that they can make it home , fitness is a must so need to be 100 % healthy , all my 500 milers have been medium to large in size but very boyant and light and balanced , wing tips must touch if they dont they wouldnt go i like to see good wing musle a tight second wing end flights near same size , and medium to large flight width with tail feathers with good feathering down to tail flight , i like a slightly longer cast bird , and two eyes are a bonus