Big Oaks is a birder's paradise with a great diversity of species in a variety of habitats. The largest population of Henslow's sparrows east of the Mississippi River nest in refuge grasslands. Federally endangered cerulean warblers nest in mature woodlands, and many other uncommon species, including worm-eating warblers, can be found seasonally. Winter brings saw-whet and short-eared owls, and occasionally a golden eagle to the refuge. A Christmas Bird Count is conducted annually on the refuge and gives visitors an opportunity to enjoy the birds of Big Oaks while doing citizen science with refuge staff.