Frenchy said:
Birds don't grow new feathers underneath exsting ones,
You could be right and I would have agreed with you until fairly recently. Note this shot on Surfbirds, for example:
Note the brand new secondaries that have just about reached the old juvenile feathers on this 2nd calendar Red Foot. This isn't the only example I have found.
However, I've had another look at the pics of the Benacre bird and I'm not as convinced as I was that there are new feathers. But still, to my eye it looks like there are some darker, fresher secondaries on this bird. I am probably wrong.
However, if this bird hasn't begun moult, then that could be a very strong argument for it being an escape as I believe captive birds are more likely not to undergo moult when they are supposed to. 2nd calendar to adult Marsh Harriers should have undergone remex moult by now.
This bird cannot be a juvenile - have a look at the Winterton Pallid Harrier shots on Surfbirds for a good example of how fresh and unworn juvenile large BOPs will look.
Anyway, back to the ID. This bird has primary fingers that appear to thin for Marsh Harrier. Marsh harrier has 'chunky' fingers that result in a 'hand' that is as broad as the 'arm'. In other words, the wings are broader and are as broad to the primary tips on Marsh Harrier whereas to my eye, the Benacre bird has a hand that narrows.
The Chestnut on the underparts looks just a little too reddish for Marsh Harrier. I've tried to find a Marsh Harrier via Google that matches this bird for colour and I can't find one. By all means prove me wrong. This is, after all, a very difficult bird to ID.
The shape of the body on the Benacre bird doesn't look right for a Harrier. It has a barrel-like body with a 'domed' back more like a buteo? Have a look on Google and again try to match the Benacre bird to other pics of Marsh Harrier.
I have found one species, so far, that could be a match for this bird. It shows white on the vent, a white throat and the underparts are a match for the colour on the Benacre bird, I feel. Try having a look at the 'intermediate' form of Swainsons Hawk (Sibley). This species can show considerable variability, it seems, and that could explain the white assymetrical blotches on the underwing etc.
Cheers,
Andy.
Ps: Nice Grosser.