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Cryptic petrels

Alex Lees is an undergraduate in Biological Sciences from UEA, England and is currently studying at UCI in California.

Madeiran Storm-petrel/Band-rumped Storm-petrel Oceanodroma castro remains one of the most enigmatic birds on the British and Irish List to be whispered in the same breath as the likes of Black-capped petrel Pterodroma hasitata and White-faced Storm-petrel Pelagodroma marina. The two records of this species both involve dead birds, the first occurring at Milford in Hampshire on the 19th November 1911 and the second at Blackrock Lighthouse Co. Mayo on the 18th October 1931. There have been occasional whispers across seabirder’s lips subsequently but no record has gained acceptance. Madeiran/Band-rumped Storm-petrel is the ecological counterpart of Leach’s Storm-petrel O. leucorhoa in tropical and the adjoining temperate sector of Western Palearctic seas (Cramp & Simmons 1977). Within the Western Palearctic there are colonies on the Desertas and Selvagens and in Graciosa and Santa Maria on the Azores as well as on the Canaries. There is also a relatively recently discovered population of 200-400 pairs on the Fairlhoes off Portugal.

Its presence outside the breeding season is subject to speculation owing to the difficulty of separation from other Oceanodroma species. North Atlantic breeders are said to occur regularly well offshore from the bulge of west Africa, given the occurrence of records south to 10 degrees N in the North Atlantic (Cramp & Simmons 1977). More interesting for European seawatchers is the presence of Madeiran/Band-rumped Storm-petrels off the North American eastern seaboard. Most observations have come from pelagic trips, with North Carolina holding the lion’s share of the records. The pattern of European records of this species is somewhat messy, there have been three French records: two in October and one in August and two Spanish records in January and February. Additionally there have been two recent records of wrecked birds at Suonenjoki Finland on the 19th January 1993 and at Sion Wallis in Switzerland in December 1999. These scant records offer few clues to ascertaining this species dispersal patterns.

As a further complication this bird has another trump card to disguise its status, for research by Luis Monteiro and colleagues have documented the existence of two temporally segregated populations sharing the same nest sites on the Azores. Analyses of adult morphology indicated highly significant phenotypic differentiation between the sympatric hot- and cool-season breeding birds, whereas an almost complete phenotypic uniformity characterised allopatric breeders within the same season. The hot-season birds are 10% smaller in egg and body mass but have longer wings and tails than cool-season birds. They interpreted the preference to breed in the cooler season as a consequence of greater food availability in that period. They hypothesised that the hot-season population has evolved from the cool-season population owing to density-dependent constraints on crowded colonies, forcing birds to time-share nest sites (Monteiro and Furness 1998).

Peak fledging time for chicks from the hot season populations is the 8th September whereas the cold season birds fledge around the 28th January (Monteiro and Furness 1998). Dispersion by cold season birds (both adults and young) is likely to occur in mid winter thus explaining the occurrence of European records at this time. Cold season birds may also be responsible for the records off the US Atlantic seaboard. This theory is supported by the recovery on the Florida coast of a cool-season adult ringed in the Azores (Luis Monteiro in litt. in Sangster 1999). These populations, (and others exhibiting the same behaviour in the Galapagos and elsewhere) may represent a case of sympatric speciation following temporal partitioning of reproduction and could be treated as sibling species (Monteiro and Furness 1998). Ascertaining the non-breeding distribution in the field could be impossible without widespread colour ringing of the two forms.

However, the fledging date information does not indicate that an Oceanodroma petrel in winter is more likely to be a Madeiran/Band-rumped, as some Arctic populations of Leach’s Storm-petrels do not fledge until December. Even so careful scrutiny should be paid to any late Oceanodroma, storms at this time of year are often more potent that ones earlier in the autumn and more likely to dump a hapless petrel miles from the sea. In recent years the presence of this species in the southern part of the Bay of Biscay in autumn has been suspected (N. Symes pers com), along with small numbers of Little Shearwaters Puffinus assimilis although problems exist with the identification of both species in the field. Prolonged south-westerly winds could encourage both the shearwater and the petrel north, the former is claimed annually and a Madeiran/Band-rumped Storm-petrel was reported in mid July 2000 155 miles W of St. Mary’s on the Scillies.

Tubenoses are widely renowned for their longevity; a study on the Galapagos revealed that Madeiran/Band-rumped Storm-petrels do not return to their breeding islands for at least five years (Cramp & Simmons 1977), so individuals from the Atlantic Island populations will be roaming the Atlantic (or even further afield?) for many years before settling down. The best place to search for this species would perhaps be with other tubenoses on the shelf edge; such seamounts with associated upwellings are widely recognised as important foraging areas (Monteiro et al 1996). The Wilson’s triangle may yet hold another secret to give up for birders with a keen eye and a strong stomach!

