The bird nobody was watching

I joined Birding Bob's group in Central Park on Saturday morning. The Ramble was misty and cold. Everyone had their binoculars up, looking for warblers and rarer birds.
I don't know much about birds. There was this little one walking along with us — hopping on the trail, pecking at the dirt, flying a few feet ahead and waiting. It did this for a good twenty minutes!
At some point it found a string hanging off a branch and started doing something with it. Tugging at it, turning it around, lifting one wing and then the other. I couldn't tell if it was playing or trying to figure it out. I kept looking around to see if anyone else was watching. Nobody was.
Meet the American Robin of Central Park.

Goes for the string.
Goes for it again.