Coots Building 'Second Nests'

Over the last week, I have on two occasions seen coots on the river Kennet building what appeared to be 'second nests'. 

The first time I saw this was when I saw one of the pair that had brought up two chicks on the nest near Highbridge Wharf started piling up strips of greenery under the boat traffic light just east of the Duke Street bridge.  The pair's two chicks seemed to be joining in, well one of them was standing on top of the pile.  At first I thought they had decided to move nest but after a day or so I saw that the new nest had disintegrated and they were back at their old nest again.

Then, this morning I saw that one of the coots from the nest at the mouth of Gunter's Brook was carefully draping strips of greenery over a log that had become fixed near the river bank.  It was placing the strips apparently quite deliberately, while its solitary chick looked on.

My first thought was this building of second nests could be the way that the parents teach their chicks how to build nests.  Another possibility is that these piles of greenery are food stores, however, this is less likely because there seems no reason to build them away from the main nest.