Your first thoughts of trees and birds may be for nesting and cover, but trees provide an equally important role: FOOD.
Even birds that primarily eat seed or fruit feed their young soft-bodied insects, especially caterpillars. Dr. Tallamy’s work, listed below, identifies the number of different species of caterpillars that use certain tree foliage for their host plant.
So while supplemental feeders are good, especially to bring the birds into easy viewing range, natural food will help fully sustain your bird habitat. Plant trees!