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Guadalupe Murrelet Synthliboramphus hypoleucus Scientific name definitions

David N. Nettleship and Guy M. Kirwan
Version: 1.0 — Published March 4, 2020
Text last updated July 3, 2015

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Introduction

Three very similar species of small, gray and white alcids (Synthliboramphus) occur in southern California and in northwestern Mexico. Guadalupe Murrelet has the most distinctive appearance of these three; it also is the rarest, most geographically restricted, and least known member of the group. Guadalupe Murrelet is known to breed on Guadalupe and San Benito islands, off the Pacific coast of Baja California; after breeding, it disperses north to waters off of central California. The natural history of Guadalupe Murrelet otherwise is poorly documented, but presumably is similar to that of Scripps's Murrelet (Synthliboramphus scrippsi) and Craveri's Murrelet (Synthiloboramphus craveri). Guadalupe Murrelet is distinguished from both of these species by the greater extent of white on the sides of the face, especially the broad white crescent around the eye. Until recently, Guadalupe and Scripps's murrelets were classified as a single species, "Xantus's Murrelet", but they now are recognized as a separate species, as they are locally sympatric as breeders on San Benito Island, differ in face pattern and in bill shape, and have different vocalizations. Guadalupe Murrelet is classified as Endangered, based on its small breeding range and global population, and because its population is believed to be decreasing.

Field Identification

23–25 cm; mean 130–185 g; wingspan c. 40 cm. Small auk with short, slender black bill  ; black upperparts  with faint greyish cast, including top of head, neck, back, wings and tail; prominent white crescents  above and below eye, meeting white of throat; mostly snowy white underparts  , with flanks varying from white to mottled grey and white; legs and feet bluish grey, with black webs and claws. Female usually significantly larger than male in culmen, wing chord and weight (averages 15 g heavier), but not bill depth or tarsus length. In winter, often paler greyish black above due to feather wear. Juvenile similar to adult, but with scattered dark barring along flanks.

Systematics History

Sometimes placed in Brachyramphus or in Endomychura. Sympatric (as is S. scrippsi) with closely related S. craveri at San Benito Is (Baja California), where further study required in order to determine degree of mixing and gene exchange; may be conspecific with S. craveri. Often considered conspecific with S. scrippsi. Proposed race pontilis considered untenable. Monotypic.

Subspecies

Monotypic.

Distribution

Offshore rocks and islands of NW Baja California (San Benito Is, Guadalupe I); status on San Martín I, Baja California, and San Clemente I and Santa Barbara I (Channel Is), off S California, uncertain (1, 2). Winters offshore presumably within the breeding range along the Pacific coast of Baja California, rarely further north.

Habitat

Offshore and along sea coasts, occupying warmer waters than all alcids except S. craveri and S. scrippsi. Breeds on steep sea cliffs, slopes and canyons on islands. Winters primarily well offshore, though distribution at this season not well known due to difficulties of separation from S. scrippsi, albeit with definite evidence of some degree of overlap.

Movement

Post-breeding dispersal from nesting islands is largely offshore to W, and to N; individual pairs and their flightless offspring swim on water. Movement to wintering areas probably slow, with earliest dates off California in early to mid Jul, and probably generally occurs to S of S. scrippsi, with many more records of the latter species off Monterey, California, than of S. hypoleucus. Nevertheless, the present species has exceptionally been recorded as far N as Oregon and Washington, like S. scrippsi, but records so far N are many fewer (at most 10% of records of the two species). Southernmost record (in mid Jul) is from Cabo San Lucas, at southernost tip of Baja California. Has usually returned to breeding islands by Dec, although there are exceptional records well away from nest-sites as late as early Dec.

Diet and Foraging

Not well studied, but no known differences from S. scrippsi.

Sounds and Vocal Behavior

No known differences from S. scrippsi.

Breeding

Very poorly known compared to S. scrippsi. Appears on sea near colonies mid- to late Dec, and breeding can continue until Jul. Nocturnal at colony; laying asynchronous and variable between years, but season is generally very similar to, perhaps marginally later than, S. scrippsi; on San Benito Is, chicks of this species recorded as early as late Feb and eggs as late as early May. Probably monogamous. Breeds mostly in small colonies and at low densities, possibly determined by patchy distribution of suitable nesting habitat: rock crevices, caves, burrows and shrubs. Nests often placed under foliage of maguey (Agave shawi), on sandy slopes facing sea, while a few few nests are in earthen caves or burrows of other species, e.g., rabbits. Clutch two eggs. No further information, but recent study on San Benito Is found no evidence for differences in timing of breeding among monotypic pairs (of both this species and S. scrippsi), intermediate and mixed pairs, but did observe possible evidence for differences in hatching success: in 2003, percentage of nests that hatched was 100% (3/3) for S. scrippsi pairs, 100% (1/1) for intermediate × intermediate pairs, and 0% (0/2) for hypoleucus pairs; in 2004, the percentage of nests that hatched eggs was 86% (6/7) for S. scrippsi pairs, 25% (1/4) for the intermediate × one subspecies pairs, and 100% (1/1) for a mixed S. scrippsi × S. hypoleucus pair.

ENDANGERED. Total population estimated at 5000 breeding birds, on the three San Benito Is, Guadalupe I and at least two associated offshore rocks. The relative numbers of this species versus S. scrippsi and S. craveri at the San Benito Is have been variously estimated at 20–38% hypoleucus, 40–62·5% scrippsi and 0–40% S. craveri, with numbers of intermediates between the first two estimated at 7–15%. At-sea surveys in 1975–2003 estimated the North American population of S. hypoleucus/scrippsi to be 39,700 individuals (including non-breeders). Cat predation is thought to have caused the extirpation, or at the very least, significantly reduced the population on the main island of Guadalupe, which is considered the most important historical site for the species. Other likely former breeding colonies (Cedros, Natividad, Asunción and San Roque) thought to have been extirpated by invasive animals. Breeding unconfirmed on San Martín I, Baja California, and San Clemente and Santa Barbara Is, California. Guadalupe I has been declared a Biosphere Reserve and the other Mexican islands with current or former breeding colonies are either in existing biosphere reserves (Natividad, Asunción and San Roque) or in a proposed new biosphere reserve. Other than introduced mammalian predators, other threats are drowning in drift gill-nets, nest-site disturbance, bright lights used by the squid fishery that cause disturbance and mortality and possibly organochlorine pollution, and changes in sea temperature associated with global climate change.

Distribution of the Guadalupe Murrelet - Range Map
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  • Year-round
  • Migration
  • Breeding
  • Non-Breeding
Distribution of the Guadalupe Murrelet

Recommended Citation

Nettleship, D. N. and G. M. Kirwan (2020). Guadalupe Murrelet (Synthliboramphus hypoleucus), 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.xanmur1.01