Such a impressive moment! Today it is a light windy day (the forecast says 13km/h, in other areas this is probably not even considered as wind :D ) I connected the prototype I built yesterday to a big branch of our birch and it immidiately began to light up the LED I connected as a load. I made a short video of it:

Here you can see the whole setup:

Below there is a stone to hold the stool to the ground (otherwise the pull of the tree would tip it over), then you can see the red bungie cord, the stepper motor with the connected led and the wood holding everything together. The string ist tied to the bungie cord, winds around the stepper motor axle and goes the to branch. If the branch moves, the string is pulled and the stepper motor rotates and generates electricity. This is enough to light up a green led. When the branch moves back, the streched bungie cord pulls the string back and there is again energy harvested.

The next interesting thing is to measure how much energy is collected over the day (and that with just an old floppy disk stepper motor as a generator!!!)