While waiting for an Osprey to appear I spent an hour or so sketching a Great White Egret which had landed directly out from the main viewing screen.
There was hardly a second when the egret wasn’t moving and this obviously makes sketching tricky. It would often readopt a position briefly so that I could add a bit more detail to that sketch. It’s definitely better to draw what you see and not what you think is there and so waiting until you see shapes well before guessing is good advice.
If the movement is particularly fleeting and into a position that it doesn’t subsequently readopt then you just have to hope that you saw it accurately and even if it’s not perfect then you hope that you’ve captured the feeling of the movement.