Buller’s Shearwater

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P5. BULLER’S SHEARWATER
Ardenna bulleri L 43–45.5 cm, WS 96–104 cm, tail 115–136 mm (graduation 50–60 mm), bill 38–45 mm Figures 29–31, 46, 65, P4.13, P5.1–P5.11, P7.1, P7.11, P8.14, P37a.1

Identification Summary Pacific. A fairly large, smartly patterned, and ostensibly unmistakable shearwater. Gray above with clean-cut dark cap, blackish M pattern, and pale-silvery panel on the greater coverts; bright white below. Flight buoyant and graceful, usually fairly low over the water.

Taxonomy Monotypic. Names For Ardenna, see under Sooty Shearwater; bulleri and the English name commemorate the New Zealand ornithologist Sir W. L. Buller (1838–1906).

Status and Distribution Vulnerable. Breeds (late Nov–Mar) in n New Zealand, ranges to temperate Pacific waters off South America (mainly Mar–Jul) and North America (mainly Aug–Oct). Migration routes and location of prebasic molt not well known; birds in wing molt off Chile in Mar–Jul perhaps prebreeding immatures. Pacific. Generally a fairly common nonbreeding visitor off the West Coast (mainly Sep–Oct). Departs New Zealand in Feb–Mar and moves n to subarctic waters of n Pacific (mainly 40–50°N) by Jun–Jul (Wahl 1985); spreads e through Aug but mainly well offshore, whence adults return to New Zealand by Sep. Rare to uncommon and irregular off West Coast in mid-Jun to Jul, increasing in Aug (occasional influxes off cen California in mid-Aug may be staging adults; pers. obs.1), with main arrival (nonbreeders) in Sep. Peak numbers off West Coast mid-Sep to mid-Oct, with stragglers into Dec, exceptionally Jan. Considerable interannual variation in numbers related to oceanic conditions; numbers off Washington decreased markedly after 1977 (Wahl and Tweit 2000), but off cen California may have increased since late 1980s, perhaps linked to an overall ocean warming trend. Farther n, considered uncommon or rare off s Alaska (late Apr to late Sep), but few observers in that region. Some northbound birds (juveniles and prebreeding immatures?) may wander n along Pacific coast of the Americas, accounting for Jun–Sep records off n Middle America, in Gulf of California, and a sick bird at Salton Sea in Aug 1966. Atlantic. Exceptional (Oct 1984) off New Jersey.

FIELD IDENTIFICATION

Similar Species Unlikely to be confused if seen well, but at a distance or in certain lighting conditions several pitfalls exist. In bright sun, beware of the effects of lighting on larger and heavier-bodied Pink-footed Shearwater (which see). On the water, sharply black-capped and white-sided appearance can suggest stockier Great Shearwater and even Manx Shearwater; Great is browner overall with deeper cap, more extensively white hindneck, scaly brown back, dark undertail coverts; Manx is smaller (but size harder to judge on swimming birds than on flying birds) and more compact with shorter bill, blackish upperparts. Light morph Wedge-tailed Shearwater in fresh plumage can have frosty-edged greater coverts, pale-edged scapulars, darker M pattern above, and even dark-capped appearance, all suggesting Buller’s; note browner upperparts of Wedge-tailed, deeper cap to well below eyes, and dingier underparts with broad dark wing margins and dark undertail coverts. Plumage pattern of Buller’s suggests various gadfly petrels, especially much smaller Stejneger’s, but upperparts of Buller’s more contrasting, with broad pale panel on greater coverts; gadfly petrels relatively larger headed (the larger species with thicker black bills) and fly with more strongly arcing flight. Stejneger’s is small and snappy vs. large and leisurely Buller’s. Overeager observers could confuse Buller’s with Cook’s Petrel, but this brain fart should be fleeting given the very different sizes and flight manners of the two species. Juan Fernandez Petrel similar in size but has longer and narrower wings, white forehead, blackish eye patch, and more uniform upperwings (gray back is palest area of upperparts, vs. broad pale greatercovert panel on Buller’s); note thicker black bill and stronger flight.

Habitat and Behavior (see Fig 29, p. 27) Pelagic, favoring warm-temperate waters from shelf to offshore, and often concentrating at temperature fronts. Singles may be seen alone or among flocks of other shearwaters, especially Pink-footed, but more often gregarious, found in loose to fairly tight aggregations feeding over the ocean or rafting in flocks that number tens to hundreds of birds (occasionally thousands in Sep–Oct). Feeds by wheeling, dashing, and skittering over the surface, seizing prey near the surface and rarely if ever submerging completely. Notable for aerobatic “ballets,” whereby tens to hundreds of birds wheel in formation, alternately flashing uppersides then undersides. Flight buoyant and generally low, with shallow, slightly flicking wingbeats and frequent glides on wings pressed slightly forward and crooked. Even in moderate to strong winds does not usually wheel very high or steeply, unlike heavier-bodied Pink-footed Shearwater and gadfly petrels.

Description A fairly large, lightly built shearwater with fairly long neck and small head, broad-based wings, and medium-long, distinctly graduated tail; low wing-loading. Adult. Upperparts gray with dark slaty cap, black tail (central rectrices relatively truncate), and bold blackish M pattern with contrasting palegray outer greater coverts, which in fresh plumage often appear as broad silvery panel on upperwings. In gloomy conditions, contrast on upperparts subdued but still apparent at moderate range. Patterns and contrast notably duller when plumage worn and brown-toned (mainly Mar–Apr, when unrecorded in the region). Throat, foreneck, and underparts clean white; narrow black trailing edge to underwing and even narrower leading edge visible only at close range. Bill dark gray with blacker tip, legs and feet pinkish overall. First cycle. Following presumed PF molt of head and upperparts, first-cycle birds in fall often show slight contrast between fresh gray (or mixed gray-and-brown) back and faded browner upperwings (including tertials); tail browner in fall than adult and central rectrices relatively tapered. On the water. Throat, foreneck, chest, sides, and undertail coverts bright white, contrasting with dark cap and hindneck, dark chest spur (when neck hunched), gray back, and blackish wings. Molt Adult wing molt probably Mar–Aug and PB2 wing molt Jan–Jun, but few data; some individuals off California finish growth of p10 and inner secondaries in Aug–Sep (pers. obs.2). Apparent PF molt of head and back mostly completed by Sep (CAS specimens). Notes 1. Daily counts of 900–1000 birds, 12 Aug 2001, 18 Aug 2003, vs. usual daily maxima in August of 10–100 birds. 2. Photo of 13 Sep bird growing p10, with inner secondaries molting.

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