Many birds are victim to string-foot.

While building nests, birds will find and pick up man-made string, which becomes tangled in their feet, eventually cutting off circulation and killing the foot.

https://www.google.com/search?q=pigeon+string+foot

https://www.pigeonrescue.org/2018/09/11/destringing-pigeons-...

http://nycprc.org/StringRemoval.html

https://www.stringfootpigeon.com/