Mammals and birds are warm-blooded, which means that they can make their own body heat even when it is cold outside. Whether it is sunny and hot outside or there is a snowstorm and it is very cold, warm-blooded animals have body temperatures that usually stay the same.
Just so, are birds classified as reptiles?
So birds aren’t just closely related to dinosaurs, they really are dinosaurs! This is what most people mean when they say that birds are reptiles, although technically according to the phylogenetic system mammals are also reptiles. You may wonder why biologists have two systems of classification.