The First and Second Records of Ashy Storm-Petrel (Hydrobates homochroa) in British Columbia and Canada. By Rick Toochin. Published: June 3, 2023.

The Ashy Storm-Petrel (Hydrobates homochroa) is a small is a medium-small storm-petrel that is occurs year-round in the waters of the California Current along the continental slope (200–2,000 m deep) and slightly farther to sea from Cape Mendocino, California, south to northern Baja California, Mexico in the vicinity of Islas San Benitos (Ainley et al. 2021). Based on sight records of 2,122 Ashy Storm-Petrels on 23 oceanographic cruises of the eastern North Pacific, 1980–1995, none were seen farther than 260 km from the coast and only a few were seen over the continental shelf (Spear and Ainley 2007). Therefore, the species concentrates in a narrow band corresponding to the shelf break front (Ainley et al. 2021). A sighting of 2 individuals farther south than Islas San Benitos was unusual (Crossin 1974). Large numbers—4,000–6,000 regularly, but occasionally up to 10,000 birds—formerly occurred in flocks each fall in Monterey Bay, south-central California (Stallcup 1976, Roberson 1985, Briggs et al. 1987), and included birds from both northern and southern California (Adams and Takekawa 2008). More recently the large flocks in autumn have shifted north to Cordell Bank region, and in north-central California (Ainley et al. 2021). During spring and summer in the northern part of their range, this species concentrates in the vicinity of the Farallon Islands, San Francisco County, and in the southern part in the vicinity of Channel Islands, Santa Barbara County; during fall and winter, it disperses slightly to occur in offshore waters north to Del Norte County, California (Briggs et al. 1987, Spear and Ainley 2007, Carter et al. 2016)

The Ashy Storm-Petrel is only known conclusively to nest, at a minimum of 33 sites, mostly in small numbers (Carter et al. 2016). These colonies range from the rocks off Mendocino County, California south to Islas Todos Santos, in Baja California, Mexico with at least 15 other, smaller sites, where their presence is known, but breeding has yet to be confirmed. (Carter et al. 2016, Parker 2016, Ainley et al. 2021). Due to the substantial interchange among locations, some far apart which has been discovered through band-recoveries, the definition of an Ashy Storm-Petrel “colony” is problematic as they move around (Ainley et al. 2021). 

The Ashy Strom-Petrel has no true migration (Ainley et al. 2021). Instead, individuals disperse, but not to great distances and only during the period of molt in the autumn (Spear and Ainley 2007). Molt is much slower than in the more migratory storm-petrels and extensively overlaps the breeding and dispersal periods (Ainley et al. 1976). Large flocks of molting birds once congregated in offshore waters of Monterey Bay, but now congregate to the north in vicinity of Cordell Bank (Ainley et al. 2021). 

North of California, the Ashy Storm-Petrel is a casual to accidental species. There are 10 records accepted by the Oregon Bird Records Committee with 8 of 10 records in occurring between the years 2014–2016 (OFO 2020). When looking at the months of when all the records combined these occurrences come from late April (1), May (4), August (2), September (1) and October (2) (OFO 2020). 

 

This species is accidental in Washington State with 3 accepted records by the Washington Bird Records Committee (WRBC 2020). These records are as follows: 1 photographed off Westport, Grays Harbor County on June 24, 2006 (Aanerud 2011); 1 well observed 126 nautical miles west of Westport, Grays Harbor County on April 6, 2008 (Merrill and Bartels 2015); and 1 photographed well offshore of Westport (46.994, -124.951), off Grays Harbor County on June 15, 2019 (WBRC 2020). This species is accidental in British Columbia with a single record from just inside Canadian Waters well offshore from Tatoosh Island in Washington State (Toochin et al. 2018).

The identification of the Ashy Storm-Petrel is covered in all standard North American field guides. This is a medium-small storm-petrel measuring 20 cm in length, with a wingspan of 46 cm, and weighing 37 grams (Sibley 2000, Dunn and Alderfer 2011). 

