Why do birds fly so high and some are not?

to know this u will have to learn physics

the answer lies on the design or purpose of being such a kind...other birds like eagles can soar up high and stay that way for hours is because they were made and design for this, they have that great leap and a great wing that span longer than their body length giving enough thrust and air buoyancy to soar and glide in the air effortlessly, but on land they find it hard to run faster as it was not their design for running making the big wings their disadvantages for this, other birds cannot fly so high because of the wings design again, the size of their wings cannot hold to much for their body weight causing them to make a lift but only for short time just like chicken, turkey and some others..other birds cannot even fly, why? it's the wing again and its somehow the proportion of the wings in their body weight which disable them to fly,,like an ostrich but this was compensated by their ability to run so fast actually they are the fastest runner of all the birds...i hope i did address your concern....happy posting