Black sunflower seed hulls are allelopathic meaning they exude a chemical that inhibits the growth of other plants near / around them (think "survival of the fittest"). Many plants - black walnut and butternut trees, for example, and some weeds like quack grass - have some level of allelopathy. In some cases, allelopathy in certain plants - like sunflower - is used to our advantage for weed suppression in agriculture. Here is a good article from our colleagues at University of Florida about allelopathy.

Unfortunately, we really cannot predict how long the soil will be affected in this area. Until which time the chemical has degraded, I would consider creating a bird feeding garden bed in this area. Edge it in stone and cover the ground with wood mulch. Plant some annuals in pots and nestled them into the mulch to add color and interest. Your bird seeds can fall to the ground and any that sprout can be pulled easily.

In the rest of your landscape, you might also consider planting flowers that grow seed heads for birds to feed on and ultimately ditching the bird feeders. It's a lot less expensive, squirrels aren't a big problem and you can have establish a pretty nice garden that benefits birds as well as pollinators. Landscaping for Wildlife by MN DNR's Carrol Hendersen is a great book. We also have great resources on our Extension Yard & Garden webpages.