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If birds were once dinosaurs, then why dont they dominate the planet today? The vast majority of bird species are just small flyers who eat bugs and small animals. You dont see giant grazing birds or large bird carnivores anymore (though there were some not too long ago, as in the terror birds). Why dont you see birds today that are like the raptors (as in the dromaeosaurs). Why have the never re-evolved the features that some dinosaurs had? A raptor-like bird isnt really out of the question, considering the relatively recent terror birds, who only went extinct like 2 million years ago (more than 60 million years after the dinosaurs died out), if i’m not mistaking.
Yeah, you have some large birds like ostriches and emus (who are probably the most dinosaur-like birds that exist today), but all the large carnivores on this planet are mammals (lions, bears, tigers, wolves), with the exception of crocodiles and komodo dragons. There are no large carnivorous birds anymore. Come to think of it, reptiles are mostly small animals too. The only giant-sized animals since the dinosaurs vanished have all been mammals. Why can’t birds achieve the sizes of the dinosaurs, or fill the same niches as the dinosaurs once did?

also…
b.) What was the critical evolutionary event which led to the tremendous diversity of grazing animals?
c.) What can be said regarding the spread of mammals into various niches of life [air, fresh water, oceans, land]?