The shining wire also known as a snare is an animal-trapping device set by humans. It is made of a thin wire tied with a noose-like knot with an end fastened down securely. The one that the rabbits meet is a very thin copper wire with its end nailed to the ground.
During their visit to Cowslip's Warren, Bigwig's neck gets caught in it and he is nearly strangled to death. He ends up with a small cut on his neck.
Strawberry's mate, Nildro-hain, Died from a snare, Causing him to leave the warren and join the Watership Down Warren