The short answer: Nothing.
The longer and more accurate answer, however, is far from simple.
Let's start here: It's not about feeders and feed. Always, bird population--migrant or otherwise--depends on habitat, area that provides food, water, shelter, and nest sites. All except water must, in nature's way, come from plants.
Step aside for a moment from the feeders and feed in your yard and consider this: Of the 166 bird species on our yard list, fewer than a third of them visit feeders.
Mostly, feeders are for us--to attract birds for our pleasure. I thoroughly enjoy my feeders, serving up a typical assortment of seed, suet, and syrup.
Unfortunately, many folks would probably prefer to rid their yards of bugs. But here's the straightforward truth: To attract the birds, you must first attract the bugs.
Some warblers--like black-throated green, chestnut-sided, and magnolia warblers--pick their way across bald cypress and pine limbs, exploring undersides of tulip poplar, black locust, and sweetgum leaves, snacking on insects, insect eggs, and little green worms. Some warblers, like northern parula and yellow-throated warbler, range higher, bugging in tall oaks and maples, almost out of sight. Others, like Nashville and Tennessee warblers, forage lower, often among head-high blossoming goldenrods which draw insects to their sweetness. Still others, like ovenbirds, forage ground level, searching leaf litter and weed clumps.
In every case, it's native plants that draw native bugs that feed our migrants.
Given their high-energy needs during migration, however, many migrants--certain warblers among them--prefer feasting on sugar-rich berries like native flowering dogwood, gray dogwood, and common beautyberry. Once again, birds recognize native berries as food because they've evolved with the plants.
Another migrant group also shuns feeders. Think catbirds, summer and scarlet tanagers, migrating thrushes (Swainson's, gray-cheeked, hermit, wood, and veery), gnatcatchers, and kinglets. These lovely charmers look too for bugs and berries.
In our yard, a bubble rock has become the number-one draw for migrants. It's small, no bigger than my arms can reach around, with a recirculating pump in a 40-gallon reservoir buried to ground level. A heater keeps it bubbling all winter. Find directions online at http://www.houzz.com/ideabooks/47964929/list/bring-in-the-birds-with-a-homemade-bubble-rock.
Then plant for berries and bugs and create a migrant's haven.