PLEASE don't put thread out for the birds! It seems like a nice thing to do, but the thread can tangle around babies' feet, wings, etc and they die. They can't snip them off as they can with grasses. In the 60's it was common to cover pet birds' cages at night with a towel, and there were incidents of limb strangulation and even choking. I've seen a bird with foot gangrene from thread- it's horrible. Poly never rots, and cotton takes a long time unless buried.