Birds (Aves) are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds or, less accurately, as songbirds.

The fossil record indicates that birds are the last surviving dinosaurs, having evolved from feathered ancestors within the theropod group of saurischian dinosaurs. True birds first appeared during the Cretaceous period, around 100 million years ago. DNA-based evidence finds that birds diversified dramatically around the time of the CretaceousPaleogene extinction event that killed off all other dinosaurs. Birds in South America survived this event and then migrated to other parts of the world via multiple land bridges while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.

What are the answers to this following set of questions:

1. What do fossil records say about birds?
2. What did they evolve from?
3. What group?
4. When did they first appear?
5. How long ago was that?
6. What group are they classified as?
7. What are their characteristics?
8. Anything else?
9. How many chambers does their heart have?
10. Where do they live?
11. How big are they?
12. What is the smallest?
13. How big are they?
14. What is the largest?
15. What is their size?
16. What is their class rank?
17. How many living species are there?
18. What are half known as?
19. What is another name for that?
20. When did they diversify dramatically?

Numbered answers:
1. are the last surviving dinosaurs
2. from feathered ancestors
3. theropod group of saurischian dinosaurs
4. during the Cretaceous period
5. around 100 million years ago
6. endothermic vertebrates
7. feathers, toothless beaked jaws,
8. the laying of hard-shelled eggs, a high metabolic rate,
9. Four
10. Birds live worldwide
11. range in size
12. bee hummingbird
13. 5 cm (2 in)
14. ostrich
15. 2.75 m (9 ft)
16. tetrapods
17. approximately ten thousand
18. passerines
19. perching birds
20. CretaceousPaleogene extinction event