A 500mm L is popular for birds in flight, but not something to carry everywhere. The 400mm DO can be carried with you and is a good lens, but expensive.
I like my 100-400mmL, but it is not that good for flying birds, just sitting ones. The 400mm f/5.6 is the low cost BIF body, but in low light, you need a 1D X.
No matter which way you turn, it takes big $$$ to cross the boundry into first class wildlife equipment.
I wonder if the small format mirrorless bodies like the Nikon 1 series coupled with a adapter and a 200mm f/2.8 lens would be a low cost way to capture wildlife in low light.
IQ might not match a 1D X with a 500mm f/4L, but neither does the price.