Least Storm-Petrel Hydrobates microsoma Scientific name definitions

Sometimes treated as the sole member of the genus Halocyptena, a compound word derived from the Greek words for sea, speedy and winged, the Least Storm-Petrel is the smallest member of the Hydrobatidae. It is a uniformly sooty black bird with a wedge-shaped tail and a paler gray-brown upperwing bar. The flight is often fluttering. Although the Least Storm-Petrel is virtually confined as a breeding bird to the Gulf of California, where it nests under rocks and in crevices on most predator-free islands, as well as on three islands in the San Benito archipelago off western Baja California, during the non-breeding season (September–June) the species wanders at sea as far south as the equator. The species’ diet consists mostly of small plankton.

13–15 cm; 14–22 g (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ); wingspan 32–36 cm (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ). Smallest seabird  with nearly uniform dark plumage and wedge-shaped tail (although latter can appear rounded in flight). Plumage entirely sooty-black  but for slightly paler and often browner greater coverts forming a usually weak wingbar  extending from carpal joint to trailing edge; iris dark brown; bill black; legs and feet black. Pale wingbar can be completely lost by late autumn (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ). Sexes alike. Juvenile as adult. Birds breeding on San Benito Is (Baja California) perhaps average larger than those breeding on islands in Gulf of California, e.g. wing 115–126 mm versus 113–121 mm, respectively (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ). Small size (obvious compared to larger and longer-winged H. melania and H. homochroa), entirely dark rump region and wedge-shaped tail  are unique among storm-petrels. Beware also possibility of confusion with dark-morph H. leucorhous socorroensis. Flight often rather fluttering (but jerkier than H. homochroa) and legs do not project beyond tail tip.

Subtropical E Pacific S to Ecuador, breeding on Islands in Gulf of California and off W coast of Baja California (NW Mexico).

Marine and often pelagic, mainly occurring over warm waters away from land. Breeds on small islands or rocky stacks.

Most move away from breeding islands between Oct and Mar (4 Tershy, B. R., E. Van Gelder, and D. Breese (1993). Relative abundance and seasonal distribution of seabirds in the Canal de Ballenas, Gulf of California. Condor 95(2):458–464. Close , 1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ). Disperses S to winter off W coast of Central America, where mainly observed off Costa Rica between Sept and Jun (5 Brooke, M. (2004). Albatrosses and Petrels Across the World. Oxford University Press, Oxford, UK. Close ), and reaching as far S as Colombia, Ecuador and northernmost Peru (Nov–Mar) (6 Schulenberg, T. S., D. F. Stotz, D. F. Lane, J. P. O’Neill, and T. A. Parker (2007). Birds of Peru. Princeton University Press, Princeton, NJ, USA. Close ). However, is recorded year-round off Central America, with significant non-breeding aggregations reported in Gulf of Nicoya and Panama Bight, these being augmented by migrants in Sept–Nov and, especially, in Feb–May (7 Loftin, H. (1991). An annual cycle of pelagic birds in the Gulf of Panama. Orn. Neotropical. 2(2): 85–94. Close , 1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ). Irregularly recorded N to C California, mainly Aug–Oct, exceptionally from Jul and until Oct (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ), e.g. following El Niño of 1992, moved N in late summer (5 Brooke, M. (2004). Albatrosses and Petrels Across the World. Oxford University Press, Oxford, UK. Close ), with most records off San Diego, less frequently N to Monterey Bay, exceptionally to Cordell Bank, also inland (late Aug to Oct), sometimes storm-driven (8 McCaskie, G. and Webster, R.E. (1990). A second Wedge-tailed Shearwater in California. Western Birds. 21(3): 139–140. Close ), with 100s occasionally recorded at Salton Sea (California) and L Havasu (Arizona) (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ).

Mostly small planktonic crustaceans, particularly larvae of spiny lobster Panulirus. Feeds on wing, by pattering, and also by surface-seizing whilst sitting on water. Gregarious at sea, feeding in groups that may total 100s, either tightly or more loosely aggregated, and readily associates with H. melania  and H. tethys in particular, but also H. homochroa (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ).

