SPECIES

Bristle-thighed Curlew Numenius tahitiensis Scientific name definitions

Jeffrey S. Marks, T. Lee Tibbitts, Robert E. Gill, and Brian J. McCaffery
Version: 1.0 — Published March 4, 2020
Text last updated January 1, 2002

Behavior

Locomotion

Walking, Hopping, Climbing, Etc

Gait of foraging bird matches prey mobility; e.g., walks slowly when feeding on berries and quickly with short bursts when chasing spiders or lizards. On wintering grounds, may perch high in trees (Holyoak 1980, Kepler et al. 1994). On treeless breeding grounds, adult defending young may perch on slender branches of willow bushes to call at intruders; maintains balance by spreading tail and opening wings slightly.

Flight

Strong and powerful. Can overtake flying falcons (Falco spp.) when mobbing and outfly pursuing falcons in level and climbing flight. On breeding grounds, performs variety of flight displays (see below). Flocks seen over land fly in loose clusters, individuals changing position often within a cluster. Flight behavior during migration unknown (but see Distribution and Habitat: Migratory Behavior, above).

Swimming And Diving

Downy young swim across narrow (1–5 m), fast-flowing creeks.

Self-Maintenance

Preening, Head-Scratching, Stretching, Bathing, Anting, Etc

Preens, head-scratches, and stretches throughout day, often while roosting. Breeding birds bathe in portions of creeks or rivers that cross their territories and in snowmelt puddles on tundra. Wintering birds bathe in tide pools and at calm portions of ocean at shoreline. Drying facilitated by ruffling body feathers and drooping wings. Most baths followed by extensive shaking and preening. Not known to ant.

Sleeping, Roosting, Sunbathing

Sleeps and roosts on territories and in communal areas. Sleeps with tip of bill tucked into scapular feathers, either while standing, sometimes with 1 leg retracted, or while sitting. Members of pair on territory may roost hundreds of meters apart or adjacent to each other; female often sleeps while mate stands nearby on tussock. In communal areas, foraging birds pause frequently, particularly on hot days, to roost for short periods (10–20 min, but up to 5 h) in loose (e.g., 40 birds within 25 m2) or tight (e.g., 5 birds within 1 m2) groups (TLT, REG). Fall-staging birds form nocturnal communal roosts of up to 120 individuals; arrive at pond roost sites about 1 h after sunset and spend next 2–7 h there. Interindividual distance (usually ≥15 cm) maintained through aggressive posturing and quick jabs with bills (TLT, REG). No apparent segregation by age or sex.

During daytime, birds on Laysan I. roost communally at rock ledges on perimeter of island. Focal-animal samples (n = 1,014) of marked individuals at rock ledges Sep–Oct 1988 revealed that birds spent 49% of time roosting (i.e., standing still or sleeping), 30% preening, 12% walking, 3% drinking, 3% bathing, 2.5% foraging, and 0.5% in agonistic interactions (JSM). Older birds (≥15 mo old) spent >80% of time roosting or preening (n = 794), whereas yearlings (3–4 mo old) spent only 61% of time roosting or preening and devoted more time to other activities, including mandibulating potential food items (e.g., feathers, gastropod shells, old crab shells) that were never swallowed. Not known how much time individuals typically spend at communal roosts, but most marked birds visited at least once/day (JSM).

Daily Time Budget

No quantitative data apart from rock-ledge roost on Laysan I.

Agonistic Behavior

Physical Interactions

Aggressive interactions occur among breeding birds at multiple social levels, including between (1) territorial males or females and intruding males or females, respectively; (2) parents of neighboring broods; (3) pairs and local communal flocks; and (4) flockmates. Physical interactions occur in air and on ground. During aerial chases, defending bird vigorously pursues intruder while in rapid normal flight. Such chases may involve actual contact, including “goosing” (attacking bird contacts posterior ventral region of attacked bird with bill), wing-striking, and/or descending on top of intruder (either in midair, or as intruder lands). Ground chases often involve ritualized threat postures (see below), may include goosing, and occasionally escalate into actual fights.

