Many sites recommend using feathers, which seems like a natural choice for bird nesting material. However, it is illegal to collect, or possess, feathers from any migratory bird species, which includes any bird species native to the United States. One can possess non-native feathers from such as birds European starlings, Eurasian collared doves, and other introduced species. Feathers from a friend’s pet bird, or chickens, could be a good source, but then again, if the feathers contain harsh chemicals it’s not a good idea as this could negatively impact developing nestlings. The dyed feathers at the craft store are not a good choice because they contain harmful chemicals.
In addition to providing nesting material, in a container of some kind, the savvy gardener can also provide the raw materials that songbirds use naturally as nesting material in other ways by including having plants in the landscape that provide fuzzy seed heads like thistle or fibrous leaves like bamboo and palms. Keeping an untidy garden can also provide them with nesting materials such as fibers from tree bark, mosses, lichens, twigs, dried grass, pieces of molted snake skin, and spider webs. When tying up plants, or vegetables, consider using natural jute to provide another source of material the birds can use.
Please don’t use dryer lint. Dryer lint seems like a good idea, but it will retain moisture, stay wet, and get compacted into a pile that will encourage mold and mildew to grow endangering the bird family, and it most likely contains fabric softener and other chemicals as well, so it is best to avoid it altogether.