Cliff Swallows Gathering Mud

Why do Cliff Swallows flutter their wings upward while collecting mud for their nests?

Cliff Swallows nest in colonies. To help prevent predation, the birds collect mud as a flock. Extra-pair copulations (or males attempting to mate with individuals other than the female with which they have pair-bonded) are a common occurrence at mudholes, especially in larger colonies. To prevent that -- and because males can be mistaken for females -- both sexes hold their wings up while on the ground.