There is an image circulating Facebook (and other social media) about scraps supplied to song birds outside in a bird feeder to help them build warm nests for their pending young. The image shows scraps of yarn. This is a very bad idea seeing that long bits of string, yarn, twine and even long hairs can pose a threat to birds and their chicks. These long strands have the ability to entangle a delicate bird potentially strangling or even cutting off circulation to vital limbs like wings and legs.
If you want to help the song birds with their nest building projects, brush your dog or cat and toss out the dead hairs. These are different than our hairs since they are much shorter and stiffer. We see a lot of bird nests consist of my old pony’s hair during shedding season when she lays down and leaves a ring of loose hair where she was.
A perfect alternative is straw or hay, very nice and warm and won’t entangle anyone.