Oystercatchers are large, striking shorebirds found along coastlines around the world. Known for their bright orange beaks and bold black and white plumage, oystercatchers use their beaks to pry open mollusks like oysters, muscles, and limpets. Their diet, however, includes more than just shellfish. Oystercatchers are opportunistic feeders that consume a variety of prey. Understanding what oystercatchers eat provides insight into their behavioral ecology and the ecosystems they inhabit. This article explores the diet of oystercatchers in depth.
Shellfish
As their name implies, oystercatchers are specialized to feed on shellfish, particularly bivalves like oysters, muscles, and clams. The oystercatcher’s stout, chisel-like bill allows it to quickly cut through the adductor muscle that keeps bivalve shells closed. Oystercatchers insert their bill into the hinge between the two shells, make one quick slicing motion to sever this muscle, and the shell pops open, allowing access to the soft body inside. Oystercatchers consume shellfish from intertidal areas as well as offshore beds and reefs. They can ingest sizeable bivalves including large oysters and muscles. Shellfish are a primary component of the oystercatcher’s diet in most parts of its range. For example, during the non-breeding season, Eurasian oystercatchers (Haematopus ostralegus) in the Wadden Sea get 90% of their calories from mollusks. On the Atlantic coast of North America, American oystercatchers (H. palliatus) eat mainly oysters, mussels, clams, and other bivalves. The prevalent use of shellfish may be an ancestral trait throughout the oystercatcher lineage.
Worms
In addition to shellfish, oystercatchers consume a wide array of invertebrates, especially worms. Species known to be eaten include ragworms, lugworms, sandworms, and earthworms. Oystercatchers probe for worms in the mud or sand. Their long, sensitive bill can detect prey buried more than 10cm deep. Vibrations made by moving prey may also be detected. While probing the subsurface, the bill is opened slightly allowing water pressure to carry worms into the bill where they are trapped and consumed. Worms tend to be more accessible at low tide when mudflats are exposed. For example, Eurasian oystercatchers nearly double their worm intake during low tide compared to high tide. Worms provide an important supplementary food source, especially in areas where shellfish are less abundant.
Insects
Oystercatchers opportunistically feed on insects ranging from beetles, flies, and butterfly larvae to crickets, grasshoppers, and cicadas. They consume insects from maritime habitats including dunes, saltmarshes, and beaches as well as inland fields and pastures. Insects become a larger part of the oystercatcher’s diet during breeding season when insect abundance increases. For example, on Germany’s Baltic Sea coast, the proportion of insects in Eurasian oystercatcher chick diets rose from 3% in May to 83% in July as insect availability increased. Adults bring insect prey back to nests to provision growing chicks. Large insects including beetles and grasshoppers are readily transported in the bill. Consuming insects provides an important source of protein and other nutrients for developing chicks.
Crustaceans
Oystercatchers feed on a variety of crustaceans such as crabs, shrimp, isopods, and amphipods. These small aquatic animals are captured in shoreline habitats. For example, Eurasian oystercatchers forage for crabs during low tide along the edges of receding water. Oystercatchers may also probe for buried crustaceans in wet sand or mud. Crustaceans can occasionally dominate the diet. One study of American oystercatchers in Virginia found that mole crabs (Emerita talpoida), a small burrowing crustacean, accounted for 95% of prey biomass delivered to chicks. Crustaceans are a high-protein food source. Their consumption may increase during breeding season to meet the nutritional needs of growing chicks.
Fish
Small fish are occasionally consumed by oystercatchers. Species eaten include eels, blennies, sticklebacks, anchovies, and juveniles of larger fish. Oystercatchers may capture fish swimming in tidal pools or stranded in shallow water during low tide. They also plunge their head and bill into water to grab fish. Fish are swallowed whole and feet first to aid swallowing. The proportion of fish in the diet varies geographically and seasonally. A European study found fish comprised only 1% of prey items for Eurasian oystercatchers overall but reached 32% of diet at one Baltic Sea breeding site. Consumption of energy-dense fish may increase during chick-rearing when demand for food is high.
