Monterey Seabirds We actually submitted and received comments from Klaus Olsen the author of the 'Skuas and Jaegers: A Guide to the Skuas and Jaegers of the World' guide about our ex-mystery Jaeger. Below is his reply: Thanks for contacting me about this interesting skua, which is identifyable as Long-tailed (longicaudus) 2nd cal. year "type".
In immature stages, plumage details are of very little help when identifying skuas. Here, the head is rather big and rounded compared to parasiticus, very important with a rather short bill IN WHICH THE LONG GONYDIAL ANGLE ENDS HALF-WAY towards the base of bill. this is perfect for longicaucus; in parasiticus, the much shorter gonys ends app. 30% towards the base of the bill, and furthermore, the bill is narrower, as well as the forehead being flatter.
Point 2 is the very long central pair of tail feathers. In 2nd cal year skuas they are much shorter than in older birds, and here we have an extension which is comparatibly much longer than seen in any -even adult - parasiticus.
The primaries have a small amount of pale at bases, in line with juvenile feathers (but rarely seen in older immatures as well, albeit with very small amount of pale).
So, from the photos, I identify this bird as longicaudus - probably a good record offshore California.