Nest building woodpigeons

On a walk in my local park, I came across two different Woodpigeons collecting nest material. This is likely to be the second brood of the season. I stopped to try and get some shots of the second one. The nest was in a large Horse Chestnut. The Woodpigeon would come to the ground, walk to the pile of sticks that had fallen from the nest, peck them until one suited him, and then fly up to the tree. Occasionally, the stick would come tumbling down a few seconds later. I wonder if they were in the early stages of nest building and the platform wasn't stable enough. Usually one of the individuals of the pair bring the nest material to the other, which is sitting on the nest, and this individual arranges the sticks.
 I had a long session while sitting at the base of the horse chestnut. Every couple of minutes or so, I would have a chance to take a shot or two of the woodpigeon choosing a stick or flying up. The woodpigeon continued its merry-go-round taking no notice of me.