Communicative Interactions

On ground, threat displays involve variety of postures and motions, often associated with chases; variation includes degree of crouching, extension of neck, ruffling of back feathers, tail-spreading, extension and elevation of wings, and (if wings extended) rate of flapping. Additional postures include (1) Tilted—hunched/crouched, tail spread and tilted (usually toward object of aggression), wing-tips drooped, and bill pointed toward ground (comparable to hunched-threat posture of Eurasian Curlew [Numenius arquata] in Cramp and Simmons 1983); (2) Arc-winged—wings open, spread laterally, and arced downward, tail cocked up and spread, bill pointed downward; (3) Sleeked—axis of head and body almost horizontal, neck retracted, plumage compressed (i.e., “sleeked”), resulting in thin, long-legged aspect; (4) Pot-bellied—head and neck upright, belly appears distended, resulting in very fat, short-legged aspect (comparable to similar displays in other scolopacids; see McCaffery and Gill 2001); posture often accompanied by “gliding” gait; (5) Upright—head high but neck not conspicuously extended, back slopes downward from anterior to posterior, wrists held slightly away from body with wing-tips held above tail, tail fanned and strongly depressed but not tilted laterally. As in Eurasian Curlew and other large scolopacids (Cramp and Simmons 1983, McCaffery and Gill 2001), territorial male Bristle-thighed Curlews engage in Parallel Walking (both along and away from territorial borders) and Vegetation Thrashing (i.e., tearing and tossing ground vegetation with bill). Threat displays often interspersed with bouts of ground chasing (or circling) and aerial chasing. During low-level aerial chases, chaser may glide with wings held over back in shallow V, while fanning tail and dangling legs. Rarely, territorial males and intruders engage in long parallel flights similar to those between rival males in Bar-tailed Godwit (see McCaffery and Gill 2001). These flights resemble Courtship Flights (see Sexual behavior, below), but after landing, birds engage in Parallel Walking and ground chases initiated by defender (BJM, TLT, REG).

Threat displays much less frequent away from breeding grounds. Arc-winged posture assumed by roosting birds on Yukon Delta staging grounds as new birds arrive at roost (TLT); threats apparently keep new arrivals at periphery of flock. For additional agonistic behavior during nonbreeding season, see Spacing, below.

Spacing

Territoriality

Nature and Extent of Territory. Size of breeding territories variable (see Demography and Populations: Range, below). Breeding males establish large exclusive territories (i.e., attempt to drive off conspecific intruders). Males initiate Territorial Flight Displays (see below) immediately upon arrival in spring. Display activity most intense over next 11–12 d, especially in unpaired males, but continues through early brood-rearing. Most activity of pairs occurs on territory before clutch completion. Once incubation commences, off-duty bird may spend extensive time foraging off territory; such behavior particularly prevalent on Seward Peninsula. After hatching, territorial defense wanes as broods move off territory; no evidence that male returns to defend territory once brood moves elsewhere. Failed breeding males, however, display on territory even after mate leaves, and along with unpaired males continue to display at least into early brood-rearing period.

Territorial boundaries flexible, particularly early in season and during incubation. Earliest males may commence wide-ranging displays centered on previous year's territory. Within days (even hours), however, other males arrive and begin displaying; display areas of first males contract in response, usually with little overt aggression. Males arriving after first week or so, however, may fight to insert themselves into territorial array. Later in season, incubation duties preclude full-time territorial defense by nesting males. While such males are incubating, adjacent neighbors may display over, and actually engage in border disputes on, incubator's territory; when off duty, intervening males re-establish control over disputed area (BJM, TLT, REG).

