This process is called photosynthesis and is used by all green plants to produce food. This food then becomes the basis of other food chains. Trees require a variety of age structures and species for a woodland eco-system to work. Seeds, saplings, standards, mature, over-mature and dead trees supply valuable food for birds, as well as the habitat which they need to survive. Redpolls and Crossbills .
Food is also available indirectly from the insects which live in the trees.
The Treecreepre picks insects out of cracks in the bark. Masses of caterpillars which feed on oak leaves in turn provide vital food for Great Tits and Blue Tits feeding their young in May and June. One in every two insect eating species depends on woodlands for its existance. Ivy is a valuable plant in woodlands and small tree covered areas. It gives nesting and roosting shelter for birds and also provides berries in late winter for Blackbirds and Thrushes .