Thursday, December 1, 2011

Intermediate Morph Soft-plumaged Petrels off Perth

The Soft-plumaged Petrel (Pterodroma mollis) is a common visitor to offshore waters of southern Western Australia, predominantly from late autumn to early spring. Pelagic seabird trips off Perth, run since 1996, have recorded the species on 27 of the 31 trips run (an 87% recording rate). The species has been recorded less commonly on pelagic trips off Albany, run since 2003, having been recorded on five of the nine trips run (a 56% recording rate).

Soft-plumaged Petrel off Perth.

Soft-plumaged Petrels are one of several petrel species to exhibit polymorphism. The pale morph is easily the most commonly occurring variant, but dark morph individuals are also regularly (but rarely) recorded. Intermediate individuals have been recorded, but these are particularly rare. Indeed, Onley & Scofield [1] report only “a few scruffy intermediate specimens … in museum collections” and “no recent records at sea or from breeding grounds”. This assertion is clouded by differences in treatment of intermediate individuals between authors. Shirihai [2] for example appears to include all intermediate morphs, along with the wholly dark morph, under the dark morphs. He describes the dark morph as being “partially (mottled) to wholly dusky-brown/sooty-grey on underparts, with variable breastband, but rest of plumage as pale morph”. Much of the confusion is likely down to the fact that in reality there is not a distinct intermediate morph; rather intermediate birds can fall anywhere on a spectrum between typical pale morph birds and fully dark individuals.

Pale morph Soft-plumaged Petrel off Perth.


In August 2010, a pelagic trip off Perth recorded two intermediate morph individuals amongst 35 or so Soft-plumaged Petrels seen on the day, and represented the first records of intermediate-plumaged birds off the WA coast. However, dark birds have been recorded previously off Perth in May 1999 (2 individuals described as ‘grey birds’ [3]) and off Albany in November 2003 (one bird, described as a dark morph [4]). Unfortunately, we haven’t been able to locate photos of either of these individuals, so it is difficult to be sure whether these records represent fully dark birds, or dark intermediates.

The two August 2010 individuals were both photographed, and apparently occupied opposite ends of the scale of intermediate individuals. The first was close in plumage to a regular pale morph bird, but with fine grey streaking on the flanks, and if Onley & Scofield are correct, may represent one of the first at-sea records of this plumage type.  The second individual approached a dark morph individual in plumage; indeed, while we describe it as a dark intermediate bird, it could equally be described as a relatively pale dark phase bird, and may fall under the category of dark morph according to both Onley & Scofield and Shirihai’s descriptions.

Pale intermediate morph Soft-plumaged Petrel off Perth, August 2010. Note the fine grey streaking along the flanks that distinguishes it from a pale morph.

Dark intermediate morph (or pale dark morph) Soft-plumaged Petrel off Perth, August 2010.
See also Rob Morris's photo on Eremaea.

A subsequent pelagic trip in the same area a month later recorded another intermediate morph individual, although unfortunately on this occasion very rough conditions precluded any photos from being obtained.

Dark morph Soft-plumaged Petrel off Gough Island, part of the Tristan da Cunha island group in the southern Atlantic.
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UPDATE:
Another Soft-Plumaged Petrel with grey flank streaking was photographed during an organised pelagic trip, approximately 45km SSW of Albany, on Saturday 28th July 2012 (see report here). While at the 'light end' of the spectrum, the presence of flank streaking again marks this bird as intermediate morph according to some definitions. 

"Light intermediate morph" Soft-plumaged Petrel with fine flank streaking,
photographed on an Albany pelagic trip,  July 2012. 
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UPDATE 2:
Another intermediate-morph Soft-Plumaged Petrel, this time a more 'classic' intermediate-morph with 'dirty' or dusky underparts and an indistinct collar, was observed on the Albany pelagic trip Saturday 4th May 2013 (see report here). 

Intermediate morph Soft-plumaged Petrel 
photographed on an Albany pelagic trip,  4 May 2013. Note dusky underparts and indistinct collar.
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We would welcome any comments on these birds, either in the comments section below, or privately via e-mail at wabirdingblog@gmail.com.

References
[1] Onley D & Scofield P (2007) Albatrosses, Petrels and Shearwaters of the World. Christopher Helm, London, p. 185.
[2] Shirihai H (2008) The Complete Guide to Antarctic Wildlife 2nd Edition. Princeton University Press, Princeton.
[3]
http://birdingwa.iinet.net.au/pelagics/p19990523.htm
[4] http://birdingwa.iinet.net.au/pelagics/p20031102.htm

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