Plants provide three essential resources for wild birds.
The first, often overlooked, is cover. Birds need shelter and protection from predators and weather for themselves and especially for their nests.
Secondly, wild birds need invertebrates, such as tiny caterpillars, aphids, beetle and sawfly larvae, etc., to feed themselves but most importantly to feed their chicks. Even house sparrows, which eat almost entirely seeds as adults, require huge numbers of invertebrates to survive their first few days after hatching.
Thirdly, the seeds, berries and fruits of plants are vital food for birds in winter. It follows that to attract and help the most birds; your plants should offer all three of these resources.
Cover means hedges and bushes, dense, preferably thorny and absolutely not pruned in summer when thrushes and finches are rearing late broods.
Robin boxes, by the way, are useless in the open but readily used if hidden behind a spiny shrub pyracantha and hawthorn are ideal, not only excellent at concealing nests but being native it supports invertebrate larvae and provides the added bonus of autumn berries.
Other plants that tick all three boxes include ivy, holly, yew (females for berries), blackthorn and bramble.
For invertebrates, native plants are best, but be tolerant; cotoneasters feed moths as well as providing cover and fruit, buddleia from the Himalayas feeds the butterflies while they search for native plants to lay eggs on.
Of course, starting from scratch it’s far better to plant native species; barberry, thyme and marjoram, an oak if you have room. For autumn fruit, elder is the king with enough berries for us to share. Apple trees (unsprayed, of course) are also excellent for both invertebrates and fruit. Don’t forget seeds: teasels for goldfinches, honesty for bullfinches, sunflowers for greenfinches.
Planting for wildlife can become an absorbing hobby; butterflies and moths, for example, require specific food plants which are easily researched in books or on the Internet. Many of these plants are attractive; a garden that provides a healthy food web for wildlife, rather than a sterile display of alien curiosities, can still be a beautiful garden to enjoy.