They are most likely warm blooded, strong evidence of that is obviously the fact they live up in the cold north and also with some species such as Gronckles and Scauldrons, being able to take in things from their environment like rocks and water and produce enough internal heat to melt and/or boil the materials. The Book of Dragons shows that there are five stages in a dragon's growth: Egg, Tiny Tooth, Short Wing, Broad Wing, and the rarely seen Titan Wing.
While the natural diet of most dragons is fish, they will happily eat other things, such as vegetables, and man-made food like bread. Changewings and Monstrous Nightmares have been shown hunting wild boars, so it may be that at least some of the other larger, land-based species do also. The Flightmare eats algae suspended in water, which other dragons can also eat. The trained dragons on Berk will not hunt farm animals, but they will accept their cooked meat if given to them by humans. Some can also eat other substances, such as Gronkles eating rock for food and to convert to the lava they fire, and Scaldrons enjoy the blue oleander flower, which is poisonous to reptiles and other dragon species. During the war with the dragons, it is failry clear that they were also willing to prey on humans, as a Monstrous Nightmare swallowed Gobber's arm and enjoyed it.
Dragons have three major predators: healthy adults can be in danger from rival dragons and humans, and wild boars prey on young and injured dragons.
Dragons are social animals, living in large colonies with their own kind and other dragon species. This behavior has also moved on to other species such as humans and domestic animals such as sheep, cattle and chickens thanks to training and semi "domesticating" of dragons.
- Dragon species such as the Terrible Terror and Night Fury were created during the first film.
- The Grapple Grounder appears to be exclusive to the How to Train Your Dragon (game)