Birds’ Eggs
Oystercatchers occasionally eat the eggs of other bird species. Most documented cases involve consumption of eggs laid directly on the ground by species that nest in scrapes in sand or gravel such as plovers, terns, and gulls. Oystercatchers patrol the shoreline and may opportunistically devour eggs they encounter. Egg predation is most common early in breeding season before many chicks have hatched. For example, Eurasian oystercatchers in Wales were observed eating eggs of ringed plovers, common terns, and little terns in May and June but not later in summer after chick hatching. Predation on eggs provides oystercatchers with a nutritious source of protein, lipids, vitamins, and minerals.
Carrion
Oystercatchers have been observed scavenging on fish, crabs, and other animal carcasses washed up on shore or left behind by the receding tide. Carrion is not a dietary focus but provides an occasional supplement. In Germany, carrion accounted for less than 1% of Eurasian oystercatcher prey items. However, consumption of marine animals killed by harmful algal blooms or diseases and washed onshore could potentially expose oystercatchers to toxins in some cases. The ecological role of carrion feeding is likely minor for oystercatchers overall.
Seeds & Plant Material
While not a major component of the diet, oystercatchers have been documented consuming plant material including seeds. Eurasian oystercatchers breeding inland on grasslands in the Netherlands were found to ingest grass seeds which accounted for up to 8% of food brought to chicks. Seeds may provide an important alternate food source in such inland habitats. Consumption of marine plants including algae and eelgrass has also been noted on occasion. However, shellfish, worms, and insects make up the bulk of the oystercatcher diet in most populations.
Differences Between Species & Populations
The two oystercatcher species, Eurasian and American, exhibit some differences in diet composition. One study directly comparing the species found that mollusks made up a greater proportion of prey items for American oystercatchers (74%) compared to Eurasian oystercatchers (47%). Meanwhile, Eurasian oystercatchers consumed more annelid worms (39% of prey items vs. 10% for American). Still, both have highly similar overall diets focused on shellfish supplemented by worms, insects, and other marine invertebrates. Within a species, geographic differences in habitat lead to local variation in diet. Eurasian oystercatchers shift from a shellfish-dominated diet along the seashore to a more insect and worm-based diet when breeding inland. American oystercatchers eat proportionally more bivalves on the Atlantic Coast but more polychaete worms on the Gulf Coast where shellfish beds are limited. However, all populations consume a wide mix of intertidal invertebrates.
Variation Between Seasons
Oystercatcher diets undergo seasonal shifts, especially during breeding. At nesting sites, the proportion of larger, energy-rich prey items like shellfish and crabs increases to meet the nutritional needs of eggs and chicks. One Portugal study found shellfish and crabs comprised just 10% of prey items brought to Eurasian oystercatcher chicks even though these were the main prey taken by adults in the same area during winter. Adult oystercatchers selectively bring chicks bigger, more nutritious food items. The proportion of insects in chick diet also increases seasonally as insect abundance rises. Adults may consume insects opportunistically for their own needs but selectively provision more to chicks. So while the overall composition of prey species remains similar year-round, diet shifts toward chick provisioning during breeding.
Influence of Prey Availability
Geographic and seasonal differences in oystercatcher diet reflect variations in the availability of different prey types. When marine mollusks are abundant, they dominate the diet. But at inland sites or where shellfish beds are limited, oystercatchers eat more worms and insects. Individual oystercatchers show dietary flexibility and will take advantage of whatever prey is most readily accessible. Within a site, they shift toward consuming more worms during low tide when shellfish beds are less reachable. Reductions in common prey sources lead to increased dietary diversity. For example, mass mortality of intertidal shellfish from disease outbreaks may prompt temporary increases in consumption of alternate prey like worms. Overall, oystercatchers are adaptable, generalist foragers that exploit the most profitable prey in a given habitat at a given time.
