Begonia semperflorens leaf damage

The leaves on my begonia semperflorens plants have been torn to shreds.  The remnants of leaves can be found all around the plants so don't think it is slugs or snails.  Could it be birds pecking at the leaves?  The culprit is destroying my plants.  Never had this problem before and have grown begonias for more than 20 years.  Any suggestions on a remedy please?  Many thanks