SPECIES

Pink-footed Shearwater Ardenna creatopus Scientific name definitions

Ryan D. Carle, Valentina Colodro, Jonathan Felis, Joshua Adams, and Peter J. Hodum
Version: 2.0 — Published April 9, 2022

Plumages, Molts, and Structure

Plumages

Has 10 full-length primaries (numbered distally, p1 to p10), 19–20 secondaries (numbered proximally, s1 to s16 or s1 to s17, and including 3 tertials, numbered distally, t1 to t3), and 12 rectrices (numbered distally, r1 to r6, on each side of the tail). Shearwaters are diastataxic (see 7) indicating that a secondary has been lost evolutionarily between what we now term s4 and s5. No geographic variation in appearance (see Systematics) or molt strategies have been reported, although the species is variable in plumage. Some individuals (described as dark-morph) are much darker than others, with variation in plumage observed in between.

See Molts for molt and plumage terminology. The following is based primarily on detailed plumage descriptions of Murphy (8), Palmer (9), and Howell (2); see Pyle (3) for specific age-related criteria. Sexes show similar appearances in all plumages. Definitive appearance is assumed at Second Basic Plumage.

Natal Down

Present primarily January–April, in the natal burrow. Hatchlings were described by Murphy (8) as being clad in thick down, "Quaker drab or almost purplish gray" above, around the neck, and in a patch on the lower belly, and the down on the remainder of the underparts being dirty white. Down of the face and throat is shorter than elsewhere.

Juvenile (First Basic) Plumage

Present primarily March–August or March–February, assuming the Preformative Molt can be absent or limited (see Molts). Similar in coloration to Definitive Basic Plumage, except that plumage is fresher than in adults in March–August, with thin fringing to the upperwing and more distinct underwing coverts. Juvenile outer primaries and rectrices also average narrower, and become increasingly brownish and abraded through the first spring and summer with wear, as compared with basic feathers. Axillars are narrower, more tapered or rounder at the tips, and average more whitish coloration (Figure 200 in 3). During March–August, older birds are in primary molt whereas juveniles do not molt flight feathers during these months. Juvenile remiges are also grown at once and are consistent in wear, not showing 'molt clines' of later plumages (see below).

Formative Plumage

Present primarily November–January in some but probably not all birds (see Molts). Similar to Juvenile Plumage except that scattered body feathers replaced, with those of the upperparts being fresher, darker, and more squared in shape than retained juvenile feathers (3).

Definitive Basic Plumage

Present primarily August–July. Head, upperparts, and upperwing coverts grayish when fresh to grayish brown when worn. The feathers often have fine pale fringes, creating a scaled appearance when fresh. Feathering around the eye can be dark or variably pale, forming an indistinct eye-ring. The upperwing greater coverts usually have narrow white tips when fresh, and occasionally subterminal pale spots. Rectrices, primaries, and secondaries are darker, dusky to blackish. Face, throat, and upper breast are variably white to whitish with light gray mottling to heavily mottled gray. In most birds, remainder of underparts is white with gray wash or mottling along sides and flanks and extending to darker femoral feathers and undertail coverts. Axillar region is usually gray; underwing coverts are largely white with dark patch or central streaks among distal lesser coverts, sometimes forming a dark patch. Color of central underparts is white to whitish, sometimes mostly to nearly entirely dark gray (often referred to as "dark morph"). Rare individuals may be entirely dark gray but most show at least a hint of white patterning to the underparts or underwing coverts, and plumage can show intermediate levels between "pale morph" and "dark morph" individuals.

Definitive Basic Plumage distinguished from Juvenile and Formative plumages by broader, relatively fresher, and darker outer primaries and rectrices. Axillars are broader, squarer, and average more dark coloration (Figure 200 in 3). Following complete molts, "molt clines" are visible among primaries, reflecting molt sequence (see Molts). Primaries become subtly darker and fresher distally, and the outermost secondary (s1) is noticeably darker and fresher than the innermost primary (p1) due to a lag in the timing of replacement (3).

