Please reconsider feeding the birds, it's causing real environmental harm in multiple ways. Bird feeders bring large numbers of bitds together and are a major cause of disease transmission - Greenfinches and more recently Chaffinches have already suffered large declines as a result if trichomoniasis disease, spread through feeders.
Birds that benefit from feeding are species that are already widespread and common, and don't really need any additional help. But unfortunately, increases in these common species are putting further pressure on rare and declining species.
The species that are really declining are insect-eaters like Swallows, House Martins, Swifts, Spotted Flycatchers, Willow Warblers - feeding does nothing to help these. It's of far more benefit all round to just concentrate on improving the habitat with e.g. native trees and shrubs, wildflowers, ponds, compost heaps etc.
A lot of bird food also gets eaten by pest species like grey squirrels and rats, and invasive birds like parakeets.
Unfortunately, there is big money in the bird food industry and the negatives are being largely ignored. It's not entirely coincidental that garden centres tend to have a large aisle full of chemicals to kill off nature (weedkiller, bug killer, slug pellets etc etc), and then another large aisle of birdfood.