- Craveri's Murrelet
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Craveri's Murrelet Synthliboramphus craveri Scientific name definitions

David N. Nettleship and Guy M. Kirwan
Version: 1.0 — Published March 4, 2020
Text last updated December 13, 2012

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Introduction

Three very similar species of small, gray and white alcids (Synthliboramphus) occur in southern California and in northwestern Mexico. These three species generally replace one another, from north to south, but Craveri's and Guadalupe (Synthliboramphus hypoleucus) murrelets both breed on San Benito Island, and at sea all three may sea overlap, in part, in the nonbreeding season. Craveri’s Murrelet is the southernmost member of this group, nesting on islands in the Sea of Cortez (Gulf of California) and, very locally, off the Pacific coast of Baja California. In late summer and early fall, however Craveri's Murrelets disperse to the Pacific Ocean, north, in small numbers, to waters off of southern or central California. This occurs most frequently during very warm water years. This is not an abundant bird, as it is estimated that the global population may be as low as 6-10,000 individuals. Craveri's Murrelet breeds from January to July. It nest in crevices in rocks, near the water line, and the clutch is two eggs. Craveri’s Murrelet is very similar to Scripps's Murrelet (Synthliboramphus scrippsi) and Guadalupe Murrelet, and identification at sea can be difficult. The black on the side of the face Craveri’s extends to just under the bill (the chin is white in the two other species); Craveri's has a dark spur on the sides of the breast; and the underwing coverts are dusky (not white). In view of its small and decreasing population, the conservation status of Craveri's Murrelet is assessed as Vulnerable.

Field Identification

c. 21 cm; 128–149 g. Small alcid with short, slender black bill  ; brownish-black upperparts, with black cap  extending to bottom edge of lower mandible; white throat and lower sides of face; white underparts, with narrow black wedge on side of breast, and blackish on flanks extending back over body; legs and feet bluish grey, with black claws. Bill slightly longer, but less robust, than in S. hypoleucus; black of cap reaches farther below eye and extends to bottom of lower mandible; more elongated black patch on side of breast, and darker dusky grey underwing-coverts (white in S. hypoleucus). No seasonal variation. Juvenile similar, but with slightly shorter bill, darker upperparts, and fine blackish bars or spots on sides of breast and belly; unusually among auks, males at this age have on average slightly smaller premaxillary processes than females, but as is typical of the Alcidae these are smaller in young birds than adults (8·2–9·9 mm versus 9·1–10·4 mm) (1).

Systematics History

Sometimes placed in Brachyramphus or in Endomychura. May be conspecific with S. hypoleucus, but no evidence of hybridization in zone of possible overlap (San Benito Is), and recent work shows them to be genetically distinct (2). Monotypic.

Subspecies

Monotypic.

Distribution

Numerous islands in Gulf of California (N to San Jorge), and islands along Pacific coast of Baja California (Cedros, Asunción–San Roque, and possibly San Martín and San Benito) (2). Winters at sea in Gulf of California and along coasts N to S California (Monterey Bay) and S to W Mexico (Nayarit).

Habitat

Offshore and along sea coasts ; has southernmost and most restricted range of any alcid, and occupies warmest waters, reaching c. 24·5 ºN as a breeder and to c. 21 ºN in winter (3). Breeds on steep sea cliffs and maritime slopes of islands. In winter, occurs mostly offshore close or adjacent to breeding range, often at considerable distances from shore in relatively warm pelagic waters of California Current (probably similar to S winter population of S. hypoleucus).

Movement

Post-breeding dispersal begins Jun–Jul; most birds remain within overall breeding range, with general movement N in Aug–Oct off Pacific Baja California N to S California (Monterey) and at least moving some S to Nayarit (4) (W Mexico), where it is known at least off the Tres Marias Is (5), and offshore W to Guadalupe I. Numbers generally increase in Gulf of California during Dec–Jan, with reoccupation of breeding sites starting early Feb; however, in Canal de Ballenas (29 ºN), numbers peak in Jun–Oct, with much lower numbers in Mar–May and, especially, Nov–Feb (6). Pelagic distribution during autumn and winter uncertain due to confusion with S. hypoleucus.