Manner of Establishing and Maintaining Territory. Males perform Territorial Flight Displays to establish and maintain territories. As in other Numenius (Skeel 1978, Redmond 1984), male uses rapid, shallow wing beats to ascend to display altitude. He frequently gains altitude by flying a short distance above a ridge and gliding out over adjacent valley and also by soaring on thermals and/or catching updrafts over ridges. May climb as high as 150 m above ground level (agl), but when taking off from level or gently sloping terrain, peak altitudes usually 5–20 m. Upon reaching apex of ascent, commences gliding, either at constant altitude or descending slowly on fixed wings held parallel to ground but usually above plane of body; sometimes move head from side to side during glide. As male scans territory, neck often conspicuously kinked with head held below body. Rarely, birds kite on steady updrafts after reaching display altitude. Display typically accompanied by vocalizations (mostly Song), which do not usually commence until start of gliding phase. Display may include several Songs and last >10 min before curlew continues shallow descending glide to landing (BJM).

Territorial Flight Displays by males may be given spontaneously or in response to conspecifics and/or potential predators, including, when courting female (see below), after transient curlews pass through established territorial array, after intruding curlew has been successfully chased away, and, early in breeding season only, when male returns to territory after mobbing predator. Territorial Flight Displays frequently trigger bouts of ground singing and/or flight displays by neighboring males that can evolve into parallel flights along common borders (as in Pectoral Sandpiper [Calidris melanotos]; Pitelka 1959).

Members of established pairs, either singly or together, vigorously chase intruding curlews out of territories (BJM, TLT, REG). If female attacks alone, intruder is usually female. Occasionally, late-arriving females encounter new female on territory and with mate of previous year; in all cases (n = 3), late arrivers (i.e., previous mate and territory occupant) successfully drove off new females, but only after violent fights lasting nearly 1 h (TLT, REG). Females almost never perform typical Territorial Flight Display (BJM).

Interspecific Territoriality. In Nulato Hills, Bristle-thighed Curlews and Whimbrels often occupy extensively overlapping territories. Exclusive interspecific territoriality absent in this situation, but aggression between 2 species frequent and usually dominated by Bristle-thighed Curlews; e.g., curlews initiated and prevailed (i.e., drove off Whimbrels) in 20 of 22 interactions observed at Curlew Lake. Most cases involved curlews running or flying at and displacing Whimbrels. Occasionally, defender (of either species) chases congener in Arc-winged posture. Rarely, ground Songs and/or Territorial Display Flights in one species trigger comparable display in congener (BJM).

Winter Territoriality. Uncommon in Northwestern Hawaiian Is., typically involving defense of local food source (e.g., crab, carrion) rather than large all-purpose foraging area; conspecific home ranges overlap considerably (JSM). On Laysan I. in Oct 1994, BJM saw territorial behavior by 2 of ≥100 individuals that did not involve defense of local food source. Territorial birds engaged in repeated aggression toward intruding conspecifics; regularly assumed Tilted (Hunched) posture, gave Complex and Shrill whistles, and sidled rapidly toward intruders; latter behavior evolved into head-long chases and Spread-winged Rushes (see McCaffery and Gill 2001 for description in Bar-tailed Godwit).

Dominance Hierarchies. Not known.

Individual Distance

On breeding territories, pair members usually forage and roost in close proximity to each other, but sometimes hundreds of meters apart. Breeding birds in communal flocks associate casually; walk, forage, and roost 1–5 m from each other and form tight flocks when predator present but, conversely, rush at and jab nearby curlews. Participants in brood aggregations exhibit similar behavior; can be within meters of each other or widely spaced (TLT, REG, BJM). On wintering grounds, birds stand close together (<0.5 m) at rock-ledge roosts, especially when large flocks form in late Apr and early May just before northward migration (Marks and Redmond 1994b). Typically forage singly, but occasionally in small groups (2–6), some individuals foraging <3 m apart (JSM).

Sexual Behavior

Mating System And Sex Ratio

Socially monogamous. Sex ratio for entire population unknown; appears equal at Neva Creek (TLT, REG) and possibly is male-biased in Nulato Hills, based on presence of unpaired males through early brood-rearing period (BJM).

