American Oystercatcher Working Group

Behavior

Author: Alex Wilke (AW), The Nature Conservancy, Virginia Coast Reserve, Nassawadox, VA

Walking, Hopping, Climbing

Frequently walks or runs rather than flying. Their counter-shaded plumage blends in surprisingly well in feeding habitat, particularly mussel and oyster beds, as well as on breeding habitat (B. Lauro pers. comm.). The sharp contrast of black and white in the wings is conspicuous in flight, and an important component of the Piping Display (see Sexual Behavior; Miller and Baker 1980). Oystercatchers can jump up (aided by wings) but are unable to climb.

Flight

See Sexual behavior. Normally deep, rapid wing beats. Shallow, slow Butterfly Flight used when intruders near nest (RH) or during territorial display and courtship (Tomkins 1954, RH).

Swimming And Diving

Adults rarely swim or dive to escape (Edwards 1870 in Samuels 1870). Observations of adults diving are typically associated with either capture activities (i.e. diving underneath deployed capture nets) or escape from aerial predators such as Peregrine Falcons (AW). Mobile young frequently dive to escape harassment (i.e. banding activities) or predators (Hayes and Bennet 1985, AW). When escape diving, young propel themselves underwater using wings; after a short distance they surface and swim (RH). Larger young are able to dive at least several meters deep and can swim underwater for at least 10m (AW). On the surface, young swim using feet and can swim for ≥ 400 m (Tomkins 1954, EN).

Preening, Head Scratching, Bathing, Anting, Etc.

Spend extensive periods preening and head scratching. Preening includes indirect head scratching with leg over wings. Breeding adults spend 7% and 8% of the time preening during the pre-laying and laying period, respectively (Nol 1985). During incubation males preen more than females (13.4% vs. 7.2%), and during chick-rearing both sexes preen about 3.5% of the time (Nol 1985), suggesting that preening occurs most when parents are least “busy.” Breeding birds spend about 0.2% of their time bathing throughout the breeding season (Nol 1985).

Sunbathing, Thermoregulation, Temperature

Gular flutter when hot (EN). Adults observed assuming shading postures over eggs and/or small chicks on particularly hot, calm and cloudless days (AW).

Sleeping And Roosting

Oystercatchers sleep with bill tucked under scapulars. Breeding birds “sleep” most during pre-laying (18.7% of time), 13.6% during laying, and least during chick-rearing (2.8% of time). During incubation, males sleep more than females (8.9 vs. 5.5%), while the other member of the pair is incubating (Nol 1985). This posture also associated with territorial and predator response (see Agonistic Behavior and Social and Interspecific Behavior).

Pairs often roost together on edge of intertidal feeding areas, either standing, with bill tucked or sitting. In winter, individual birds or small flocks roost on the edges of intertidal feeding areas during periods surrounding low tide, presumably when water height is not optimal for foraging or when birds may be experiencing a digestive bottleneck (AW). Wintering birds roost in flocks during periods surrounding high tide when foraging grounds are inaccessible. Roosting birds may stand on one leg particularly during low temperatures. Comprehensive aerial surveys of wintering flocks on the Atlantic and Gulf coasts of the U.S. in the winter of 2002/2003 documented flock sizes of 1-492 birds (Brown et al. 2005). Sanders et al. (2004) documented flocks sizes of 1-752 oystercatchers in coastal South Carolina between 1999 and 2002 with annual averages ranging from 26-37 birds. Maximum flock size documented in coastal Virginia during comprehensive surveys of wintering flocks between 2004 and 2010 was 475 birds (TNC and VDGIF unpubl. data). Roost site fidelity in Virginia appears to be high, with flocks consistently using a network of preferred roosts in addition to less frequently used alternative roosts (AW, RB). Models presented in Peters and Otis (2007) suggest that characteristics that best predict American Oystercatcher use of roost sites in the Cape Romaine Region of South Carolina are abiotic roost characteristics (specifically preferring shell substrate) and evidence of a weak avoidance of boats within 1000 m. Roost sites occupied by American Oystercatchers also remained relatively constant across the years of the study (Peters and Otis 2007). Summering flocks, presumably consisting of at least a portion of nonbreeding juveniles and/or adults, have been observed in Florida (≥100 birds), SC and VA (≥30 birds) (P. Leary, FS).

