Strange Goings-on on the Ponds

The other day I was walking along beside Carshalton's famous ponds when I noticed something surprising. Sitting atop the nest - a great pile of sticks and detritus - that a pair of Coots had built to raise their young was a female Tufted Duck and her brood of five ducklings, all huddled cosily together and looking perfectly at home. The Coots had sent their young out into the watery world some while back, so the nest had served its purpose - how cheering that it was now being put to good use for a second time by an unrelated species. What a fine example of mutually beneficial co-operation in the natural world...
  Half an hour later, passing by the ponds again, I saw that all had changed. The nest was now occupied by two burly adult Coots, looking about them with a decidedly proprietorial, not to say threatening, air. The family of Tufted Ducks was nowhere to be seen.
 We should never rely on Nature to live up to our amiable fantasies.