I was watching a flock of pigeons on a flat roof after a heavy shower. They were all obviously drinking and having a sort of bath. However, they all kept lifting up their wings and holding them up in the air.
For the same reason I lift my arms in the shower, to wash their armpits, well, wing pits but you know what I mean. It seems to be something larger birds have evolved to do as they rarely find deep enough water to wash up that high.
I topped up the birdbath yesterday and a blackbird hopped on to have a drink as soon as my back was turned. It then hopped in and had a great splash around for a couple of minutes and half emptied it again, then hopped out, had a squit in it and flew off.