Greetings from a preschool classroom! Can you help us?

I’m a preschool teacher in a nature based school and we’re currently studying birds and having so much fun!

One of our interest areas is nests. I’d like to conduct a simple experiment in which we put out multiple nesting materials and see which one is the most popular. I’m having a tough time coming up with the best (and safest for birds) ideas. I have 4 suet containers I’d like to fill so if we get 4 options that would be amazing.

Here are my ideas so far, would you please correct any bad ideas and share suggestions if you have them?

  1. Very thin and small sticks/twigs
  2. Dog fur from a dog without any chemicals such as flea drops
  3. Small pieces of yarn in natural fibers only such as wool.

4.?

I’m in New England USA if that helps and thanks in advance.