The hollows serve as breeding places and shelters for birds and insects.
A natural wall for bank swallows is made of sand. Artificial walls for the birds can be created by making holes in a concrete structure. Insect walls are made from wood and reed materials. The insects can make nests and lay eggs in the gaps in the wood and reeds.
Walls for bank swallows and insects can be made in hard materials and so they are easy to include in built-up areas (including tidal parks). The BwN measures focus on nature linked to water or banks. Walls for bank swallows can be used in combination with other BwN measures, strengthening nature linked to water and banks.