From the BASC:
Methods of Control
Drey Poking
At different times of the year the control of squirrels can take different forms. With the leaf off the trees, winter is a good time for shooting.
Drey poking can be effective, especially on cold winter days,. A team of four, with two working the poles and two covering the tree can spend a day clearing a lot of dreys in a wood. When all the dreys are cleared from a wood in winter it becomes easy to spot i new ones made in the summer.
There are a few rules you must adhere to when carrying out this activity. Safety is paramount; guns should stand well back from the tree. When using the poles tap the bottom of the drey gently, this will allow the squirrel to run out slowly, it will probably stop just outside which will give the guns time to shoot. Never shoot at a squirrel running down a tree, it is better to either let it run down and run away from you or stop it and turn it back up the tree. Remember you are aiming to cull squirrels, so be efficient and effective.
Never shoot at a squirrel running down a tree
may become my new mantra