The phrase "cage bars" refers to the metal bars that make up the walls of a cage. It represents confinement or being trapped inside a restricted space. Full definition
But what can they do if there is a problem such as broken or missing cage parts, bent cage bars or they can not figure out how to put the cage together? (petbusiness.com)
I've used this myself and find it fabulous for cleaning both wet and dried poop off cage bars and trays, as well as from carpets, wood floors and sofas. (petbusiness.com)
Chewing on cage bars: «I'm bored; please get me some new toys to chew on or put me in a bigger cage so I have more room to run and play.» (smallanimalchanel.com)