The Common Kingfisher does not build a nest in a tree. It digs a tunnel, up to about one metre long, into a steep bank of sand or loam. The tunnel ends in a small chamber where the eggs are laid.
The bank must be bare. Roots and plants make digging difficult, and a bank faced with stone or concrete cannot be dug at all. River works that protect banks with hard material therefore remove nest sites along the whole treated stretch, even when the water itself is clean.