Pair Bond

Courtship Displays and Mate-Guarding. Relationships among courting Bristle-thighed Curlews variable. Experienced territorial males will court mate from previous year within minutes of female's arrival on territory; however, same male may court new female prior to previous mate's arrival. Females without previous local breeding experience range widely, apparently sampling males by visiting territories and eliciting courtship behavior. Males with and without previous local breeding experience may court but fail to pair with new females. Following divorce from previous year's mate, one experienced male courted ≥3 different females before fourth nested with him. Factors affecting females' pairing decisions unknown; among known pairs, previous year's success does not predict between-year mate fidelity (i.e., successful pairs may divorce, and unsuccessful pairs may re-mate; BJM, REG, TLT).

Courtship behavior involves aerial and ground displays. In addition to Territorial Flight Display (see Spacing, above), 4 types of aerial displays identified: (1) Paired Courtship Flights. Generally occur before establishment of pair bond and involve male and female flying around male's territory in long, sinuous curves, usually <5 m agl; male or female may sing. Flights can cover hundreds of meters and may incorporate Crazy Flights; either sex may lead, or birds may be roughly parallel (BJM). (2) Flutter-Hover. When Paired Courtship Flights end with female landing, male descends and circles female 1–2 times at 3–4 m agl, with wings fluttering (i.e., “helicoptering” with rapid, shallow wing beats above plane of body). When male lands, he often approaches female on foot and waves wings over his back (BJM). (3) Shallow Undulating Flight Display (Shallow UFD). Analogous to “bounding-SSK flight” (Allen 1980b) and “undulating flight display” (Redmond 1984) in Long-billed Curlew. Most commonly used by territorial males early in pairing process, Shallow UFD involves sequence of repeated powered ascents and gliding descents, usually performed ≤10 m agl, with undulations generally spanning ≤3 m. Wing-beats during ascent phase similar to powered ascents in Territorial Flight Displays, but ascent briefer in duration. Flight path of Shallow UFDs may be linear or circular. Perimeter of circular displays may encompass several hectares, with female at center. When Shallow UFD performed over gently descending slopes, gliding descents are of longer absolute and relative duration than powered ascents; male may cover tens of m during single descent phase, gliding <1 m agl (BJM, REG, TLT). (4) Brief Flight Displays. Comparable in form, but not function, to single cycle of Long-billed Curlew Undulating Flight Display (Fig. 2 in Redmond 1984). Once pair bond established, both sexes perform Brief Flight Displays, in which curlew ascends to low apex (2–5 m agl) and then descends immediately in a shallow glide, usually accompanied by Song. Typically given when 1 pair member rejoins mate after foraging elsewhere (usually out of sight). Shallow UFD and Brief Flight Displays may incorporate Crazy Flights into descending glides (BJM, REG, TLT).