References:

Cramp S, Simmons KEL 1977 Handbook of the Birds of the Western Palearctic Volume 1, Oxford University Press

Monteiro LR, Ramos JA, Furness RW, DelNevo AJ 1996 Movements, morphology, breeding, moult, diet and feeding of seabirds in the Azores, COLONIAL WATERBIRDS 19, 82-97

Monteiro LR, Furness RW 1998 Speciation through temporal segregation of Madeiran storm petrel (Oceanodroma castro) populations in the Azores? PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL SCIENCES 353: 945-953

Sangster G 1999 Trends in Systematics: Cryptic species of storm petrels in the Azores? DUTCH BIRDING, 21, 101-106

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	Cryptic petrels By Alex Lees Photo by Steve McConnell (Alabama Pelagic Birding) Alex Lees is an undergraduate in Biological Sciences from UEA, England and is currently studying at UCI in California. Madeiran Storm-petrel/Band-rumped Storm-petrel Oceanodroma castro remains one of the most enigmatic birds on the British and Irish List to be whispered in the same breath as the likes of Black-capped petrel Pterodroma hasitata and White-faced Storm-petrel Pelagodroma marina. The two records of this species both involve dead birds, the first occurring at Milford in Hampshire on the 19th November 1911 and the second at Blackrock Lighthouse Co. Mayo on the 18th October 1931. There have been occasional whispers across seabirder’s lips subsequently but no record has gained acceptance. Madeiran/Band-rumped Storm-petrel is the ecological counterpart of Leach’s Storm-petrel O. leucorhoa in tropical and the adjoining temperate sector of Western Palearctic seas (Cramp & Simmons 1977). Within the Western Palearctic there are colonies on the Desertas and Selvagens and in Graciosa and Santa Maria on the Azores as well as on the Canaries. There is also a relatively recently discovered population of 200-400 pairs on the Fairlhoes off Portugal. Its presence outside the breeding season is subject to speculation owing to the difficulty of separation from other Oceanodroma species. North Atlantic breeders are said to occur regularly well offshore from the bulge of west Africa, given the occurrence of records south to 10 degrees N in the North Atlantic (Cramp & Simmons 1977). More interesting for European seawatchers is the presence of Madeiran/Band-rumped Storm-petrels off the North American eastern seaboard. Most observations have come from pelagic trips, with North Carolina holding the lion’s share of the records. The pattern of European records of this species is somewhat messy, there have been three French records: two in October and one in August and two Spanish records in January and February. Additionally there have been two recent records of wrecked birds at Suonenjoki Finland on the 19th January 1993 and at Sion Wallis in Switzerland in December 1999. These scant records offer few clues to ascertaining this species dispersal patterns. As a further complication this bird has another trump card to disguise its status, for research by Luis Monteiro and colleagues have documented the existence of two temporally segregated populations sharing the same nest sites on the Azores. Analyses of adult morphology indicated highly significant phenotypic differentiation between the sympatric hot- and cool-season breeding birds, whereas an almost complete phenotypic uniformity characterised allopatric breeders within the same season. The hot-season birds are 10% smaller in egg and body mass but have longer wings and tails than cool-season birds. They interpreted the preference to breed in the cooler season as a consequence of greater food availability in that period. They hypothesised that the hot-season population has evolved from the cool-season population owing to density-dependent constraints on crowded colonies, forcing birds to time-share nest sites (Monteiro and Furness 1998). Peak fledging time for chicks from the hot season populations is the 8th September whereas the cold season birds fledge around the 28th January (Monteiro and Furness 1998). Dispersion by cold season birds (both adults and young) is likely to occur in mid winter thus explaining the occurrence of European records at this time. Cold season birds may also be responsible for the records off the US Atlantic seaboard. This theory is supported by the recovery on the Florida coast of a cool-season adult ringed in the Azores (Luis Monteiro in litt. in Sangster 1999). These populations, (and others exhibiting the same behaviour in the Galapagos and elsewhere) may represent a case of sympatric speciation following temporal partitioning of reproduction and could be treated as sibling species (Monteiro and Furness 1998). Ascertaining the non-breeding distribution in the field could be impossible without widespread colour ringing of the two forms. However, the fledging date information does not indicate that an Oceanodroma petrel in winter is more likely to be a Madeiran/Band-rumped, as some Arctic populations of Leach’s Storm-petrels do not fledge until December. Even so careful scrutiny should be paid to any late Oceanodroma, storms at this time of year are often more potent that ones earlier in the autumn and more likely to dump a hapless petrel miles from the sea. In recent years the presence of this species in the southern part of the Bay of Biscay in autumn has been suspected (N. Symes pers com), along with small numbers of Little Shearwaters Puffinus assimilis although problems exist with the identification of both species in the field. Prolonged south-westerly winds could encourage both the shearwater and the petrel north, the former is claimed annually and a Madeiran/Band-rumped Storm-petrel was reported in mid July 2000 155 miles W of St. Mary’s on the Scillies. Tubenoses are widely renowned for their longevity; a study on the Galapagos revealed that Madeiran/Band-rumped Storm-petrels do not return to their breeding islands for at least five years (Cramp & Simmons 1977), so individuals from the Atlantic Island populations will be roaming the Atlantic (or even further afield?) for many years before settling down. The best place to search for this species would perhaps be with other tubenoses on the shelf edge; such seamounts with associated upwellings are widely recognised as important foraging areas (Monteiro et al 1996). The Wilson’s triangle may yet hold another secret to give up for birders with a keen eye and a strong stomach! References: Cramp S, Simmons KEL 1977 Handbook of the Birds of the Western Palearctic Volume 1, Oxford University Press Monteiro LR, Ramos JA, Furness RW, DelNevo AJ 1996 Movements, morphology, breeding, moult, diet and feeding of seabirds in the Azores, COLONIAL WATERBIRDS 19, 82-97 Monteiro LR, Furness RW 1998 Speciation through temporal segregation of Madeiran storm petrel (Oceanodroma castro) populations in the Azores? PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL SCIENCES 353: 945-953 Sangster G 1999 Trends in Systematics: Cryptic species of storm petrels in the Azores? DUTCH BIRDING, 21, 101-106

Cryptic petrels

By Alex Lees

Photo by Steve McConnell (Alabama Pelagic Birding)