I've been saving all my snipped-off ends, almost since I first started seriously crocheting, in a glass jar. I could see the different layers - threads that I'd used in an early cat blanket; all the colours from my first ripple; the blues and greens from my Skye blanket. Didn't take a photo unfortunately. Today I decided to put them into a bird peanut feeder, to see if they would be useful for nest-building. I've heard that this is what other people do, so I thought I'd try it.
I'm a little concerned about 'health and safety' - as a lot of it is acrylic rather than natural fibre, I didn't know if there were any dangers to the birds. But when I mentioned it to Chris he pointed out that they use all sorts of unnatural fibres in their nests, plastic twine etc, and it doesn't seem to do them any harm. And they're not eating it, just using it as building material, so hopefully it's ok.
I had to press it down into the feeder with a wooden spoon, there was so much of it, but it's poking nicely out of the holes, so fingers crossed the birds appreciate it. I hung it on a hook at the end of the garage, nearest to the trees at the back of the garden, as that's where we see all the bird activity.
This is where I read that you should use natural fibres, but the whole article is very interesting.