Ground displays similar in many respects to those of Long-billed Curlew (general descriptions and nomenclature from Allen 1980b and Redmond 1984; distinctive elements in, or displays unique to, Bristle-thighed Curlew described below). (1) Ground-calling. Performed primarily by unmated male as he Whines from atop conspicuous tussocks or hummocks (as in Whimbrel; Skeel 1978); patterns of mild frequency and amplitude modulation in vocalization seem similar to Long-billed Curlew (see Allen 1980b), but Bristle-thighed Curlew does not assume appeasement posture (BJM). (2) Scraping. As in other scolopacids, commonly performed by male in presence of female, from initial courtship through at least early egg-laying. Less frequently, unpaired males Scrape on territory. Among paired males prior to clutch initiation, Whine most common vocalization associated with Scraping (29 of 31 observations), rarely accompanied by Complex Whistles (2 of 29). In 1 of 2 observations of Scraping at nest where egg-laying had commenced, paired male gave Complex Whistles and Chiu-eet s but did not Whine. Female approaches male in nest scrape in nearly half of Scraping displays (n = 18 of 41 complete observations). Approaching female may walk past Scraping male (n = 8), stop adjacent to Scraping male (n = 3), or enter scrape (n = 7). Stops adjacent to male range from 25 to 30 s; if female enters scrape, she usually remains 1–3 min. Once clutch initiated, bouts of Scraping and/or Tossing (see below) often longer (≤1 h) and more clearly associated with nest construction proper; often involving both birds at nest simultaneously (BJM, REG, TLT). (3) Tossing. Male and female Toss vegetation into nest scrape, but relative importance of behavior as display versus functional nest-lining unclear. Male usually Tosses while crouching over or standing next to nest scrape; may perform Tossing when up to 1 m upwind of scrape. Female may Toss while standing at or sitting in scrape (BJM, REG, TLT). (4) Pointing. Not described in other North American Numenius (Allen 1980b, Skeel and Mallory 1996), but common in other scolopacids (e.g., Byrkjedal et al. 1989). When female approaches Scraping male, male usually (8 of 14 observations) steps out of and stands adjacent to scrape and assumes Pointing posture in which he tilts forward with his bill pointed toward ground. Male usually motionless while Pointing, but may perform Choking. Choking includes repeated downward jerks of head, with return to initial position following each one, or may involve series of repeated upward jerks in which bill and head are gradually lifted out of typical Pointing posture. Male may remain Pointing for up to 20 s after female leaves scrape; when Point maintained after female departure, male usually does not follow female for further courtship (BJM). (5) Tilted. Typically used in aggressive interactions (see Agonistic behavior, above), and observed once in courtship context. Male Scraped and then Pointed as new female (i.e., not mate from previous 2 years) approached but did not enter scrape. After brief Point, male Tilted with body, drooped wing, and fanned tail, all tilted toward female. She ran off, and after a brief flight, he ran after her with waving wings (see below); she flew off and he pursued (BJM). (6) Pot-bellied Pursuit. Similar to aggressive display (see Agonistic behavior, above) and also observed once in courtship. Male began walking toward mate of ≥3 consecutive years in a crouched, flat-backed, pot-bellied posture; unlike similar threat behavior, head and neck held low at level of back. When male closed to 1 m, female walked off, tracing sinuous path as male followed for 30 s. She then flew, and he followed, performing 4 Crazy Flights (BJM).

Mate-guarding poorly quantified. Pairs spend vast majority of time together during prelaying period. When together on territory, paired birds may be >100 m apart and out of sight of each other. Frequent vocalizations serve to maintain contact. Paired males chase off intruding males, but such chases, as well as wide-ranging Territorial Flight Displays, may leave female alone for minutes at a time. Response of paired males to predators suggests important additional function of proximity to mate is to warn her of approaching danger (BJM, REG, TLT).

Copulation. Occurs on territories, first observed 1–7 d after arrival in spring (TLT, REG). Scraping and Pointing often segue into more advanced courtship behavior; 4 of 5 attempted mounts and 6 of 10 successful mounts preceded by Scraping and/or Tossing. Once female leaves scrape, male may remain Pointing (n = 3), leave scrape and resume feeding or preening (n = 6), or pursue female (n = 10); 9 of 10 pursuits led to further courtship (BJM, REG, TLT). Male approach to female comparable to Courtship-Run of Long-billed Curlew (Allen 1980b), but male's bill may be pointed slightly upward. As in other large shorebirds (Allen 1980b, Skeel and Mallory 1996, McCaffery and Gill 2001), approaching male usually (17 of 18 cases) begins waving his wings, particularly if female slows or stops (BJM, REG, TLT). As male approaches rear of female, he often performs display analogous to “shaking” in Long-billed Curlew (Allen 1980b). Male Bristle-thighed Curlew may peck at female's tail, back, or sides, the pecking sometimes grading into rhythmic stroking of female's sides (R. Redmond pers. comm., BJM, REG, TLT). Simultaneously, male may begin prancing behind female, often describing semicircular arc behind her just prior to mounting. Eight of 10 successful mounts, but only 2 of 6 attempted mounts, included either Shaking and/or Prancing. Receptive female leans forward, but head may be either upright or lowered beneath plane of body; rarely, female may sit for mounting. Male flutters up and lands gently on female's back; often treads conspicuously for portion of mount. Vocalizations during precopulation display and mounting extremely variable; Whines most frequent (but see Sounds and Vocal Behavior: Vocalizations, above). Mounts average 57 s ± 27.8 SD (range 18–95, n = 9); cloacal appositions (up to 2/mount) last 2–10 s. Male rarely grabs feathers on head or neck of female when mounted. During mount, female may alternate bouts of standing and walking; latter often triggers dismount of male (BJM, REG, TLT).

