Fewer Marsh Harriers than last year?

I was told by one of the wardens last year that the quarrying operations at Ouse Fen had involved the removal of a mature lone tree out to the Eastern side of the fen that was being used as a winter roost by up to nine resident Marsh harriers. As far as I can make out (subjectively), the number of Marsh Harriers over the reserve this Spring are still down, which is a pity.

Photo taken on the bridleway from Brownshill Staunch to Earith, not on the path across Cuckoo Fen (which remains closed).