Final answer:
The fossil record for birds is less extensive due to their delicate, hollow bones and small size, making bird bones more fragile and less likely to fossilize compared to those of reptiles and mammals.
Step-by-step explanation:
The fossil record of birds is less extensive than that of reptiles and mammals because birds have delicate, hollow bones and birds are mostly small animals.
Birds are endothermic amniotes with feathers for insulation and flight, and their pneumatic bones are hollow which makes them fragile and less likely to fossilize compared to denser bones. The fact that birds evolved from dinosaurs, specifically from a group called archosaurs within the theropod dinosaurs, further indicates that the bird fossils we find are exceptional. Not only do birds have a high metabolic rate to maintain their body temperature, but they also digest food rapidly, requiring them to eat often. Their adaptation for flight with lightweight, hollow bones means that their remains are more prone to decompose without fossilizing, resulting in a sparser fossil record.
The fossil record of birds is less extensive than that of reptiles and mammals because birds have delicate, hollow bones and birds are mostly small animals. Birds have adapted to their environment by evolving hollow bones to reduce weight for flight. This adaptation makes their bones more fragile and less likely to fossilize compared to the denser bones of reptiles and mammals. Additionally, birds are often small in size, which means that they have a smaller chance of being preserved in the fossil record.