It's no surprise that sparrowhawks take advantage of feeders where their prey are unnaturally concentrated and exposed. Birds are their only food and are very hard to catch. In the woods, most chases are unsuccessful; each carries the risk of disabling injury. Hawks starve in hard winters, especially the small males, and patches of woodland that provide enough prey are scarce in modern landscapes. The sparrowhawk is a native species that has coexisted with our songbirds for millennia and most of the species it takes at feeders are common; hawks are hungry wild birds too, why would we try to limit their activities?
The only place small birds are safe is within cover, such as a hedge or bush that the hawk is too big to penetrate. Try not to force small birds to feed in the open. If possible, place feeding stations next to bushes, although if cats are a problem as well, the bushes should be cut clear of the ground and remember too that feeders next to a continuous hedge are vulnerable if the hawk can use it to screen its high-speed approach. Providing cover above feeders, for example by hanging them under a pergola or from tree branches, seems to be helpful; hawks often strike horizontally so escaping upwards is a good tactic.
Sparrowhawks adopt regular hunting routes and although they quickly adapt to obstacles and scarers, changes to their route can temporarily disorientate them, giving prey precious extra seconds. For example, simply moving feeders around the garden every few days can be surprisingly effective, it is also good for hygiene. However, the only really effective method of reducing the 'hit rate' at feeders is by placing them inside a permanent physical barrier such as a wire mesh or netting cage that allows small birds in but keeps hawks out. Alternatively, seed, nut or fat feeders can be pushed right into hedges - small birds will find the food and move along inside the hedge to safety. Place food underneath the hedge for thrushes and blackbirds. You won't be able to watch the birds feeding, but more of them will stay alive.
Picture by David Marney