“But whatever the reasons may be, small stature was likely a useful precursor to flight. Though larger animals can glide, true flight powered by beating wings requires a certain ratio of wing size to weight. Birds needed to become smaller before they could ever take to the air for more than a short glide.”
“Over time, they discovered, the face collapsed and the eyes, brain and beak grew. ‘The first birds were almost identical to the late embryo from velociraptors,’ Abzhanov said. ‘Modern birds became even more babylike and change even less from their embryonic form.’ In short, birds resemble tiny, infantile dinosaurs that can reproduce.
This process, known as paedomorphosis, is an efficient evolutionary route. ‘Rather than coming up with something new, it takes something you already have and extends it,’ said Nipam Patel, a developmental biologist at the University of California, Berkeley.”
Why would paedomorphosis be important for the evolution of birds? It might have helped drive miniaturization or vice versa. Changes in size are often linked to changes in development, so selection for small size may have arrested the development of the adult form. ‘A neat way to cut short a developmental sequence is to stop growing at smaller size,’ Benton said. A babylike skull in adults might also help explain birds’ increased brain size, since baby animals generally have larger heads relative to their bodies than adults do. ‘A great way to improve brain size is to retain child size into adulthood,’ he said.