 

The following identification criteria for identifying the Ash Storm-Petrel is taken from Ainley et al. (2021).

 

The body and limb proportions are typical of Hydrobates Storm-Petrels, although wings and tail are proportionally longer. Birds are entirely dusky gray when in fresh plumage, except for a paler gray uppertail covert band and paler transverse bar across the upperwing. The primaries and secondaries darker than other feathers. In the freshest plumage, especially in newly fledged individuals, scapulars, flanks, and wing coverts are edged with light, and pearly gray, but this darkens with wear. Larger underwing coverts are silver-gray, contrasting with darker surrounding underwing feathers. Ages and sexes are all similar in appearance. Fresh juveniles often show more-distinct or whiter pale wing-covert fringing and become browner and more worn in spring/summer due to lack of first-cycle molts. Juvenile outer primaries and rectrices are relatively narrower and more worn, and juveniles lack 'molt clines' in feather freshness among and between primaries and secondaries, which older birds show following protracted Definitive Prebasic flight-feather molt.

At sea, size differences and tail shapes are difficult to discern, and apparent color varies with light conditions and feather wear, so that any of these storm-petrels may appear browner or grayer (Onley and Scofield 2007). Under these conditions, Ashy Storm-Petrel is best recognized by flight action: except when taking off from the water, it raises its wings only to about the horizontal before each downstroke; other storm-petrels in its range raise their wings much higher (Stallcup 1990, Howell and Zufelt 2019). Other similar-sized storm-petrels glide more than Ashy Storm-Petrel (Ainley et al. 2021). From September to November, molting Ashy Storm-Petrels can appear browner due to wear, can fly differently due to missing feathers, and can have tail shapes resembling Least Storm-Petrel when outer rectrices are molting (Ainley et al. 2021).

Within British Columbia, the Ashy Storm-Petrel is most like the Leach’s Storm-Petrel (Hydrobates leucorhous), which is also a primarily dark species. However, the Ashy Storm-Petrel lacks the bold white rump patch of Leach’s Storm-Petrel, which should immediately distinguish the two species under most circumstances (Ainley et al. 2021). It also has a longer and more deeply forked tail than Leach’s and is overall greyer (dark sooty-grey in Ashy, dark blackish brown in Leach’s) (Pollet et al. 2021). The Leach's Storm-Petrel (Chapman's) (Hydrobates leucorhous chapmani) breeds on islands off the Baja California peninsula (Isla Los Coronados and Islas San Benito), and which range north into southern California and could potentially occur as a vagrant into British Columbia, lack the white rump patch of the more northerly-breeding Leach’s and are therefore much more like Ashy Storm-Petrel (Pollet et al. 2021). For these individuals it will be necessary to rely on flight style as well as the more subtle structural and plumage differences between the species (Pollet et al. 2021). The other species of Storm-Petrel occurring in British Columbia is the Fork-tailed Storm-Petrel (Hydrobates furcatus) which is overall a much paler grey with a bold black “M” across the upperwing that is formed by the blackish primaries and the blackish bar across the upperwing coverts (Howell and Zufelt 2019). In addition, the Fork-tailed Storm-Petrel shows a distinct dark patch through the eye that is not shown on the much darker sooty-grey head of the Ashy Storm-Petrel (Dunn and Alderfer 2011).

