Problem:

I was walking through a park this afternoon and observed a few birds having fun flying around and it got me thinking why would natural selection favour birds with flesh at all if it hinders their flight.

We started first with single-cell and micro-organisms flourishing the ponds and then they evolved into massive dinosaurs. It therefore seems, there are some unique reasons for growing to such a large size.

However, I thought it's counter intuitive because the large body slows down locomotion, requires lots of food and energy upkeep, is a big flesh target for predators, makes hiding ineffective.

Although it is obvious that the flesh protects the internal organs, but it seems like a chicken and egg problem because without a massive body, perhaps then there is no need for the internal organs.

On a different perspective, perhaps those animals that evolved without flesh are called plants.
That sums up my naive train of thoughts.

So, any expert can shed some light? Please explain