Are birds cold blooded like snakes and fish ?

 

All living birds are endotherms, meaning they burn energy internally to maintain a high body temperature. In fact, birds maintain a higher body temperature than mammals. Therefore birds actually feels warm to our hands. If anything, our own blood is colder than the blood of birds. If our blood is as warm as birds, we would be having a fever. 

That said, there were indeed some birds that were closer to snakes in not being endothermic. Those birds had growth rings in their bones, just like other ectothermic animals. Those birds were known as enantiornithine or opposite birds, and they are different from modern birds in having leg bones that fuse from the opposite direction as modern birds. 

They were also very successful, and they dominated the earth during the age of the dinosaurs, when the climate was much warmer than today. Because it was so warm, there was no need to waste energy to maintain a warm body temperature. They were also warm blooded without burning energy to stay warm. Further, if there were to burn energy to stay warm, they may overheat after exercising. In fact, one of the reasons why lions can only chase a zebra for a short period of time is because they build up so much heat that they cannot keep up the chase. Therefore being endothermic in a hot climate is actually a disadvantage. Theropod dinosaurs, because they were ecotherms, can probably chase their prey for much longer periods of time without overheating. 

The enantiornithine birds, despite their success, were unfortunately wiped out completely at the end of the Cretaceous, when a giant meteor struck the earth and created world wide forest fires and high atmospheric temperatures, killing all the dinosaurs and the opposite birds. The modern birds survived the mass extinction because they were unable to compete and they were forced to occupy shoreline habitats, near the edges of seas, like today's sandpiper. Because shoreline habitats were cooler, they evolved endothermy in order to maintain a high enough body temperature to survive. Because the shoreline habitats were cooler, they were spared from the heat and survived. All living birds evolved from this small group of survivors, and that is the reason why all living birds are endotherms. Other groups of animals that survived because they lived in or near the water include the crocodilians and the turtles.