As the day gets busier, a tmux session could really need some little re-organization. To tidy things a bit, you can move a pane from one crowded window to another using join-pane.

It requires two steps:

1. break the pane out via break-pane. This is bound to <prefix> ! by default.
2. Execute this in the tmux command-line: :join-pane -t <int>, where <int> is the index of the window you want to move it into. You can optionally specify -h or -v to explicitly tell tmux to attach the current pane as a horizontal-split or a vertical-split on the target window, respectively.

For more info, consult man tmux.