While at first glance it may seem silly, these jumpers are actually being made because the chickens have lost their feathers. Battery chickens in poor conditions tend to bully each other by tearing out feathers or lose them as a stress response.
These are rescued chickens, so they're probably fairly traumatized. It'd be the same sort of thing as making coats for long-haired dogs who for some reason of abuse had lost their hair.
Really though, they need a "C" in a crest, like Superman, on those. Up up and away!