One owl shot and we saying AF is as good as 600mm f4 IS? Sorry I would love to see some more BIFs shots with a sequence shown here. BTW - Nobody buys 600mm f4 IS for BIF shots as it is not HH lens. That is where 500mm f4 shines. And newer version through lot more expensive is lighter.
The guy problably shot quite a number of other photos too during the day.