Finally, a species that occurs in California and could potentially range north into British Columbia as a vagrant is the Black Storm-Petrel (Hydrobates melania) which is larger and longer-winged than Ashy Storm-Petrel and is much darker and blacker overall (Everett et al. 2021). Like Ashy, however, it has a solidly dark rump and could potentially be a source of confusion. The tail of Black Storm-Petrel is not quite as deeply forked as in Ashy Storm-Petrel (Everett et al. 2021). The flight style is slow, deliberate, with deep wing beats interspersed with long glides; flight comparable to that of a Black Tern (Chlidonias niger) or a nighthawk (Chordeiles species) which should easily help distinguish this species from Ashy Storm-Petrel (Everett et al. 2021). Other southern all-dark storm-petrels are unlikely to reach British Columbian waters. These include the recently split endangered Townsend’s Storm-Petrel (Hydrobates socorroensis) with a population of less than an estimated 10,000 birds, breed off western Mexico on the south end of Guadalupe Island, but is known to range only between 35º N and 10º N in the eastern Pacific, being fairly-common to uncommon off southern California between June and early November (mainly from July–September), at least in some years (Kirwan 2021a). This also include the recently split Ainley’s Strom-Petrel (Hydrobates cheimomnestes) which breeds on islets (Negro, Afuera, and Gargoyle Rock) around Guadalupe Island, off northwestern Mexico and apparently ranges south in the non-breeding season (May–Sept) at least as far as Galapagos Island, west of Ecuador with records coming from the waters off western Mexico (May – October) (Kirwan 2021b). This species has not yet been detected in North American waters (Kirwan 2021b). Both Townsend’s Storm-Petrel and Ainley’s Strom-Petrel are extremely unlikely to occur in British Columbia and their identification and separation from Ashy Storm-Petrel is not discussed here.

The Ashy Storm-Petrel is accidental in British Columbia with a single record of 1 adult observed by experienced observers G. Scott Mills and Tim Shelmerdine at 93 nautical miles and 2 adults 101 nautical miles northwest of Tatoosh Island, but in Canadian waters, from the deck of the NOAA ship David Starr Jordan on July 6, 2008 (S. Mills Pers Comm.). This record was included on the BC Rare Bird List by Toochin et al. (2012) at the request of the observers who also provided excellent field sketches, notes, and documentation of their observations. The original observer, G. Scott Mills, was very clear this record was from Canadian waters and not American waters. G. Scott Mills writes “First Bird: Canadian Waters (British Columbia) about 93 nm [172km] northwest of Tatoosh Island, Clallam County, Washington.  Second and Third birds:  Canadian Waters (British Columbia) about 101 nm [187km] northwest of Tatoosh Island, Clallam County, Washington”. As for his experience G. Scott Mills writes “I have extensive experience with northeastern Pacific seabirds.  I have spent more than 200 days conducting seabird surveys since 2000, more than 100 of them aboard NOAA vessels.  I have seen thousands of Leach’s and Fork-tailed Storm-Petrels.   I have seen a number of Ashy Storm-Petrels, including the first Washington state record in 2006, a second bird in Washington in April of this year [2008], and 5 and 16 birds, respectively, on April and August NOAA trips in California this year [2008]”.  Away from its breeding colonies, the Ashy Storm-Petrel occurs only in pelagic waters in association with areas of upwelling, such as along the continental slope or near seamounts or islands (Ainley et al. 2021). It prefers deep water and tends to avoid shallower pelagic waters over the continental shelf (Ainley et al. 2021). The 2 records from British Columbia both pertain to individuals that were recorded seaward of the continental shelf (G. S. Mills Pers. Comm.). In recent decades, the oceans have had large fluctuations in temperature due to large El Nino and La Nina events that could at least theoretically push more southerly occurring species like the Ashy Storm-Petrel northward (M. Meredith Pers. Comm.). It should be pointed out there are very few pelagic trips that take place in the late spring and summer months off British Columbia from late April, May, June, July, and August making the likelihood of encountering this species in the future a much lower possibility since records from September and October are far fewer north of California which when most pelagic trips occur in the province.

I wish to thank Don Cecile for editing the original manuscript. I also want to thank G. Scott Mills for forwarding his incredibly detailed field notes and sketches of the Ashy Storm-Petrel records. Also, with thanks to meteorologist Mitch Meredith for is information on El Nino and La Nina.

 

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