Carolina Wren behavior

We have a pair of Carolina wrens that have for years nested in a birdhouse in our carport. About 5 feet from the birdhouse is a small bucket filled with bottle caps, which hangs about 3 feet off the ground. Before they build a nest, one (or maybe both) of the wrens hops into the bucket, picks up a bottle cap, and drops it nearby. This happens every time they nest. They drop 5 or 6 in the area. Anyone well-versed enough in the behavior of Carolina wrens to have an explanation for that? Thanks.

My first thoughts were maybe the little Wrens wanted to put a "toy" into the nest for the young to peck at and play with, but found the caps too heavy to get into the nest. Some birds do this, the reasoning isn't entirely known other than it might help the young to develop skills later needed.

I looked at Sialis.org and they brought up the wide variety of nesting places the Carolina Wrens use and I wonder if they originally want to nest in that bucket, but find the removal of the bottle caps too much work.
http://www.sialis.org/nestscarolinawren.htm

I am just speculating, but find this really interesting. Sometimes I wish I could know what birds are thinking.

This may also be way off base, but I've read that many birds like to "decorate" their nests. Some will deliberately add shiny items, for instance.

I don't really understand it as it seems counter-productive since they want their nests to be hidden, but I've read this many times over the years. Perhaps your Carolina Wrens are just attracted to those bottle caps.