Wow, I looked up mourning cloak caterpillars and here is what I found.  This is a very informative site.  I love mourning cloak butterflies but never knew the caterpillars looked like this. 

Mourning Cloak Caterpillars, Not Gypsy Moth Caterpillars

Do they have red dots, kerc? If they are mourning cloak caterpillars, they won't kill your tree, and soon they will all go away without your doing anything.

My neighbor, being in a new house with new gardens to her, noticed many caterpillars crawling on a garden plant. She started to kill them until she realized, with my prompt, that they were Monarchs on a milkweed plant, the garden being a butterfly garden. Always check before freaking:-)