Living in Cornwall I have had plenty of sea gull (aka beach chicken) poo on my car. By the size of the offending poo you would be forgiven for think some of the guls were ostriches or prehistroric teradactyls. However I have had many occasions where ther has been massive gul splats on my car baking in very hot sunshine for a long period of time as we enjoy the beach. I have to say I have not experienced any etching yet to my knowledge. My current and 2x previous car were white, and the cars before that were black. But I cannot remember anytime where I have found a etch mark from gul or other bird poo. However what I did find one was a bad scour mark which went right through the paint from bones in gul poo as I tried to remove it from the roof of my old Type-R many years ago. I would say if bird does etch paint. Just a musing thought, maybe it is a locale related affair dependent on the diet of the birds in the area?