Originally Posted by
Longarm
PLEASE PLEASE DO NOT DO THIS!!!!
There was an article in "Birds and Blooms" about the fabric and thread getting wet causing the baby birds to die of the cold and damp.
I don't know who thought of this but they are terribly wrong if they think it is a good idea. If you want to give the birds nesting material then use pet or human hair. I'm sure if you went to a local beauty salon or pet groomer they would let you have hair to put out for bird nests.
I enjoy looking at the beautiful photos in Birds and Blooms. It's a nice publication, but no serious birder I know considers it an authoritative resource.
Abandoned nests we've inspected over the years (empty--no dead baby birds) have had at most four or five of my tiny fabric strings like the ones shown in the OP. The amount of moisture they could retain would be no more consequential than that retained by the twigs and leaves and bits of moss that also make up the nests.
Just a reminder - John James Audobon killed all the birds he used for his pictures, they weren't painted from live birds.
That isn't relevant to this subject.