I recently visited Slimbridge to shakedown my two new wildlife lenses, my EF 400mm f2.8 L IS II and my EF 100-400mm f5.6 L IS II. Both exceptionally sharp lenses that are suprisingly different even though their focal lengths over lap.
I shot Swans, Avocets, Plovers, Waders, Flamingo, Ducks…lots of different types of ducks…more ducks, Gooses, a Coot and a few Jackdaws.
I shot less with my big 400 and my keeper rate was far higher. But most of my keepers were taken with the very versatile zoom lens. It’s hand holdable, light and very versatile.