From the Cornell Lab of Ornithology:

Birds spend lots of energy building nests—some songbirds can build an entire nest in the space of a single day! One thing you can do to help birds during the nesting season is to gather nest materials in your yard for easy access. Each species has their own preferences for material, so providing items that are already found in nature is the best way to keep birds safe and give them the best chance at success. NestWatch, our nest monitoring project, has this advice:

DO provide:

Twigs, moss, untreated grass clippings (free of pesticides, fertilizers, or other such chemicals), straw, leaves, lichen, rootlets.

Try to AVOID:

Dryer lint, yarn, string, hair, pet fur (especially if the fur has been treated with shampoos, flea/tick treatments, etc.).

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