Duration and Maintenance of Pair Bond. Within a season, successful pairs at Neva Creek remain together through early brood-rearing (Lanctot et al. 1995) and unsuccessful pairs for 1–3 d after loss of eggs or young; at Curlew Lake, unsuccessful pairs may remain on territories for nearly 4 wk (BJM). Among 4 yr each at Neva Creek and Curlew Lake, 67–100% of pairs (n = 3–15 marked pairs/yr) reunited following year and 0–13% divorced; 0–20% changed mates when previous mate did not return; TLT, REG, BJM). Pairs not observed together in fall or winter, but sample sizes small; 3 banded birds from Neva Creek observed on Yukon Delta without mates (TLT, REG), and 6 banded birds from Curlew Lake seen on Laysan I. without mates (JSM).

Extra-Pair Mating Behavior

Early in season, territorial males court and attempt to copulate with females to which they do not end up paired; trespassing males will court resident male's mate until driven off by territory holder (BJM). Not known if these interactions lead to copulations. At 1 site in Nulato Hills, flocks form after territorial establishment but before end of egg-laying (BJM); for details of similar flocks slightly later in season see Social and interspecific behavior, below. Not known if such flocks include fertile females away from their mates and territories. Needs study.

Social and Interspecific Behavior

Degree Of Sociality

After clutch completion, off-duty breeders travel to traditional sites away from territories to forage and roost in loose flocks (typically 6–12 birds) for several hours at a time. Flock sizes peak (sometimes 75–100 birds) around hatching in late Jun as early failed breeders mix with off-duty birds. Smaller flocks composed of deserting parents and late failed breeders persist into early Jul. At Neva Creek, flocks form on upper slopes and ridge tops of 5 main hills in the 93-kmstudy area; however, over 5-yr period, 1 ridge top received several times more curlew use than others. Banded individuals travel 0.5–7.0 km one way to visit sites, and both members of an incubating pair visit on alternate schedules. Flocks aggregate loosely, i.e., sometimes birds flush together, but often some stay while others flush. Birds that arrive together often do not depart together, and small flocks coalesce after hazing a predator.

Flocks generally form uphill of established territories, but territorial pairs will chase flock members. Flock members often fly toward overflying predators, giving Complex Whistles and Whines but not attack-mobbing. When nearby adults with broods attack-mob predators or humans, flocked birds often fly toward or land near source of disturbance (TLT, REG).

Migrants on Yukon Delta occur as singles or couplets in spring and in flocks averaging 3.1 birds ± 0.39 SE (range 1–33, n = 148) in fall (Handel and Dau 1988). Form flocks year-round at rock-ledge roosts on perimeter of Laysan and Lisianski Is., with numbers peaking (80–160 birds) during spring and fall migration periods; flocks also form during same periods at water-catchment area on runway at Midway Atoll (JSM).

Play

Not reported.

Nonpredatory Interspecific Interactions

Bar-tailed Godwits, Whimbrels, and Long-tailed Jaegers often within curlew flocks on breeding grounds. Heterospecifics associated with 60% of curlew broods and brood aggregations at Neva Creek, most common associates listed in order of occurrence: Bar-tailed Godwit, American Golden-Plover, Pacific Golden-Plover (Pluvialis fulva), Whimbrel, Long-tailed Jaeger, and Western Sandpiper (Calidris mauri; Lanctot et al. 1995). Adults of all these species jointly defend young in brood aggregations but also aggressively chase each other away from chicks (see above). These species forage within meters of adult curlews in nesting territories and at communal foraging areas (TLT, REG), generally without antagonism. Infrequent observations of Willow (Lagopus lagopus) and Rock (L. mutus) ptarmigan and Lapland Longspur (Calcarius lapponicus) displacing foraging curlews, and conversely, of territorial male curlews chasing Willow Ptarmigan from vicinity of foraging mate (BJM, TLT).

