-I sure hope you don't get your biological education from films only. "Dinosaurs are birds" is so simplified that it's implications are incorrect. Lose the habit of believing reality can be expressed very simply.
-A more accurate expression would be that large bodies are inefficient and require more ideal conditions. Once those conditions disappear, those with smaller (or otherwise more efficient bodies) were far more likely to survive. As conditions change and pressures change, preferred forms change. To better understand this (in great detail) study up on the Galapagos Finch. A good book for removing the misunderstandings of evolutionary theory would be The Ancestor's Tale. It's a long series of stories that each explain an aspect of the theory (or theories).
-One thing you cannot deny, however ignorant you may be: the fossils are there. More to the point, the fossils are always found with simpler forms further down and more complex forms further up. It doesn't take a genius to figure out the implications; it just takes a willingness to accept that pre-existing explanations (poofed into existence by a magic man in the sky) may have been wrong.
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