Foraging Behavior
Oystercatchers utilize a variety of foraging techniques to locate and capture different prey types:
- Shellfish – Insert bill into gap between valves and cut adductor muscle.
- Worms – Probe bill into mud or sand to detect buried worms.
- Insects – Move bill from side-to-side in grass to flush insects.
- Fish – Plunge head underwater to grab fish.
- Eggs – Patrol shoreline and consume unattended eggs.
Specialized morphological adaptations like the chisel-shaped bill allow oystercatchers to access certain prey. However, they remain behaviorally flexible and employ different techniques as needed to take advantage of available food sources.
Digestive Adaptations
Oystercatchers have digestive adaptations to process their mollusk-heavy diet:
- Powerful jaw muscles to crush shells.
- Thick-walled gizzard for grinding up shells.
- Ability to expel sharp shell fragments through the bill.
While the gizzard and jaws allow shell contents to be digested, oystercatchers avoid ingesting entire shells which have low nutritional value. The internal processing of hard-shelled prey comes at an energy cost. Oystercatchers maximize energy intake by consuming soft-bodied prey like worms when available in addition to shellfish.
Energy Requirements
The daily energy expenditure of oystercatchers has been estimated at 400-500 kcal for an ~600g Eurasian oystercatcher. Shellfish provide an energy-rich food source. For example, common blue mussels may provide ~5 kcal per gram. Meeting daily energy needs would require consumption of ~100g of mussels. However, oystercatchers also expend considerable energy processing hard-shelled prey. The exact amount of food intake required is influenced by prey quality, foraging effort, digestive efficiency, and other factors. Consumption must increase during breeding season to meet the added energy costs of egg-laying and chick provisioning. Across seasons, oystercatchers aim to maximize energy gain by preferentially feeding on energy-dense, readily accessible prey.
Biomagnification of Contaminants
As predators of intertidal invertebrates, oystercatchers are susceptible to bioaccumulation and biomagnification of contaminants like heavy metals that become concentrated in coastal food chains. Analyses of tissues from Eurasian oystercatchers breeding in the United Kingdom found elevated levels of various metals including mercury, arsenic, cadmium, and lead compared to control levels. High concentrations of selenium and chromium were found in American oystercatchers from California. Common prey sources like filter-feeding mussels and algae can sequester metals dissolved in the water. Consumption moves these contaminants into oystercatchers which get exposed over years of foraging. High levels of accumulated metals could potentially impact physiology and reproduction. More research is needed, but diet exposes oystercatchers to hazardous substances present in coastal zones.
Importance as Prey for Other Species
As consumers of intertidal invertebrates, oystercatchers play an integral role in coastal habitats. In turn, they also serve as prey for a number of predators. Documented oystercatcher predators include various gull species, sea eagles, owls, foxes, mink, raccoons, and weasels. Nestlings and eggs are especially vulnerable to predation but adults may also be taken. Most predation happens during breeding season when oystercatchers are concentrated at nesting sites. For example, western gulls (Larus occidentalis) were found to preferentially prey on American oystercatcher eggs and chicks over other shorebirds in coastal California. Oystercatchers help support coastal food webs as both predator and prey.
Conclusion
Oystercatchers consume a varied diet centered on shellfish but supplemented by worms, insects, crustaceans, fish, and other prey. Their bill morphology allows specialization on mollusks but they remain behaviorally flexible and will exploit whatever foods are readily available in their coastal habitat. Geographic and seasonal differences in diet reflect variations in local prey assemblages and the nutritional needs of breeding. As important shorebird predators and prey, oystercatchers are an integral part of coastal ecosystem structure and function. Their dietary patterns provide insight into the ecology of intertidal zones in different regions. Further study of oystercatcher foraging ecology will aid conservation efforts for this distinctive shorebird.
