Posted by Bjorn Johansson (62.20.93.130) on October 22, 2001 at 23:55:34:
Moult in jaegers
Long-tailed start a post-juvenile moult january-march (1st winter= 2nd calenderyear) including primaries and tail. The new central tail-feathers in Long-tailed is pointed. They are about the same length as in juvenile. In Parasitic the tail feathers are still growing in june - july. When outgrown in fall they are about the same length or a little longer than juvenile. In 2nd summer (3rd calenderyear) the central tailfeathers in Long-tail is 2-9 cm longer than the rest of the tail.
A 1st summer jaeger (all species) is very similar to a juvenile. Only determinable on the absecence of pale fringes to coverts on upperwing and on the dark indivuals the belly becoming white. Underwing as in juvenile. 1st summer birds are extremely rare on the breeding grounds. For instance on the 1994 expedition along all the Russian artic coast from the Kola peninsula to Chuckee no 1st winters were seen and only 1 imm/subad Long-tailed and 2 Parasitic.
What is puzzling with the bird in the photo is that the body looks adult, but the coverts are like a 1st summer. A 2nd summer, which this bird must be, usually have some adult-type dark coverts.
As for proportion diffences between Parsitic and Long-tailed it is useless if not in direct comaparison, especially in just one photo. I suppose a tele-lens also distorts proportions to some degree. As for P10 it is variable in all three jagers and at best just an indication towards Long-tailed in juvenile plumage, I don't know what's it worth in imm. birds…
I think the bird in qustion is a Parasitic, but normally I don't want to identify a jaeger on a single photo from this angle. I would be nice to see a shot showing the upperwing!