Nol 1997b). During the non-breeding season a higher percentage of oystercatchers were seen foraging during rising and falling tide than at low tide (Cadman 1980, Hand 2008). During nesting, foraging is more frequent during a falling tide than during a rising tide (Sabine et al. 2008).
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 extracts 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 the 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 an oystercatcher’s bill becomes caught by shellfish and held so that the bird drowns with rising tide (Terres 1980). Oystercatchers also locate marine worms by probing in intertidal flats. Nocturnal foraging has never observed even during moon lit nights (Tuckwell and Nol 1997b). Searching times and diversity of prey eaten increased in response to declines in density of oysters (Nol and Tuckwell 1997b).
Oystercatchers experience both conspecific, (i.e., juveniles stealing from adults and other juveniles), and inter-specific kleotoparasitism. Inter-specific kleptoparasitism by gulls can depress intake rates and size of prey (Tuckwell and Nol 1997a). Primary interspecific kleptoparasites of oystercatchers in South Carolina were Willets (Hand et al. 2010).
Northern part of range (Massachusetts south to New Jersey): blue mussels (Mytilus edulis), ribbed mussels (G. demissa, Modiolus plicatus), soft-shell clams (Mya arenaria), surf clams (Spisula solidissima), stout razor clams (Tagelus plebeius), sandworms (Nereis pelagic), mole crabs (Emerita talpoida), razor clams (Ensis directus), hard clams (Mer cenaria mercenaria; (Post and Raynor 1964 RH, T. Virzi pers. comm.); Southern part of range (Virginia, N. Carolina, S. Carolina, Georgia, and Florida): oysters (Crassostrea virginica), soft-shell and razor clams, stout razor clams, ribbed mussels, mole crabs, polychaete worms, limpets (Aemaeu sp.), jellyfish (Coelenterata), sea urchins (Strongylocentratus sp. ), starfish (Asteria spp.), false angel wings (Petricola pholadiformis), northern quahog (Mercenaria mercenaria), blood ark clams (Anadara ovalis), coquinas (Donax variabilis),knobbed welk (Busycon carica) and crabs: hermit crabs, lady crabs (Ovalis ocellatus), speckled crab (Arenaeus cribrarius), (Bent 1929, Tomkins 1947, Cadman 1979, Johnsgard 1981, Nol 1989, Glatt 2002). In South Carolina in the autumn and winter, diet composition was 94% oysters and 4% mussels (Hand et al. 2010). Diet may change seasonally. In Virginia, oystercatchers foraged on mussels and oysters in autumn but primarily on oysters in winter. Oysters consumed in the winter were larger than oysters eaten in the autumn. Diet also changed during rising and falling tides (Tuckwell and Nol 1997b).
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) during laying; by the end of the season, however, only slightly higher for females than males (54,747 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). In South Carolina nesting areas adjacent to extensive shellfish reefs had higher daily survival of chicks (0.989+ 0.007) than nest sites with less extensive shellfish reefs (0.966+ 0.012). Brood success was positively correlated to extensive shellfish reefs adjacent to nest sites by allowing greater parental attendance and less commuting to foraging sites (Thibault et al. 2010).
Food items collected in South Carolina and Georgia had exceptionally high levels of vitamin A, levels that would be toxic to other species (Terry Norton, DMV, pers. comm.).
In Virginia, food profitability at distant feeding areas 1.2-4.2 g wet weight/min (mean 2.4g); 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) 6.17g (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). Foraging proficiency differs between immature and adults (Cadman 1980). During the non-breeding season in South Carolina immature oystercatchers had longer prey handling times, shorter searching times, and equivalent intake rates and diet composition compared to those of adults (Hand et al. 2010). Prey handling times increased with an increase in size of prey within prey type (Cadman 1980, Tuckwell and Nol 1997b, Hand 2008).
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 birds are inactive and sitting in the sun on the nest (EN). No significant differences between the sexes.