I wanted to make an activity for the students in my middle school science class when we studied center of mass. I found a few "balance bird" models here. I really liked gianfranco's "a present for you !" version (https://www.thingiverse.com/thing:2674741) where you can place coins on the bird's wings and move the wing structure to demonstrate the change of center of mass.

I found the tree didn't print with a flat surface on top. So, I cut the top to a flat surface. Also, I found the wings slid too easily with my prints (PLA+). So, I closed the gap in the body a bit to hold the wings in place once you adjust them.

I printed them for all my kids and they had to adjust and mark the wing location with US Quarters, Nickels, Pennies, and Dimes. They enjoyed the activity.