I am lucky to live in an area where we have a lot of birds and so I love to help them if I can. I saw a 'nester' sack in the garden center the other day with the kids and we decided this would be a simple thing to make ourselves.
You will need:
Netting sock mine was re-purposed from a box of plums I had bought
Natural fibers - cotton balls, pet fur, feathers
Tie the netting at the bottom if you need to. Cut your yarn into pieces 2-4 inches in length. Stuff the fibers into the sock, have the yarn, twigs etc stick out the side holes in places. Tie up the top and string it up to a tree in a safe area of your yard where you see a lot of birds.
Now you just need to wait and see if any of your helpful building tools end up in a nest. We added our twigs and straw after this picture was taken - at the time it was still too snowy but a couple of 50 degree days and we have boggy brown looking grass appearing again. Come on spring...