Birdfinding.info ⇒ The little-known Gulf of Guinea form of Band-rumped Storm-Petrel appears to be very locally common by day in waters around São Tomé and Príncipe and at night in the interior highlands of São Tomé. Since its discovery at inland sites it has been observed regularly at Monte Carmo in Obô Natural Park.
“Gulf of Guinea Storm-Petrel”
Hydrobates castro ssp. nova
Breeds on São Tomé; apparently resident in Gulf of Guinea.
“Gulf of Guinea Storm-Petrel” observations at sea (A, B, C & D). © Flood et al. 2019
“Gulf of Guinea Storm-Petrel” observation sites on São Tomé. © Flood et al. 2019
Suspected of nesting in burrows in mountain forests of São Tomé, and possibly also on Annobón and/or Bioko.
Breeding activity apparently occurs year-round, as it has been recorded at inland sites throughout the year, and specimens in breeding condition have been collected in July and December.
Not known from outside the Gulf of Guinea. It has been observed at around São Tomé and Príncipe, and in offshore waters of Nigeria.
Identification
As a form or potentially cryptic species within the Band-rumped Storm-Petrel complex, “Gulf of Guinea” is extremely similar to other band-rumped-type storm-petrels.
Compared to closely related forms, “Gulf of Guinea” averages slightly larger overall and tends to have a narrower white rump band, which makes its tail appear proportionately longer. Geographical probability is the strongest indicator, as no other band-rumped-type storm-petrel is known to occur in the Gulf of Guinea.
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova. © William H. Wagstaff
Like other band-rumped-type storm-petrels, “Gulf of Guinea” is dark-brown overall, with an even white band across the rump that usually extends partway down the sides of the rump to the undertail. Its tail usually appears either square-tipped or shallowly notched.
As with most dark storm-petrels, all band-rumped-types typically show a paler brown or whitish diagonal stripe on the wing coverts, but the boldness varies depending on molt-stage and lighting. So the apparent color and boldness of the wingbar can provide clues to the age and molt-stage of closely observed individuals.
Juveniles and freshly molted adults have the most pronounced wingbars. On juveniles the bar appears silvery. On fresh adults, the bar is blond. With feather-wear, the bar diminishes and may disappear entirely by the time the next molt begins.
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova. © William H. Wagstaff
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova. © William H. Wagstaff
Notes
Monotypic form, one of seven or more potentially distinct forms of Band-rumped Storm-Petrel (castro), which is in the midst of taxonomic revisions. A review published in 2019 (Flood et al.) found indications that the “Gulf of Guinea Storm-Petrel” might be more closely related to the Cape Verde Storm-Petrel (jabejabe) than to the other band-rumped forms.
See below for a comparison of the “Gulf of Guinea Storm-Petrel” with other Atlantic members of the Band-rumped Storm-Petrel complex.
Cf. Other Atlantic Band-rumped-type storm-petrels. At least six cryptic species or forms in the Band-rumped Storm-Petrel complex have been recognized in the Atlantic. Four breed on North Atlantic archipelagos of Macaronesia and two on widely scattered islands of the South Atlantic. Apart from their breeding grounds, the ranges of these forms remain mostly unknown. Sightings are scarce and the forms are extremely difficult to distinguish from one another at sea.
The four North Atlantic forms overlap with one another to varying degrees and are known to differ vocally. In brief, all four have two similar call types—chattering and purring—which vary in length, pattern, and complexity. Differing calls have helped to confirm their respective breeding distributions, but they generally call only on the breeding grounds and only at night.
In most cases, under most conditions, the visual differences among the forms are too subtle and inconsistent for field identification. With that caveat, following is a summary of factors that might help in exceptional circumstances—the forms are listed by the average latitudes of their breeding grounds, from north to south.
Cape Verde Storm-Petrel—identification presumed based on the location, far out at sea south of Cape Verde, though any of the Atlantic band-rumped forms could occur there at any time of year. (May 4, 2011.) © Graham Ekins
“Madeiran Storm-Petrel”, H. c. castro —likely this subspecies, based on molt-stage, season, and location within its known breeding range—but these factors are also consistent with Monteiro’s, Cape Verde, and first-year “Grant’s”, all plausible alternative identifications. (Offshore northeast of Lanzarote, Canary Islands, Spain; November 1, 2014.) © Miguel Rouco
Monteiro’s Storm-Petrel—based on large samples of measured individuals at their Azorean breeding sites, Monteiro’s tends to have a more deeply notched tail than “Grant’s”, but their measurements overlap. (Offshore from Graciosa, Açores, Portugal; May 2011.) © Gareth Knass
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova, showing apparently thin rump-band and long-looking tail. © William H. Wagstaff
Monteiro’s Storm-Petrel (monteroi): Breeds in the Azores from March to October (eggs laid from early May to early July). Gives the shortest, simplest calls, with a rougher, scratchier quality than the others. Relatively small-bodied, long-tailed, and long-winged, and usually has a visibly notched tail.