Diet and Foraging

Studies lacking, but limited collections during nesting period and early post-breeding dispersal indicate mainly small-sized larval fish (largest 40–70 mm), especially rockfish (Sebastes), herring (Clupeidae) and lanternfish (Benthosema panamense), but also small Pacific mackerel (Scomber japonicus), shrimps and squid; northern anchovy (Engraulis mordax) and Pacific saury (Cololabis saira) may also be important. Appears to forage in pairs or alone, often in small groups widely scattered, rarely associated with other seabird species. Probably feeds relatively close to colony during nesting season, at or near surface over deep water. Precocial young taken to sea at 1–2 days and fed small pelagic fish.

Sounds and Vocal Behavior

High trilling whistles, sometimes loud and of high intensity, and audible up to 18 m away (7). Vocalizes primarily when nesting or when on the water with newly fledged chicks, principally when moving between nest sites and the sea during the incubation period (this exchange is only made in darkness) and the adults also call very frequently and loudly when encouraging the young to fledge (7).

Breeding

Appears in waters off breeding sites in late Dec and Jan, occupying sites early Feb; peak laying second half of Feb and early Mar, but breeding synchrony low, and nesting period extends to early Apr; most families depart Apr to early May. Mating system poorly known; probably monogamous, with high mate and site fidelity. Nests in small groups (up to several dozen pairs) (4) at low density, largely determined by availability of suitable sites; nocturnal at colony. Lays usually in scrape on bare rock or soft substrate at end of rock cavity or crevice, with narrow entrance (7), but also in ground burrows, under dense shrubs and boulders; nests may be sited from just above the high-tide mark to several dozen metres up sea-facing slopes (4). Two eggs; incubation from second egg, probably averages 31–33 days, shared by both sexes equally; hatching synchronous; downy chick c. 25 g, mostly dark brown-black above and white below (adult pattern, including sooty underwing), precocial and mobile, not fed at nest, which abandoned after 1–2 days (based on one record of fledging in 31–38 hours and another of c. 33 hours) (7) when entire family goes to sea at night; family undertakes migration by swimming away from colony; parents protect and feed chicks at sea, but time to independence not known. Nesting success probably variable, sometimes high (75%), though chick survival during first month at sea appears low (30–35%). Most egg losses due to introduced predators (small rodents), with natural predators, such as Peregrine Falcon (Falco peregrinus), Common Barn-owl (Tyto alba) and Yellow-footed Gull (Larus livens), probably taking some adults and young (4). Survival rates, recruitment and age of first breeding unknown.

VULNERABLE. Previously considered Near Threatened. In Mexico, formerly treated as threatened, but now considered Endangered (4). Total population estimated at 6000–10,000 breeding birds, with 90% at three locations (Partida, Rasa, Tiburón) in Gulf of California, and remainder scattered elsewhere in Gulf and on Pacific coast of Baja California; in all 85% of the population breeds in Gulf of California (4). At-sea estimate placed overall population at 22,000 individuals in mid-1990s. Population small and extremely vulnerable to human activities during and outside breeding season. Major threats are: disturbance at breeding locations, including introduced predators (deer mice, rats, cats), human disturbance (predation, tourism developments) and habitat alteration; and disturbance at sea, including commercial fisheries (competition for food, drowning in gill-nets), oil spills from offshore drilling and tanker traffic, and chronic disturbance caused by increase in ship traffic in Gulf of California. Population known to have declined recently, with several colonies extirpated and others severely reduced and threatened (e.g. Tiburón), mostly by introduced predators. Management needs similar to those for S. hypoleucus; introduced mammals have been eradicated from several islands (e.g. San Pedro Mártir, Isla Rasa) that are current, past or potential breeding sites, while instructive signs have been placed on many islands and there is a general move towards increasing enforcement of existing regulations. The eradication of black rats (Rattus rattus) and house mice (Mus musculus) from Isla Rasa by the mid-1990s recently (2010) led to the first confirmation of breeding by S. craveri on the island since at least the mid-1900s (4), and single pairs of S. craveri were found nesting on San Pedro Mártir in 1991 and 1993, but not during the 1992 El Niño (8).

Distribution of the Craveri's Murrelet - Range Map
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  • Year-round
  • Migration
  • Breeding
  • Non-Breeding
Distribution of the Craveri's Murrelet

Recommended Citation

Nettleship, D. N. and G. M. Kirwan (2020). Craveri's Murrelet (Synthliboramphus craveri), version 1.0. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.cramur.01