Barbule hooks hold the barbules and the barbs together, maintaining a smooth surface for air to pass over. the hooks are looped around the barbules rather than fused to them. what do you think the benefit of barbules that can move might be?

Answer : The moving barbules allows all the feathers to bind together, to keep wing stiff and intact so that it effectively acts like a single solid object.

When during the flight, air is forced over and around the wings, instead of passing through the gaps between the feathers.

The dynamic air keeps moving through the wings which decreases its aerodynamic efficiency and also its subsequent lift while flapping. 
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