What's the trick to shaking the berry trees?

Every now and then I come across a Berry tree, which I shake for Berries. You can shake the tree multiple times with more and more berries falling out of it.

However, occasionally a Pokemon encounter falls out of the tree, and once the encounter with it is done, some of the previously dropped off berries have been 'eaten by nearby Pokemon'.

Is there a trick to knowing how long to shake the tree?

The trick is to watch the tree after berries have dropped.

• If there is no movement in the tree, it should be perfectly safe to continue shaking it.
• If there is occasional movement (leaves rustling every 2-3 seconds), there is a low chance of encountering a Pokemon.
  • I have personally never had an encounter when the tree shakes like this, but @Roybin93 confirms it's possible, if a low chance.
• If there is rapid movement (leaves rustling every second), there is a high chance a Pokemon will fall out.
  • I have actually managed to shake the tree successfully again at this stage, meaning it's not a guarantee that a Pokemon will fall out.

You do seem to get a higher berry yield from shaking at rapid movement as well, so there is greater reward for the risk of losing the berries you've currently dropped.