 Last week a once-in-a-lifetime oriental turtle dove landed in a Chipping Norton garden and more than a hundred twitchers paid a fiver each to get a view from the kitchen window. I hear an echo of the orient in my own back garden right now. The "coo, coo-coo" song comes again, heard every day but rarely listened to. Two notes short of a woodpigeon, the monotonous refrain of the collared dove carries an underlying fuzzy vibrato that speaks of the tropics. This bird first arrived in Britain nearly 60 years ago.  This morning, a visibly exotic arrival turned up in the most mundane of surroundings by Girtford Bridge. a brilliant white shape pulled my eyes down from the treetops into a roadside ditch. I almost passed it off as a swan, but a narrow, steep-sided channel with only a thin stream of water was hardly the place. Besides, though it was partially obscured by a sallow bush, the bird was making animated, jerky movements and swans do not do jerky. after a moment, the little egret pranced out from behind its twiggy curtain. a stiletto beak on a vacuum hose neck made constant little stabbing motions into the water and down the sides of the banks, then the spindly legs pattered a few steps forwards and the bird repeated its gleaning ritual. It reached a concrete culvert and I expected the egret - all angles and points - to swoop up and away on great sheets for wings. Instead, it tripped off into the tunnel.  I can still - just - remember every egret sighting within walking distance. But the map is turning white, for the egret's climate change-driven spread north is happening apace. How long before the extraordinary becomes ordinary?  Derek Niemann