“Grant’s Storm-Petrel” (ssp. nova): Breeds from the Azores east to the Berlengas and south to the Canaries from August to April (eggs laid from early October to early December). Calls end with signature punctuation. Relatively large and robust, and has a squarish or shallowly notched tail. Post-breeding adults show heavy feather-wear and/or molting flight feathers most noticeably from April to June—when Monteiro’s and “Madeiran” are fresh-plumaged.
“Madeiran Storm-Petrel” (castro): Breeds in Madeira, the Selvagens, and the Canaries from April to October (eggs laid in June and July). Calls are squeaky, rapid, about intermediate in length between Monteiro’s and Cape Verde, and lack the particular ending of “Grant’s”. Relatively small and slim overall; tail can appear either square-tipped or notched.
Cape Verde Storm-Petrel (jabejabe): Breeds in Cape Verde over an extended season, possibly year-round with two peaks—one from October into winter, the other from April into summer. Gives the longest, most complex calls, and genetic analyses indicate that it is the oldest divergent lineage in the band-rumped complex. Proportions are average, and has a squarish or shallowly notched tail.
“Gulf of Guinea Storm-Petrel” (ssp. nova): Breeds on São Tomé over an extended season, possibly year-round. Calls are relatively long and complex, suggesting an affinity with Cape Verde. Appears slightly larger than other forms, with a narrow white rump band and long-looking squarish or shallowly notched tail.
“St. Helena Storm-Petrel” (helena): Breeds on St. Helena and Ascension in two seasons—possibly multiple distinct forms or cryptic species—one from October to March, the other from April to September. Attends the nest diurnally. On some individuals the white rump band extends far down undertail coverts. Slightly larger than most other forms; usually shows a visibly notched tail.
Additional Photos of “Gulf of Guinea Storm-Petrel”
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova. © William H. Wagstaff
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova, museum specimens, dorsal view. © Angus Wilson & Robert L. Flood
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova, museum specimens, ventral view. © Angus Wilson & Robert L. Flood
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova. © William H. Wagstaff
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova, flying over montane forest. (Monte Carmo, Obô Natural Park, São Tomé; August 22, 2021.) © Ross Gallardy
“Gulf of Guinea Storm-Petrel”, H. c. ssp. nova. © Nik Borrow
References
BirdLife International. 2018. Hydrobates castro. The IUCN Red List of Threatened Species 2018: e.T132341128A132433305. https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T132341128A132433305.en. (Accessed December 22, 2021.)
eBird. 2021. eBird: An online database of bird distribution and abundance. Cornell Lab of Ornithology, Ithaca, N.Y. http://www.ebird.org. (Accessed December 22, 2021.)
Flood, R.L., R.F. de Lima, M. Melo, P. Verbelen, and W.H. Wagstaff. 2019. What is known about the enigmatic Gulf of Guinea band-rumped storm petrels Hydrobates cf. castro? Bulletin of the British Ornithologists Club 139:173-186.
Harrison, P. 1983. Seabirds: An Identification Guide. Houghton Mifflin, Boston.
Howell, S.N.G., and K. Zufelt. 2019. Oceanic Birds of the World. Princeton University Press.
Onley, D., and P. Scofield. 2007. Albatrosses, Petrels & Shearwaters of the World. Princeton University Press.
Taylor, R.S., M. Bolton, A. Beard, T. Birt, P. Deane-Coe, A.F. Raine, J. González-Solís, S.C. Lougheed, and V.L. Friesen. 2019. Cryptic species and independent origins of allochronic populations within a seabird species complex (Hydrobates spp.). Molecular Phylogenetics and Evolution 139:106552.
Xeno-Canto. 2021. Band-rumped Storm Petrel – Oceanodroma castro. https://xeno-canto.org/species/Oceanodroma-castro. (Accessed December 22, 2021.)