As promised, some photos from an evening at an amazing harrier roost - we estimated about 250 birds, and counted 200 in the air at one time. 90% were Eastern Marsh Harriers, with a small minority being Pied Harriers. Though spectacular to witness it wasn't particularly easy to photograph, as the birds were spread widely and rarely came within photographic range.
Eastern Marsh have a distinctively long head projection and powerful bill compared to Pied Harriers. Chaiyan Kasorndorkbua's comment on this bird: "A second calendar year (2cy) male." Pointers include the grey central tail feathers, male-type upperwing coverts and pale iris.
This one is certainly a male. Chaiyan aged this as a full adult (despite brown barring on rear flanks).
A white leading edge to the wing from wing-base to carpal is a good feature of Pied, but one needs to be careful - Eastern Marsh can show a very pale buff-brown leading edge which can look white!
The same bird a lot closer.
The bird is mostly in 1st year plumage (I think) but the central pair of tail feathers are new and presumably 2nd calendar year feathers. Chaiyan's comment: "2cy female." [EDIT: Comment from Teruaki Morioka: This bird is probably a 2nd winter female, because most tail feathers have already been replaced. But I cannot elininate the possibility of it being a 1st winter bird, as most remiges appear juvenile-like. A view of the upperside would be helpful!]
An adult male.
Male and female. The female is a 3cy bird.[EDIT: Comment from Teruaki Morioka: This is likely to be a third winter bird because of dull grey. But are there no older females with dull grey barrings on remiges? The females breeding in Japan have very little grey and the males of the Japanese population have less grey than the females of continental EMH.]
This bird has strongly barred black and pale grey outer primaries on the left wing. Chaiyan's comment: "A 1st w or 2cy male." Note the other birds starting to arrive in the background!
A few extra images which Chaiyan has aged for me:
A 3cy female.[EDIT: Comment from Teruaki Morioka: This bird is in at least her 3rd winter. The barring on the primary underwing-coverts appear for the first time in third generation feathers.]
A near adult female.
An adult female.
A juv Pied Harrier looks petite in comparison with the bulky Eastern Marshes.
Adult male Pied is one of the best raptors there is, but sadly none came close today.
A few Purple Herons flew by!
In the last moments of daylight, the birds all took off and performed a spectacular wheeling 'sky-dance'.
Bedtime for the harriers, and time for me to start my 3-hour drive home!