The more that we learn about these animals the more we find that there is basically no difference between birds and their closely related dinosaur ancestors like velociraptor. Both have wishbones, brooded their nests, possess hollow bones, and were covered in feathers. If animals like velociraptor were alive today our first impression would be that they were just very unusual looking birds.
Really the best way to understand anything about dinosaurs is by looking at living animals. You look at birds and then look at the closest living ancestor of birds, which is the crocodile. If you look at characteristics that birds and crocodiles have in common, the explanation is that the trait was in the common ancestor that birds and crocodiles had at one time.
... if you saw a baby tyrannosaur you would probably think it was a weird looking bird. A full grown one might have had feathers too, maybe not on its whole body though, maybe more of an ornamental display sort of feathers. So traits in the theropod dinosaurs were more birdlike than say, crocodiles.
We have as much evidence that T. rex was feathered, at least during some stage of its life, as we do that australopithecines like Lucy had hair.