Daily Time Budget

During pre-laying, most time spent in standing (17%), sleeping (18.7%), or sitting (7%); only about 0.075% of the time in territorial defense; about 10% of the time in searching for food (Virginia; Nol 1985). During the laying period birds of both sexes spend less time standing or sleeping (33% combined) and from 26% to 39% of the time sitting on eggs. The amount of time in chasing and Piping is still small (0.044 vs. 0.083 for females and males, respectively). During incubation, females spend significantly longer incubating than males do (56.9% vs. 38.7%), and males spend more time resting and preening (Nol 1985). At nests with no recorded human disturbance during incubation, McGowan and Simons (2006) documented active incubation by both birds during 90% of the time the nests were videotaped. At other nests that experienced varying levels of human disturbance, filming documented incubation of the nests by both birds 82% of the time (McGowan and Simons 2006). The same study also showed that the time spent incubating by oystercatchers in coastal North Carolina was negatively correlated with ATV traffic but was not significantly affected by other forms of human disturbance, such as truck, pedestrian and airplane traffic and aggression with neighboring oystercatchers. During chick-rearing, both sexes spend about 24% of the time searching for food, 31–37% of the time standing on the territory, presumably being vigilant, and about 5% sitting, with no differences between the sexes (Nol 1985). In two regions of coastal South Carolina, combined adult attendance at nest sites during the chick-rearing phase and during low-tide foraging periods averaged between 81-88% percent of the time, suggesting that the remaining proportion of time was spent foraging to provision chicks (Thibault 2008). During chick-rearing, males chase other oystercatchers more than females do, and are chased more by other oystercatchers (Nol 1985).

Six sessions (726 instantaneous scan samples) of behavioral observations of one chick on Cumberland Island, Georgia documented a time budget of 48% at rest, 24% in locomotion, 19% foraging, 7% in self-maintenance, and 3% in parental care (Meliopoulos and Schweitzer 2010).

Physical Interactions

Both sexes defend territories but males are more aggressive (Nol 1985). Piping Display used in intra-specific interactions with neighboring pairs (common early in the breeding season), in courtship, and greeting (called “cocked tail display” in Kilham 1980, Miller and Baker 1980). Evasive behavior includes the bird throwing itself sideways against the ground to dodge an opponent’s bill during territorial conflict (Kilham 1980). They may also bob their head when disturbed. Displacement activities include pseudo-sleeping posture, in which bird stands on one or both legs with eyes open and bill hidden beneath scapulars, mock-brooding (see below), and grass-pulling (B. Lauro pers. comm.).

Nature And Extent Of Territory

Establish territories soon after arrival on breeding grounds. Size and spacing of territories varies according to habitat, with sandy beaches (e.g., Virginia) having larger territories than dredge spoil areas (e.g., New York, New Jersey; Lauro and Burger 1989, Humphrey 1990, Lauro et al. 1992). Densities range from 0.7 pairs/ha (Virginia) to 13 pairs/ha (New York; Lauro et al. 1992). Averages of 10.6 and 0.60 breeding pairs per kilometer of beach shoreline were documented on islands near the mouth of the Cape Fear River and on barrier beach habitat, respectively, in North Carolina (McGowan et al. 2005b). Mean nearest neighbor distances to individual breeding pairs in South Carolina were 200m, 107m, and 577m for barrier islands, estuarine islands and edge shell mounds, respectively (Sanders et al. 2008).

Interspecific Interactions

Oystercatcher nests have been documented in Common Tern (Sterna hirundo), Least Tern (Sterna antillarum), Gull-billed Tern, Royal and Sandwich Tern, Black Skimmer (Rynchops niger) and Brown Pelican colonies, which form after oystercatchers have initiated nesting. In many cases, oystercatchers maintain their territories within the colonies but are occasionally displaced. Although Herring (Larus argentatus) and Great Black-backed gull (Larus marinus) colonies are established prior to or concurrent with oystercatcher nest initiation, oystercatchers are often observed establishing nesting territories within and/or on the periphery of these colonies where the ranges of these species overlap (AW). In Massachusetts, oystercatchers generally nest closer to terns than to conspecifics (RH). Near Long Island, NY, a Canada Goose (Branta canadensis) displaced an American Oystercatcher pair with its nest. It renested only 1 m away (B. Lauro pers. comm.).

Oystercatcher depredation of the eggs of other shorebird and colonial waterbird species is occasionally observed (Burger and Gochfeld 1990), as well as the harassment of young terns, skimmers and Piping Plovers nesting nearby, particularly if disturbance causes the young to enter or cross through oystercatcher territories (AW). In Virginia, an adult oystercatcher was observed attacking and injuring an adult Piping Plover (PD).

Winter Territoriality

No territoriality in roosting flocks. Weak territoriality on foraging grounds (Cadman 1980).

Dominance Hierarchies

Appear to be dominance hierarchies within a brood, with one chick consistently being fed earlier than others, but whether this is always a particular chick in the hatch order is unknown (Nol et al. 1984). The second egg and chick is larger than the first or third, and so it may be more dominant (Nol et al. 1984). Dominance hierarchies exist in broods of H. ostralegus and H. bachmani (Safriel 1981, Groves 1984).

Mating System

Typically monogamous. Polygamy (polygyny; RH) and communal nesting (Chapman 1982, Lauro et al. 1992) recorded. Communal nesting (usually 1 male, 2 females) appears to be a consequence of high nesting densities and limited nesting habitat (EN). Lauro et al. (1992) observed communal nesting to be more common in New York than in Virginia. At least five cases of communal nesting observed on the Virginia barrier islands. Clutches of up to six eggs observed, often with distinguishable differences between eggs from different females (AW).

In Massachusetts, one female was documented having mated and successfully produced eggs with her offspring (E. Jedrey, pers. comm.).

