As long as the temperature in the room where they are kept is not below 65 F, it should be OK without any cover. I actually keep the temperature a little higher than that -I try to make it no less than 68 F in the birdroom but, if it goes down to 65, I don't worry. I use the heating pad for sick birds. I put it at the bottom of the cage with a newspaper covering (so it doesn't get poop on it) and they look like this: https://www.amazon.com/s/?ie=UTF8&keywo ... vtojvjg6_e
And you just have to make sure it's not the kind that shuts itself off every two hours.