I was working in the Strumpshaw office, when Ben the warden wandered in. "I've got some photos for you," he said, waving his camera. I peered at the camera screen - a tiny image of a distant marsh harrier swooping over the reedbeds. To be frank, it was a rubbish photograph. But Ben is a brilliant photographer, so why was he showing me this blurred speck of a marsh harrier?
"Look closely," he said. "Can you see? - its cream cap is in the wrong place. It's on its neck instead of its head."
Consult any bird book and it will tell you that female marsh harriers are dark brown with a cream head, while males have a brown head and body and grey wings. At least that's what I've always believed. But nature has ways of surprising us - reminding us to delve below the surface. For all is not what it seems in marsh harrier world.
Ben had been watching this oddly marked bird for several weeks. Its actions, he said, were even stranger than its plumage. It was clearly making amorous advances to its fellow female marsh harriers. We've all heard of male-male penguin pairings, so was this a case of lady-lady love, or something even more intriguing?
Ben had an inkling what might be going on. Marsh harriers - it turns out - are one of only two bird species in the world that display 'female mimicry', where some males develop female plumage as adults. I had heard of this happening in fish and insects, but never in birds. That's because it's exceptionally rare in birds (other than marsh harriers it's only recorded in ruffs).
Scientists in France have been probing the possible reasons for female mimicry in marsh harriers. Using cunning experiments with decoy birds they discovered that the lady-like males are far less likely to be attacked by other territorial males and can have sneaky liaisons with the ladies without being fought off.
This is still a new area of research, with many unanswered questions, but I for one feel rather excited that such a strange and rare phenomenon is playing out above the reedbeds of Strumpshaw Fen. (And might have slipped us by unnoticed, but for hawk-eyed Ben).