Hummingbirds and wood-pewees skillfully camouflage their nests by using sticky spider webbing to affix chips of bark and lichens to the outside. Their nests look just like the tree branches on which they are built. Thrushes and some warblers use mosses for structure and lining. This matters because birds know their flosses. Many warbling vireos gather willow floss throughout their nesting period, adding fresh floss each day.
Song sparrows have a knack for finding last year's milkweed floss.
They must use old floss because their nesting period ends a full month before the new milkweed follicles open to release their floss-borne seeds. American goldfinches frequently select thistle floss, which they use as both structural and lining material. Some thistles mature before the goldfinches nest, thus providing fresh new floss, and some thistle heads never open completely so last year's floss remains trapped and available for an energetic goldfinch. Every item of these nest-building materials has to be collected somewhere. If you live in a dry climate or a newly developed neighborhood, don't wait for natural processes to establish mosses and lichens in your yard.