Here's a short story:

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 Cretaceous–Paleogene 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. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: What do fossil records say about birds?
Guess: are the last surviving dinosaurs

Question: What did they evolve from?
Guess: from feathered ancestors

Question: What group?
Guess: theropod group of saurischian dinosaurs

Question: When did they first appear?
Guess: during the Cretaceous period

Question: How long ago was that?
Guess: around 100 million years ago

Question: What group are they classified as?
Guess: endothermic vertebrates

Question: What are their characteristics?
Guess: feathers, toothless beaked jaws,

Question: Anything else?
Guess: the laying of hard-shelled eggs, a high metabolic rate,

Question: How many chambers does their heart have?
Guess: Four

Question: Where do they live?
Guess: Birds live worldwide

Question: How big are they?
Guess: range in size

Question: What is the smallest?
Guess: bee hummingbird

Question: How big are they?
Guess: 5 cm (2 in)

Question: What is the largest?
Guess: ostrich

Question: What is their size?
Guess: 2.75 m (9 ft)

Question: What is their class rank?
Guess: tetrapods

Question: How many living species are there?
Guess: approximately ten thousand

Question: What are half known as?
Guess: passerines

Question: What is another name for that?
Guess: perching birds

Now answer the following question in the same format.

Question: When did they diversify dramatically?

Cretaceous–Paleogene extinction event