The plumage of Western Marsh Harriers / Rohrweihen (Circus aeruginosus) can be amazingly variable thanks to different morphs, individual variation and depending on sex and age.
Some variation in adult male Western Marsh Harriers:
I recently came across the cracking bird below at Lake Neusiedl, Austria. A very extreme adult male that recalls Eastern Marsh Harrier (C. spilonotus) at first sight. It appears grey, white and black and lacks almost all brown tones on the upperparts. The eastern cousin has a range across much of eastern Asia and hasn’t been recorded in Europe so far.
Eastern Marsh Harrier lookalike from Seewinkel:
Since there seem to be no papers dealing with the identification/separation of Western Marsh Harrier and Eastern Marsh Harrier, here are the differences that I can see in adult males of both species:
- underparts (flanks, belly, vent): brown in WMH, pure white or light grey in EMH (resembling Hen Harrier); underwing largely white except for dark fingers in EMH
- fingers: black in WMH, paler with irregular barring or mirrors in most EMH
- upper wing coverts: scaled or spotted pattern in EMH, greater coverts grey with dark tips; usually uniform and brown in WMH
- head: dark hood with streaked upper breast in EMH, in WMH often paler than back or with pale
- tail: pale and uniform light grey in EMH, with brown tone in WMH and more contrasting to pale base
Eastern Marsh Harrier from China:
Thanks to Terry Townshend (Birding Beijing) for letting me post his photos above.
Another post on this topic from the Netherlands.