Occasional individuals may retain secondaries during Second or Definitive Prebasic Molts (most often among s8–s14; see Molts). These individuals can be identified as in Second Basic Plumage if retained feathers are juvenile (narrower and relatively more worn) or, more likely, in an older basic plumage if retained feathers are basic (3).

Molts

Molt and plumage terminology follows Humphrey and Parkes (10), as modified by Howell and Corben (11) and Howell et al. (12, 13). Pink-footed Shearwater exhibits a Complex Basic Strategy (cf. 12, 14), including complete prebasic molts and an inserted preformative molt in at least some individuals, but no prealternate molts (9, 15, 3, 14, 2; Figure 1). Most Procellariiformes with documented preformative molts are species that breed in the Southern Hemisphere and migrate to nutrient-rich waters in the Northern Hemisphere, whereas those that breed and/or molt in tropical or subtropical latitudes may be less likely to have a preformative molt due to lack of available nutrients to first-cycle individuals (3).

Prejuvenile (First Prebasic) Molt

Complete, primarily March–May, in the nest burrow. No information on pennaceous feather development. Presumably is completed or nearly completed by fledging in late April or early May, with final growth of outer primaries and rectrices sometimes completing at sea, as reported for other species of shearwaters.

Preformative Molt

Absent to limited, primarily in October–January (Figure 1) in the Northern Hemisphere. Includes at least some scattered body feathers of the upperparts (often in the back and among scapulars) but no wing coverts or flight feathers.

Second Prebasic Molt

Complete, primarily January–June (Figure 1) and primarily in the Northern Hemisphere, along the Pacific North American coast; molt can commence in the southern hemisphere and possibly may be suspended for northbound migration. Flight-feather replacement sequence is similar to that of Definitive Prebasic Molt, but begins earlier and completes quicker due to lack of breeding constraints.

Definitive Prebasic Molt

Complete, primarily March–August (Figure 1) in the Northern Hemisphere, or northern Humboldt Current, depending on final migration destination (see Migration section; 16). Primary molt proceeds distally from p1 to p10; secondary molt proceeds from 3 centers: proximally from s1, proximally from s5, and bidirectionally from the second tertial (t2). The last feathers to complete growth are p10, s4, and s9. Molt of rectrices is also likely similar to that of Ashy Storm-Petrel (Hydrobates homochroa) and Leach's Storm-Petrel (Hydrobates leucorhous); commonly, rectrices are molted in pairs in the order r6 (outermost), r4, r2, r1, r3, r5, with much variation in the sequence. Among nonbreeding subadult or adult birds captured at sea off Santa Barbara, California (n = 12), all 12 had regrown their inner five primary feathers by the first week in June, and only 1 of 12 individuals had lost all primaries by 30 June (JA). Timing of primary molt off California is consistent with timing off Peru based on a single individual collected on 4 July in Peru with two outer primaries still sheathed (8). Occasionally one or more secondaries may be retained during molts, most often among s8–s14, the last feathers replaced in sequence. This may occur more often in breeding birds that have successfully fledged chicks and lack as much time to complete the molt.

Bare Parts

Bill and Gape

The bill is grayish blue in hatchlings (8). In adults, bill is dull pinkish to pale pink, with dusky gray on the tip and sometimes on the culmen.

Iris and Facial Skin

Iris is blackish brown at all ages.

Tarsi and Toes

In hatchlings, legs and feet pale pinkish (8). In adults, pinkish with dusky toes.

Measurements

Linear Measurements

Based on birds (sexes combined) measured from Isla Mocha and Juan Fernández Islands in Chile (1), except as noted.

  • Total length (cm) for Isla Mocha only: 47.0 ± 1.11 SD (range 44.5–50.0) (1).
  • Wing length (cm): 33.3 ± 0.75 SD (range 31.5–35.0, n = 146) (1).
  • Tail length (cm): 11.4 ± 0.38 SD (range 10.5–12.3, n = 146) (1).
  • Tarsus length (mm): 56.0 ± 2.12 SD (range 44.9–59.8, n =146) (1); range 44.9–59.8, n = 200 (3).