Shorebirds that nest within curlew territories at Neva Creek include American and Pacific golden-plovers, Bar-tailed Godwit, Whimbrel, Wilson'sSnipe (Gallinago delicata), and Western Sandpiper. Not known whether proximity to curlews is due to similar habitat preferences or response to curlews themselves, but aggressive interactions do occur between curlews and these species (see Agonistic behavior, above). Avian predators of adults, eggs, and chicks also breed adjacent to or within curlew territories, including Parasitic (Stercorarius parasiticus) and Long-tailed jaegers, Rough-legged Hawk (Buteo lagopus), Northern Harrier (Circus cyaneus), Merlin (Falco columbarius), and Short-eared Owl (Asio flammeus). Not known if such nesting proximity generally detrimental for curlews, but curlews usually interact aggressively with these potential predators. In Nulato Hills, curlews often place nests <100 m from Short-eared Owl and Long-tailed Jaeger nests. Curlews appear to benefit from such associations via active nest defense from other predators by jaegers and owls; 4 of 5 nests placed near defensive species hatched versus only 5 of 15 nests placed >100 m from defensive species (McCaffery and Gill 1992, BJM).

During fall staging on Yukon Delta, only 1.4% of 350 flocks contained Bristle-thighed Curlews and Whimbrels (Handel and Dau 1988), although many observations of curlews foraging in tight flocks with Mew Gulls (Larus canus) and Long-tailed Jaegers (TLT).

In Northwestern Hawaiian Is., communal roosts at rock ledges often attended by Pacific Golden-Plovers, Wandering Tattlers (Tringa incana), Ruddy Turnstones, and Brown Noddies (Anous stolidus), and occasionally by other species (e.g., Wedge-tailed Shearwater [Puffinus pacificus], Bar-tailed Godwit) with no overt signs of aggression (JSM). May forage in same area with other shorebirds without interaction, but chases Ruddy Turnstones from local bonanzas such as carrion pile and crab carcass (JSM). Black-footed Albatrosses (Phoebastria nigripes) landing among staging flocks occasionally trigger migratory departures (Marks and Redmond 1994b).

Predation

Kinds Of Predators

During migration periods in Alaska, only 2 instances (1 spring, 1 fall) of predation documented, both involving adults taken by large falcons (J. Wright pers. comm., TLT, REG). On breeding grounds, known predators of adults include Gyrfalcon (Falco rusticolus); of eggs, Parasitic Jaeger and Common Raven (Corvus corax); and of chicks, red fox (Vulpes vulpes), Northern Harrier, Gyrfalcon, Sandhill Crane (Grus canadensis), and Long-tailed Jaeger (Lanctot et al. 1995, BJM, REG, TLT).

Aside from take by humans, no known observations of predation on wintering grounds. Presumably, no terrestrial predators occurred before settlement of Oceania by humans, but now potentially preyed upon by human commensal animals, especially cats and dogs; curlews also vulnerable to predation by diurnal raptors that occasionally reach remote islands. Rough-legged Hawk present from early winter to late spring on Laysan I. ate terns but apparently no shorebirds (JSM).

Manner Of Predation

Pairs of Parasitic Jaegers and Common Ravens may quarter over curlew nesting habitat looking for eggs or chicks. When curlew focuses attack on one member of predator pair, other member approaches nest to take contents. In one case, pair of Parasitic Jaegers dragged incubating curlew off nest (C. Harwood pers. comm.). Long-tailed Jaegers hover above young chicks and grab them with bills, and red foxes crisscross brood area sniffing for hidden chicks (BJM, TLT, REG). Northern Harrier that disregarded mobbing parents flew into brood-aggregation area and circled for about 3 min, apparently waiting for fledglings to flush or move, then attacked fledglings (J. Pearce pers. comm.).

