Friday, March 29, 2013

Pirates Parade: The Autumn Jaeger Migration at Woodman Point

Many birders are familiar with the annual migration of waders between the Arctic tundra and Australia, but some may not realize a similar migration takes place just offshore this time of year – that of jaegers head north towards their far northern summer breeding grounds. An excellent place to witness this spectacle is at Woodman Point south of Fremantle, where the long sandy spit jutting around 1.5kms into Cockburn Sound provides a perfect platform for jaeger-watching. 
 
A patient seawatch here during the migration (which builds through March to peak around mid-late April) will usually reveal a steady procession of jaegers flying northwards, as well as some hanging around to feed by kleptoparasitic attacks on gulls (chasing them until they regurgitate food). Woodman Point is a perfect site to witness these spectacular aerial chases, sitting directly next to a seagull ‘highway’ between Carnac Island and a local rubbish tip – for the jaegers, a somewhat different diet to their summer fare of lemmings and voles! 
 
Two intermediate-morph Arctic Jaegers Stercorarius parasiticus close in on a Silver Gull at Woodman Point.


The same birds are joined by an adult summer pale-morph with one central tail projection. These feathers, diagnostic of species when present, are frequently missing or damaged in birds on autumn migration 
 
Most of the jaegers at Woodman Point are so-called 'Arctic Jaegers', our antipodean name blending the American preferred ‘Parasitic Jaeger’, and the European name of ‘Arctic Skua’. The term jaeger is derived from a German word for ‘hunter’, and is used to distinguish the three smaller skua species from the larger, browner ex-‘Catharacta’ skuas (‘skua’ is an old-Norse term referring to the northern hemisphere Great Skua). Brown or Subantarctic Skuas (Stercorarius antarcticus lönnbergi) winter along the Australian coast, arriving in Cockburn Sound in early April just as their smaller cousins begin to leave.

Adult summer pale-morph Arctic Jaeger with a single central tail projection. Arctic Jaegers typically show more of a white 'flash' on the primaries than this bird
 
Arctics are dimorphic in their plumage, with about half the local birds being ‘light morph’ (white-bellied), and half the darker-brown ‘dark morph’. The stability of these plumage variations is an intensively-studied ornithological puzzle, particularly a latitudinal phenomenon whereby dark morphs more common from the more southern breeding areas. Added to the fact that most passage birds are in transition from winter to summer plumage, and that immature birds can look very different (heavily barred and often rufous plumage), the plumage of Arctic Jaegers is extremely variable and rarely matches the illustrations in field guides. Even size can be variable, with a noticeable dimorphism between males and females. 

Adult pale-morph Arctic Jaeger in transition to summer plumage – but lacking tail projections.

An older immature dark-morph Arctic Jaeger. Note the scruffy moult of the underwing coverts – typical of passage birds in April.
 
A jaeger watch at Woodmans will often also yield a few Pomarine Jaegers, heavier and more ‘pot-bellied’ birds with greater underwing ‘flash’ and a larger black hood. Pomarines are at the centre of a very interesting taxonomic debate concerning multiple lines of evidence (behaviour, nuclear and mitochondrial DNA, shared feather lice) that Poms are most closely related to the Great Skua Stercorarius (Catharacta) skua [1]. The most likely explanation for this seems to be relatively recent hybridization between the two lineages after the origin of the otherwise monophyletic Catharacta group, to which Pomarines are a sister taxon and more closely related than they are to their smaller cousins [2]. Most taxonomies now combine the whole group into a single genus Stercorarius, with Long-tailed and Arctic as basal splits – thus skuas and jaegers now differ in English name only. 

Pomarine Jaeger at sea. Note the bigger body, broader wings with more extensive white ‘flashes’, larger head and bill, and deeper black mask compared to Arctic.
 
The third jaeger species, Long-tailed, may also be a possibility at Woodman Point. Long-tailed Jaegers are more pelagic in their habits than the other two species, and spend most of their lives far out to sea. However several were observed at Woodman Point in April 2007 [3], raising the possibility that the concentration of other jaegers in the areas may attract them in from offshore. The observers concluded: 
“Given the difficulty in picking out pale/intermediate phase immature Long-tailed Skuas (let alone dark-phase birds) it may be that the Long-tailed Skua is a very scarce migrant which just isn't picked out, rather than being a "true" rarity. Only more systematic and careful observations will clarify this."
 
The number and diversity of jaegers at Woodman Point, plus the chance of these rarer species, offers a highly recommended annual birding challenge. To reach Woodman Point, turn off Cockburn Road (Munster) at O’Kane Ct, follow the road around the left, then head west on Woodman Point View. From the last parking area, it is about a 300m walk to the end of the spit where the closest views are usually afforded. 

Immature (second summer) pale-morph Arctic Jaeger at Woodman Point. Note the barred rump and underwing. This small and slender bird demonstrates the broad range of body size possible within Arctic Jaegers.

An older immature dark-morph Arctic Jaeger in winter plumage (note barred rump and pale fringing on mantle) closes in on a Silver Gull in an attempt to steal a meal.

References: 
[1] Cohen BL et al. (1997). Enigmatic phylogeny of skuas (Aves: Stercorariidae). Proc Royal Soc Lond 264: 181-90. 
[2] Andersson M (1999a). Hybridization and skua phylogeny. Proceedings of the Royal Society of London, Series B, Biological Sciences 266, 1579–85 
[3] Darnell J & Collins A (2007). Skuas at Woodman Point. WA Bird Notes 122 (June 2007)  

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