Sooty Shearwater - some questions about moult

I've been working on some preliminary drawings for a plate of Sooty and Short-tailed Shearwaters for The Tubenose Handbook (Shirihai & Bretagnolle in prep). Looking through my photos of Sooties from California I was reminded of a mental note I made at the time but, as usual, forgot about until now. By a happy coincidence it's now approaching peak Sooty season so perhaps it's timely to chuck this particular pebble in the pond and see what happens.

Sooty Shearwater - 5/9/2011 off Point Loma, California

A fairly ordinary Sooty, the flight feathers look fresh - it's hard to tell from this angle - but the body feathers appear to be more worn/faded.

Sooty Shearwater - 6/9/2011 Channel Islands, California

Sooty Shearwater - 6/9/2011 Channel Islands, California

This one has almost completed primary moult - P10 is still growing - but everything else looks worn and the underparts are particularly pale and faded. Of the several hundred Sooties I saw this one was not unusual and in fact many looked even tattier, I just didn't get useful photos.

For comparison, here's a Sooty I photographed off West Cornwall a couple of months earlier.

Sooty Shearwater - 25/7/2011 Cornwall

Sooty Shearwater - 25/7/2011 Cornwall

This is typical of the birds I've seen off Britain over the years, uniformly dark, greyish or brownish depending on the light and in fairly fresh plumage; I can't recall ever seeing a Sooty as pale and faded as the ones off California. Admittedly most Sooties are seen at distance and often in poor light, not close and from a boat like this.

Some background: Sooties are, of course, southern hemisphere breeders, mostly SE Australia, New Zealand and Chile with much smaller numbers in the South Atlantic (Marchant & Higgins 1990). Adults and just fledged young disperse in late April to early May and most follow a figure of eight migration route into both the North Pacific and North Atlantic, returning south during August to October. Breeding adults begin wing moult immediately after the young have fledged (body moult begins earlier) and typically complete by September. Juveniles do not start moult until January or February of their second calendar year, throughout subsequent years this begins later and later until the typical breeding adult moult cycle is reached (Marchant & Higgins 1990, Howell 2012).

So birds not in wing moult during the northern summer (May to August) would be expected to be birds of the year. By September, moderately worn juveniles should differ from freshly moulted adults.

Where does this leave the very worn September birds? In these birds primary moult is nearly complete but secondary and body moult appears to have hardly begun. The moult cycle is behind, not ahead of, breeding birds so I presume these are not immatures. Are they sick or just not in breeding condition so delaying moult?

Curiously, most Sooties seen in summer in the western North Atlantic appear to be in fresh plumage, 99% of birds seen off North Carolina in late May and June are not in moult (Howell, 2012) so presumably are juveniles or recently completed second year birds. I get the impression, from personal observation and from published photos, that most birds seen off the UK are not in moult either although, since the peak time is August-September (Cramp & Simmons 1977), this may include some completed adults and immatures.

As ever, I started looking in the books and then had to chase up some references, which led to more references and so on. Here's a summary of some of studies to date on Sooty Shearwater moult: of 603 birds recorded in the North Atlantic between May and November by Brown (1988), none were in primary moult. However, Keijl (2011) found 35 of 76 (46%) of birds photographed near Rockall in July to be in primary moult. Cooper et al (1991) recorded moult in 653 beached Sooties from the southwestern Cape, South Africa; numbers (and percentage in moult) peaked strongly in February and March, the timing indicating these were either juveniles, young pre-breeders or failed breeders. Few were found during May to December as would be expected if most birds were in the northern hemisphere.

This post has turned into rather more than I intended (and apologies for getting a little bogged down in references to moult studies) but has only reinforced my impression that there's something puzzling going on. Are most North Atlantic birds juveniles or immatures as suggested by Brown (1988)? Do South Atlantic adults tend to travel north in the Pacific rather than the Atlantic as might be expected? Are juveniles more likely to be seen close to shore, do older and/or moulting birds feed in different areas? As ever, more research needed...

Sooty Shearwaters

References:

Brown, R. G. B. 1988. The wing-moult of Fulmars and shearwaters (Procellariidae) in Canadian waters. Canadian Field Naturalist. 102:203-208
Cooper, J., Underhill, L.G. & Avery, G. 1991. Primary molt and transequatorial migration of the Sooty Shearwater. Condor 93: 724-730
Cramp, S., & Simmons, K.E.L. (eds.)1977. Handbook of the Birds of Europe, the Middle-east, and North Africa. Vol. 1. Oxford University Press
Howell, S.N.G. 2012. Petrels, Albatrosses & Storm-petrels of North America. Princeton University Press
Keijl, G.O. Sooty Shearwaters Puffinus griseus in the North Atlantic - moult studies using digital cameras. Marine Ornithology 39: 141-142
Marchant, S., & Higgins, P.J. (eds.) 1990. Handbook of Australian, New Zealand, and Antarctic Birds, Vol.1. Oxford University Press