Response To Predators

Adults respond defensively and/or aggressively to most potential avian predators, including Sandhill Crane, Osprey (Pandion haliaetus), Bald Eagle (Haliaeetus leucocephalus), Northern Harrier, Northern Goshawk (Accipiter gentilis), Red-tailed [Harlan's] Hawk (Buteo jamaicensis), Rough-legged Hawk, Golden Eagle (Aquila chrysaetos), Gyrfalcon, Merlin, jaegers, Short-eared Owl, Snowy Owl (Bubo scandiacus) and Common Raven (McCaffery and Gill 1992, BJM, REG, TLT). Curlews on nests rely on cryptic coloration to avoid detection; off nests, they actively scan for predators and crouch slowly when a raptor flies overhead and give alarm calls when attacked (presumably to warn mate; McCaffery and Gill 1992).

During breeding, response to predators depends mostly on phase of cycle and predator species involved (McCaffery and Gill 1992, Lanctot et al. 1995). In general, curlews respond by tracking aerial predators visually without vocalizing or moving (e.g., Osprey, Harlan's Hawk); squatting silently (e.g., some raptors and jaegers); calling from ground (e.g., Sandhill Crane, Bald Eagle, Golden Eagle); flushing in response to predators that pass nearby (e.g., Northern Harrier, Northern Goshawk, Golden Eagle, Merlin); feigning injury to entice predators away from nests or chicks (e.g., red fox, humans); or approaching directly, circling above predator while alarm-calling (e.g., red fox, Gyrfalcon); and attack-mobbing (BJM, TLT, REG). Parasitic Jaegers and Common Ravens may elicit any of above responses. When adults on ground are attacked by Gyrfalcons or red foxes, escape by flushing with unidirectional climbing flight (BJM).

Response to natural predators by incubating birds seldom observed; general tactic is for curlew to sit tight until attack imminent (McCaffery and Gill 1992). In response to approaching humans, median and modal flushing distance 2 m and 1 m, respectively (n = 33 approaches). Curlews flushed from nest usually depart silently with wings spread and tail fanned widely and slightly depressed, revealing bright rump and upper tail. Retreat may be in slight crouch, and wings may be waved slowly at level of body; less frequently, wings drooped with tips nearly scraping tundra. Typically, flushed curlew runs about 10 m, folds wings and tail, and begins alarm-calling; may then follow and/or lead intruder from nest in generally upright posture.

From about 1 to 2 d before hatching through the middle of brood-rearing, adults exhibit aggressive antipredator behavior, attack-mobbing humans, Northern Harriers, Rough-legged Hawks, Merlins, Parasitic Jaegers, Long-tailed Jaegers, Mew Gulls, and Common Ravens. Most avian predators attacked when they fly within 400–500 m (up to 1,000 m) of the brood. Behavior different for red fox; adults circle above foxes, make repeated shallow dives (carefully remaining out of striking distance of fox), and give frantic alarm calls (usually Chiu-eet calls and Shrill Whistles). Foxes and humans generally responded to when they come within 200 m of young brood. When foxes or humans in close proximity of brood, adults approach closely, drop to ground, feign injury, and make several short runs away, presumably to entice predator to follow (Lanctot et al. 1995, BJM, REG, TLT).

From 2 to 5 wk after hatching, frequency and intensity of attack-mobbing progressively lessens; at fledging, adults seldom attack-mob but stand on prominent landscape feature near chicks and alarm call (Lanctot et al. 1995, BJM, REG, TLT).

Recommended Citation

Marks, J. S., T. L. Tibbitts, R. E. Gill, and B. J. McCaffery (2020). Bristle-thighed Curlew (Numenius tahitiensis), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.brtcur.01