In a sample of individuals measured from both the Juan Fernández Islands and Isla Mocha, males were generally larger than females, though females had relatively longer tails than males (1). The most useful measurement for determining sex in the field was head length (1). Males had longer head lengths than females, though not all birds could be classified unambiguously with this measurement because of size differences between individuals from different breeding colonies (1). Birds from the Juan Fernández Islands were overall larger than those from Isla Mocha, with proportionally stouter bills and thicker legs, and shorter tails (1). A combination of sex and breeding colony best predicted morphometrics of birds measured at both the Juan Fernández Islands and Isla Mocha, with males from Isla Mocha and females from the Juan Fernández Islands having similar body sizes (1). In a sample of 13 museum specimens from the Juan Fernández Islands and Isla Mocha, males had significantly greater bill depths than females (17).

Measurements by sex:

  • Total length (cm) for Isla Mocha only: males 47.3 ± 1.17 SD (range 45.0–50.0, n = 37), females 46.4 ± 1.12 SD (range 44.5–49.6, n = 25) (1).
  • Wing length (cm): males 33.5 ± 0.74 SD (range 33.8–35.0, n = 51), females 33.1 ± 0.86 SD (range 31.5–35.0, n = 38) (1).
  • Tail length (cm): males 11.2 ± 0.35 SD (range 10.5–12.0, n = 51), females 11.5 ± 0.39 SD (range 11.0–12.5, n = 38) (1).
  • Tarsus length (mm): males 56.4 ± 2.00 SD (range 46.8–59.8, n =51), females 55.5 ± 2.82 SD (range 44.9–59.4, n = 38) (1).
  • Head length (mm): males 58.2 ± 1.98 SD (range 55.3–65.8, n = 51), females 55.8 ± 1.39 SD (range 52.9–58.9, n = 38) (1).
  • Bill depth at base (mm): males 16.75, females 15.65 (n = 13 birds total; based on museum specimens from Juan Fernández and Isla Mocha) (17)

Measurements by breeding colony for males:

  • Maximum bill height: Isla Mocha 17.2 ± 0.66 SD (range 15.3–18.6, n = 37), Juan Fernández 18.0 ± 0.47 SD (range 16.8–18.8, n = 14) (1).
  • Tarsus width: Isla Mocha 4.4 ± 0.34 SD (range 3.9–5.8, n = 37), Juan Fernández 4.8 ± 0.24 SD (range 4.4–5.2, n = 14) (1).
  • Tail length: Isla Mocha 11.3 ± 0.34 SD (range 10.5–12.0, n = 37), Juan Fernández 11.0 ± 0.31 SD (range 10.5–11.5, n = 14) (1).

Measurements by breeding colony for females:

  • Maximum bill height: Isla Mocha 16.3 ± 0.56 SD (range 15.0–17.5, n = 25), Juan Fernández 17.1 ± 0.43 SD (range 16.4–17.9, n = 13) (1).
  • Tarsus width: Isla Mocha 4.4 ± 0.32 SD (range 4.0–5.3, n = 25), Juan Fernández 4.8 ± 0.18 SD (range 4.4–5.1, n = 14) (1).
  • Tail length: Isla Mocha 11.5 ± 0.41 SD (range 11.0–12.3, n = 25), Juan Fernández 11.7 ± 0.32 SD (range 11.0–12.0, n = 14) (1).

Mass

Mean mass (g) of birds from Isla Mocha and Juan Fernández Islands in Chile: 723 ± 66.3 SD (range 576–889) (1). Mass of nonbreeders captured off Santa Barbara, California, in late June (n = 5): 746 g ± 39 SD (JA).

  • Mean mass (g) by sex from Isla Mocha: males 733 ± 63.7 (range 628–865, n = 37), females 674 ± 64.7 (range 576–776, n = 25) (1).
  • Mean mass (g) by sex from Juan Fernández: males 781 ± 54.0 (range 677–879, n = 14), females 738 ± 34.4 (range 660–788, n = 25) (1).

Recommended Citation

Carle, R. D., V. Colodro, J. Felis, J. Adams, and P. J. Hodum (2022). Pink-footed Shearwater (Ardenna creatopus), version 2.0. In Birds of the World (P. G. Rodewald and B. K. Keeney, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.pifshe.02