I was talking to my literary agent at the time, but I hung up on her. Something to do with royalties. The call came through when I was at the Hen Reedbeds, the Suffolk Wildlife Trust's wonderful wet spot near Southwold. I had gone there mainly for marsh harriers, and there was a stunning male, startlingly pale, gleaming with silver highlights. But I wouldn't put the phone down on Georgina Capel for that.

We were having a fascinating chat about me (and you know how soothing that is) when it happened. “I'm sure the book is going to be absolutely marvellous, and what's more - .” “George, I have to go.” And did. Because three royalties were parading before me: two avian princes and a princess emerging from the reeds and taking to the skies. Talking, even on my favourite subject, was no longer an option.

We are all familiar with the spectacle of the completely impossible: when someone you know very well behaves in a way that you would never, not in a million years, expect, something that makes you doubt the evidence of your senses; a sober-sides dancing madly; a plain woman suddenly beautiful; a shy person holding the floor. It happens with wildlife too.

I know bitterns. I have lived with bitterns for 20 years. I know where they live, and I know what they do. Above all, I am familiar with the fact that you don't see them. They are big, burly things in shades of black and gold, and they lurk in the reedbeds. In spring they make a weird noise: if there was time travel I would go back 200 years to the fens to listen to the booming of a hundred bitterns.

But you don't see them. They hate to be seen. Occasionally, just occasionally, a single bittern will fly low over the reeds from one food source to another. Rarely, rarely, you will see them sneaking about on the edge of the reeds. They are not birds for seeing. You just know they are out there, and feel richer for knowing.

Then all of a sudden the sky was full of them. There were three: you never see them together. And they were going nuts. Round and round they flew, wrapped up in some profound botaurine drama, a wild pursuit, with evasions and jinks and a huge concentration on each other. Eventually, one of them dropped to the ground and disappeared into the reeds. The remaining two completed a few more circuits, and then there was one. There was a brief heron-like croak, something I have never heard before, and then there were none.

What prompted all this mad behaviour? Perhaps the same thing that prompts the dramatic and inexplicable changes in your friends. Sex, of course. I rang a few bittern experts: yes, they said, sex, what else?

There were three bitterns over-wintering at Hen Reedbed, and two into three won't go. So this was a negotiation between two males and one female for the right to make more bitterns. Perhaps this was an inflection in a long-running drama, perhaps it was the moment of decision.

The point was that it mattered so much to the bitterns that they shrugged off their normal bittern selves and became something else. Life presents these sudden extraordinary dramas to an audience of one.

And the sea levels rise, and I wondered how much longer these fine marshes would hold fresh water and bitterns. But the wild trio took no thought for that. For the winners, a tryst in the wet reedbeds; for me, a phone call full of apologies.

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