- Townsend's Storm-Petrel
 - Townsend's Storm-Petrel
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 - Townsend's Storm-Petrel
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Townsend's Storm-Petrel Hydrobates socorroensis Scientific name definitions

Guy M. Kirwan
Version: 1.1 — Published August 18, 2021
Revision Notes

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Introduction

Townsend's Storm-Petrel shares with Ainley's Storm-Petrel (Oceanodroma cheimomnestes) a very restricted breeding distribution: both species nest only on a few islets at the southern end of Guadalupe Island, off of the northwestern coast of Mexico. Although these two species share the same islets, they scarcely overlap, as each breeds at a different time of year. Townsend's occupies the site in the warmer months, with egg laying from late May to the end of June, and the nestlings fledge in late September and early October. In contrast, egg-laying by Ainley's Storm-Petrel is believed to occur from mid-November through December, with most nestlings fledging from early March through to mid-April. In addition to this temporal segregation, Ainley's and Townsend's storm-petrels have different vocalizations, and also have subtle size and plumage differences. Very little is known about the nonbreeding distributions of these species, due to their similarity to each other and to Leach's Storm-Petrel (Oceanodroma leucorhoa), and to the rarity of both species, but Townsend's Storm-Petrel disperses both north and south at sea, extending from the waters off of southern California, United States, and at least to a latitude of 10º N.

Field Identification

16·5–18·3 cm; wingspan 41·1–44·4 cm. Medium-sized blackish storm-petrel with forked or deeply notched tail (fork 11–17 mm), usually with white patch on rump, but has both white-rumped (70–90% of those breeding on I Afuera) and dark-rumped populations (80–90% of those on I Negro). Head and most of upperparts including upperwing and tail sooty-black, upperwing with large and often contrasting pale panel formed by greyer to buffier inner, central and bases of outer greater coverts, as well as on central and outer median coverts and tips to outer lesser coverts, this panel widest towards rear base of wing, with narrow whitish fringes on innermost secondaries and longest tertials, white on lower rump and uppertail-coverts forming sharply contrasting horseshoe, but this is variable, many having partial or complete longditudinal dark bar dividing white patch into two, and many birds lacking any white in this region; underwing usually plain sooty-black, sometimes with some contrast between browner greater coverts and blacker secondaries; underparts sooty-black except white sides to rump extending onto rearmost flanks and lateral undertail-coverts; iris blackish brown; bill black; legs and feet black. Sexes alike. Juvenile similar to adult. Flight manner in calm to light winds is fairly fast and strong, with rather deep, clipped wingbeats and a more direct and steadier (less three-dimensional) flight than the jerkier, more confident bounding flight of H. leucorhous; compared to the latter (including chapmani), present species is smaller and shorter-tailed, with a shallower tail fork, producing a more compact appearance. Plumage is darker overall, more blackish than brownish, and the white rump patch is often solidly white and more extensive, relatively, than in H. leucorhous, with a shorter tail projection beyond the white; in dark-rumped birds, the pale upperwing band of averages duller than the relatively bolder, brighter band of H. leucorhous chapmani. Unclear, on present knowledge, whether present species and the larger and longer-tailed H. cheimomnestes can be separated at sea, but latter is generally slightly paler overall, with a duller white rump that usually has a dusky median stripe or dusky markings that appear subtly but qualitatively different from H. leucorhous.

Systematics History

Has been included in H. monorhis. Until recently, was considered a race of H. leucorhous, but separated on basis of vocal evidence, as well as finding that present species and H. cheimomnestes (which two are sister taxa) are strongly differentiated in mtDNA from H. leucorhous. Nevertheless, further evidence is highly desirable, and could reverse this conclusion, if a large sample of vocalizations were to demonstrate overlap or if birds in burrows were to react to playback of H. leucorhous. Proposed race kaedingi (from sea near Guadalupe I) of H. leucorhous synonymized with present species. Monotypic.

Subspecies

Monotypic.

Distribution

EC Pacific Ocean, breeding (in summer) on islets (mainly I Afuera and I Negro) off S end of Guadalupe I, off W Mexico.

Habitat

Marine and pelagic, rarely coming near land except at colonies. Breeds on offshore islands on high ground or slopes, usually among rocks, but also in burrows in soil.

Movement

At-sea range poorly known, but is known to range only between 35º N and 10º N in E Pacific, being fairly common to uncommon off S California between Jun and early Nov (mainly Jul–Sept), at least in some years. Nothing further known.

Diet and Foraging

Diet undescribed. Foraging behaviour unknown, primarily due to challenge of identifying this species at sea.

Sounds and Vocal Behavior

Like most storm-petrels, only regularly vocalizes around colonies, both on ground and in flight. Flight calls of both this species and recently recognized H. cheimomnestes differ markedly from those given by both races of H. leucorhous: that of present species is slightly rasping or scratchy (nominate H. leucorhous has more chuckling quality) and consists of 11–20 notes (typically 17), with emphasis on single note c. ⅔ of way through; burrow calls of both this species and H. cheimomnests are faster-paced than those of nominate H. leucorhous and H. r. chapmani, the purring notes (26–27 per second) being punctuated every c. 5 seconds by scarcely audible (but abrupt, plaintive and gulping) inhalation lasting c. 0·6 seconds and first note of following purr louder than rest of series.

Breeding

Very poorly known. Overall season late May/Jun to Oct/Nov, with egg-laying mainly in late May–Jun, and fledging in Oct–Nov. Nests in crevices and burrows; nocturnal at colonies. Single white egg, size 28·2–31·2 mm × 21·1–23·1 mm (n = 15). Nothing further known.

ENDANGERED. Restricted-range species: confined as a breeder to Guadalupe Island EBA. Total breeding population of Guadalupe may be just 5000 pairs (present species and H. cheimomnestes combined), but that of present species has been more recently assessed at 2500–10,000 individuals, based on the suggestion that there may be 4000 birds breeding on I Negro and 3000 nesting on I Asfuera; it may also breed on the main island of Guadalupe I itself, but the species’ persistence there is uncertain. The population has probably suffered significant population declines in the past due to introduced species such as cats (Felis catus, thought to be the primary cause of the extinction of H. macrodactylus), goats (Capra aegagrus hircus, responsible for large-scale habitat degradation on Guadalupe, probably also trampling burrows) and potentially mice (Mus musculus), present on Guadalupe and known to predate nests of the closely related H. leucorhous. However, goats were eradicated in the mid 2000s and, although cats are still present on the island, they are controlled around seabird nest-sites, presumably reducing the extent of predation. Further, the islets on which H. socorroensis nests are thought to be free of invasive species.

Distribution of the Townsend's Storm-Petrel - Range Map
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  • Year-round
  • Migration
  • Breeding
  • Non-Breeding
Distribution of the Townsend's Storm-Petrel

Recommended Citation

Kirwan, G. M. (2021). Townsend's Storm-Petrel (Hydrobates socorroensis), version 1.1. 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.leastp5.01.1