Sorry, but that is hogwash. First of all, there is no outside connection to the interior duct work, so no way for a bird, baby sparrow or eaglet, to get inside. Nor have I seen any exterior a/c shroud that has large enough openings for bird nesting to be a serious possibility. Mice or other small rodent is more likely, but I don't think any fabric cover is going to help. And they still can't get inside that way because the is no inside/outside air exchange.
If you want one, Camping World and other RV stores offer a selection of fabric covers for various a/c brands & models. See http://www.campingworld.com/category/air-conditioner-accessories/166