Large and old oak trees have special properties lacking in younger trees. They have rougher bark, more dead branches in the crown, and more and bigger cavities in the trunk.
These cavities become full of wood mould, which looks like reddish-brown flour and consists of rotten wood, remnants of nests, dead insects, bird droppings, leaves, fungi, etc.
Many insects are dependent on this wood mould for food and as a habitat for their larvae. The cavities of oaks also house bats and hole-nesting birds in addition to insects.
They vary in size, type of decay and amount of tree mould. They can be dry or moist, in a sunny or shady position. Trees are also an important habitat for many organisms long after they are dead, and rough trees take longer to decompose when they die.