Every summer, well-meaning people hang out balls of dryer lint, yarn, and pet fur for the birds. Most of it does more harm than good.
Dryer lint feels perfect, but it isn't. It crumbles the first time it rains and leaves holes in the nest, and it carries detergent residue and microplastics.
Yarn, string, and long hair can be worse: they wrap around a nestling's leg or wing as it grows and slowly cut off circulation, and wildlife rehabbers see the results every year.
Pet fur seems like the safe, natural choice, and it's the one I'd skip hardest. A 2025 UK study found that 100% of the animal fur lining the nests of two common songbirds contained insecticides from flea and tick treatments, and the higher the dose, the worse the chicks did. The soft lining was poisoning the brood.
Here's the good news: The best nesting material is a slightly messy yard, and it's free. Leave the twigs, dead leaves, and dried grass where they fall. Let plants stand so the seed fluff and stems are there for the taking. Leave a small patch of bare mud for the robins.
Birds were building nests for millions of years before the craft-supply aisle existed. Give them a wild corner and get out of the way.