Was the swallowtail flapping, or just hovering with wings steady?

If the former, I'd be surprised. The wings have to go across the wind to flap, so that'd be hard to flap fast enough. But if it angles its wings into the wind, a butterfly has hardly any cross-sectional area, so its weight and the small aerodynamic force is likely enough to keep it in place.

Birds have a pretty big cross-sectional area, especially larger birds with more aerofoil-shaped wings that can glide efficiently. Seagulls often get blown back by the wind because of that. But sparrows rarely do - as small birds, flapping works better than gliding (because gliding makes you a sitting target for a hawk), which means they have small wings and an already-small cross-sectional area.

Graham.