It's rule 24-2 - immovable obstruction.
You get a free drop within one club length from the nearest point of relief
Even if it seems movable, you may not treat the cage as movabe obstruction 24-1, because it is not made to be moved but to protect a tree.
Srushing is right, that you will find a local rule on the back of your score card on most golf courses, because they do not want you, to damage a young tree with your club.
If you are uncertain, you may play two balls under rule 3-3.
You would drop the ball in the cage at the nearest point of relief and another one under rule 28 (unplayable ball) with one penalty stroke.
Do not forget to announce in advance, which ball should count, if relief is possible under both rules. (Otherwise, you might pick the better score after holing out with both balls).