Ned Harr wrote: The birds stop by and take whatever they need and then fly off to build their nests with it.
Probably this is unnecessary (birds have been foraging from human-less environments much longer than there's been a human-ful one)...
I think this is particularly important in urban areas where many humans clean up all the 'mess', like grass clippings, animal fur, dead flower heads etc.
In my case, there are a number of areas where I pin up male dog fur at deer nose height to suggest they go elsewhere. (Nothing totally works to keep deer out, but dog fur helps.) I absolutely see the birds stealing it in the spring. Hummingbirds and Goldfinches in particular, but others are learning from them.
There are plenty of areas on my land which have natural materials they can forage, so I haven't gone as far as you have, but I totally agree with the concept. Many of our songbirds have a greater need for safe nesting sites than bird feeders, from what I've observed.
I also occasionally have tossed human hair from trim jobs, outside only to find some of it later in a nest, so if you do home trim jobs, be generous and share with the birds!