Vocalizations weaker and less scratchy than those of H. melania, the Burrow Call being described as resembling the persistent whirring of a revolving cogwheel (5 Brooke, M. (2004). Albatrosses and Petrels Across the World. Oxford University Press, Oxford, UK. Close ), being a relatively soft, fast-pace, rattling or snoring churr, in bursts of 3–4 seconds, punctuated by abrupt, plaintive (breath-like) inhalations; Flight Call comprises relatively low-pitched rhythmic chattering notes that frequently accelerate towards emphatic terminus, “krrih krrih krrih-krri-krri-krri” or “krruh kuh-uh krr-krr-krr-krr” (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ).

Starts Jun/Jul in N part of breeding, with newly hatched young recorded 12 Aug to 8 Sept (9 Murphy, R. C. (1936). Oceanic Birds of South America. Volume 2. American Museum of Natural History, New York, NY, USA. Close ). Speculated to arrive at San Benito Is later than those in Gulf of California, in late Apr/May, where eggs hatched between late Jul and mid Sept, and fledging period started in mid Oct (10 Bedolla-Guzmán, Y., Masello, J.F., Aguirre-Muñoz, A., Lavaniegos, B.E. and Quillfeldt, P. (2017). Breeding biology, chick growth, and diet of the Least Storm-Petrel Oceanodroma microsoma on Islas San Benito, Mexico. Marine Orn.. 45(2): 129–138. Close ), whereas arrives in Gulf of California from Mar (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ) and eggs recorded on La Lobera in latter region by mid Mar, with young fledging there in Jun (11 Carmona, R., J. Guzmán, S. Ramírez, and G. Fernández (1994). Breeding waterbirds of La Paz Bay, Baja California Sur, Mexico. Western Birds 25:151–157. Close ). On San Benito Is, timing of breeding varies from year to year: hatching period began earlier in 2013 than in 2015 and was longer in 2014, while fledging period started later in 2014 than in 2013 (10 Bedolla-Guzmán, Y., Masello, J.F., Aguirre-Muñoz, A., Lavaniegos, B.E. and Quillfeldt, P. (2017). Breeding biology, chick growth, and diet of the Least Storm-Petrel Oceanodroma microsoma on Islas San Benito, Mexico. Marine Orn.. 45(2): 129–138. Close ). Colonial; nests in rock crevices and on bare sites under rock slabs, perhaps selecting smaller crevices than larger H. melania (5 Brooke, M. (2004). Albatrosses and Petrels Across the World. Oxford University Press, Oxford, UK. Close ) and H. leucorhous chapmani (1 Howell, S. N. G. (2012). Petrels, Albatrosses, and Storm-Petrels of North America: A Photographic Guide. Princeton University Press, Princeton, New Jersey, USA. Close ). Clutch single white egg, mean size 23·4 mm × 19·4 mm (5 Brooke, M. (2004). Albatrosses and Petrels Across the World. Oxford University Press, Oxford, UK. Close ). During multi-year study on San Benito Is, hatching success (c. 80%) and fledging success (c. 90%) were consistently high in all three years (10 Bedolla-Guzmán, Y., Masello, J.F., Aguirre-Muñoz, A., Lavaniegos, B.E. and Quillfeldt, P. (2017). Breeding biology, chick growth, and diet of the Least Storm-Petrel Oceanodroma microsoma on Islas San Benito, Mexico. Marine Orn.. 45(2): 129–138. Close ). No further information, but natural predators of adults at colonies include Peregrine Falcons (Falco peregrinus) (12 Velarde, E. (1993). Predation of nesting larids by Peregrine Falcons at Rasa Island, Gulf of California, Mexico. Condor. 95(4): 706–708. Close ).

Not globally threatened (Least Concern). Most abundant seabird in Gulf of California, with 100,000s or possibly 1,000,000s, with potentially largest populations on islands of Cardinosa and Little Cardinosa (5 Brooke, M. (2004). Albatrosses and Petrels Across the World. Oxford University Press, Oxford, UK. Close ); c. 15,000 breeders on San Benito Is off Pacific coast of Baja California (1968). Introduced predators, especially rats and cats, on many islands in area. Surveys required to determine numbers, trends and threats; research on breeding biology and ecology also desirable to ensure future security.