I agree with Michael, it depends what your bird is used to and if you can go out in a T-Shirt and shorts, your bird will probably be alright.
For my birds, I judge by their body language and the temperature of their feet to see how they feel. If a bird's cold, it's feet will be. When my cockatoo is hot, his feet become very hot as well. My cockatiel gets very dramatic when she's cold, too: she puffs up and even shivers a little. Then again, she shivers of excitement and everything else, too.
