Because the work of the nest, chiefly of dirt or loam, may not, while it is soft and green, pull itself down by its own weight, but have sufficient time to harden and dry. - G. White.
Because the heat of the sun may not crack or destroy their nests. - G. White.
Because their wings and tails are shorter, wherefore they are not capable of such surprising turns, and quick and glancing evolutions, as the swallow. - G. White.
Because, in July and August, when it usually brings out its young, one rainy day, attended with wind, will moisten the earthy nest, the cement then fails, and all the unfledged ones are dashed to the ground; and there are some places to which these poor birds are unfortunately partial, though their nests are annually washed down. The parent birds at times seem aware of the misfortune that awaits them; as, before the calamity is completed, we may observe them, with great anxiety hovering about their nests. - Knapp.