1) Lots of bird droppings have gritty texture - something about birts eating gravel and not digesting it - so you need to be careful about how you wipe the initial hit.

2) Bugs and bird residue if left on the paint can etch itself into the layers of wax maybe even clearcoat.

3) The Mercedes car care products are probably Meguiar rebranded, certainly not a poor product, cost and performance wise a good way to maintain a car.

4) I have Mercedes 040 (paint code) black paint, and recently used Zaino brand swirl remover (ZP-C) and swirl reducer (Z5), and my eight year old finish looks great. I had the same bird etched marring on my hood, and now after using Zaino the marring is gone (removed or hidden by the products I don't know).

5) I think the Mercedes paint cleaner is a product intended for frequent use and therefore not as agressive as what may be needed to clean the etching.
This dosen't mean the product is poor - because you always want to use the least agressive products first.

6) Do some research and try a more agressive pain cleaner or swirl mark remover. And there is no reason hand applications won't do the job.

7) A tip, if you try to use the Mercedes paint cleaner some more, use a 100% cotton applicator, and after applying the cleaner ( and before buffing out ) use a fine spray of water on the area and continue buffing in the cleaner.

8) Too much rubbing can leave a scratch haze, but three applcations may not be enough, it could take some extensive time and frequent checking to make sure no scratching before the Mercedes paint cleaner works.