because of presence of electrolytes (like sodium, potassium in body) when any living thing comes in contact with wires, there is a chance that current may flow through it. The current needs to complete its path, or circuit, which it does by traveling through return line (in case of railways, its the rails themselves). However if a bird travels an hits two adjacent wires in such a way that its body provides a direct path for current to flow from one wire to other, current, always opting for easier paths flows through the poor birds's body and connects the two wires electrically. This leads to a huge short circuit current, and this fault is called Line-To-Line fault.
Since the amount of current...
more...

is huge, the energy involved is also significant, and these much energy can damage wires terribly.
When WR was converted, similar problems were faced as birds used to perch on top of wires back when it was DC rather safely. With conversion to AC, doing so lead to their deaths and the first few days were filled with incidents like the above ones. few days on birds realized they are dangerous and stopped perching there.
Hopefully this will happen with CR's birds too, because unlike railfans they don't have IRI for min-to-min update :D