Pair Bond: Courtship Displays, Copulation And Mate Guarding

Courtship Display . Similar to that of other oystercatchers except H. leucopodus (Miller and Baker 1980). Usually referred to as Piping. Begins with birds, usually two, walking parallel and uttering single pipe note. Birds then adopt posture with neck stretched forward and down, with back parallel to ground. Birds run side-by-side, with heads bobbing up and down, giving loud Piping Call (see Sounds: Vocalizations), which begins slowly, with notes becoming more rapid and changing in pitch. Birds frequently stop and turn 180 degrees, full circle, or at right angles and continue Piping Display (Fig. 4). Often birds take flight flying parallel or close together, both giving long Piping Call. At this point the displaying pair is often joined by residents from adjacent territories. This Piping “tournament” or “ceremony” may involve 3 or more pairs. Piping Ceremony may change back and forth from ground to air Piping Display. Flight is often with rapid shallow wing beats.

Formal Mating Sequence . Female adopts a stiff, stationary posture, bending forward with legs straight, neck drawn in, bill horizontal, and tail elevated. Male mounts, briefly waving wings, calling and pecking at female’s head, then lowers himself onto tarsi. Copulations usually last 1–2 s. Female appears to initiate all copulations (M. Cadman pers. comm.).

Duration And Maintenance Of Pair Bond

Pair bond may last as long as both birds survive (Palmer 1967), although few banded pairs have been observed for many consecutive years. Over 6 years of monitoring 23 banded pairs, male and female changes (or dies) in pair equally (EN). In 1 case female known to have moved to new mate, 1 case male known to have moved to new mate. Only 4 (17.4%) remained together for full 6 years (EN). In all other cases one or both individuals of a pair disappeared.

Extra-Pair Copulations

In 131 h of observation during pre-laying period, no extra-pair copulations observed (Nol 1985). Males and females within a pair highly synchronous in behavior: of 3,603 scans (1 min apart on 8 pairs), 3,326 (92%) showed both members of the pair behaving similarly and together on breeding territory, suggesting mate guarding. In addition, copulations nearly always occurred after Piping Display—after both male and female had been in the air, sometimes separated by other oystercatchers (EN).

Degree Of Sociality

Gregarious in winter, forming compact roosting flocks that disperse into loose flocks when feeding. Very vocal when greeting or departing. Synchronized nesting at one study area in Virginia (Nol et al. 1984). On Long Island, NY, groups of up to 100 seen in Aug just prior to migration (B. Lauro pers. comm.). Flocks of up to 200 staging/post-breeding oystercatchers have been observed in and around outer Cape Cod (Monomoy National Wildlife Refuge, South Beach, Chatham) and Nantucket, Massachusetts before migrating September through mid-October (E. Jedrey, pers. comm.).

Predation

On adults, mink (Mustela vison), red fox (Vulpes vulpes), skunk (Mephitis mephitis), domestic dog (Canis familiaris), cat (Felis domesticus), rat (Rattus rattus, Rattus norvegicus), American Crow (Corvus brachyrhynchos), Herring Gull, Greater Black-backed Gull, Peregrine Falcon (Falco peregrinus; RH,EN), Great-horned Owl (Bubo virginianus) and Snowy Owl (Nyctea scandiaca; T. French pers. comm.). Suspected predation of adult oystercatchers by coyote (Canis latrans), Northern Harrier (Circus cyaneus) and bobcat (Lynx rufus).

When eggs present, at approach of predators or humans, tending adult slips off nest, usually unseen, while intruder is still some distance away; walks rapidly away some distance before taking flight. Bird then exhibits distraction behavior, circling back over intruder in “butterfly” flight giving distress call (Tomkins 1954, Miller and Baker 1980, RH), sometimes beginning Piping Display. Different vocalizations used depending on whether adults are defending eggs or young (RH; see Vocalizations). May also demonstrate “cripple-display“ (Tomkins 1954), Crouch-run, or mock feeding. Birds often adopt mock sleeping attitude (see below) or mock brooding in an exposed location. In this posture, will allow predatory birds such as gulls to approach and rarely even touch them. Once displaced, oystercatcher will run some distance, then resume mock brooding posture again (RH).

Exact flush distances for incubating oystercatchers by predators and/or humans not well documented. Sabine et al. (2008) recommended no disturbance zones during incubation of at least 137m based on reduced reproductive behavior and increased vigilance and locomotion behavior of incubating adults exposed to pedestrian activity on Cumberland Island, Georgia. Vehicular disturbance within 300m changed reproductive behaviors of incubating adults but not significantly (Sabine et al. 2008). Buffer zones of 103m and 96m for personal watercraft and outboard powered boats respectively were recommended for oystercatcher foraging and loafing sites in coastal Florida (Rodgers and Schwikert 2002). In Virginia, incubating birds remained on the nest only 16 of 63 times (25%) when vehicles passed. Incubating adults did not react to non-human territory intruders (21 species of birds, mammals, and crustaceans) in 65.5% of 212 instances recorded (Denmon et al., in review).

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