Not much in the way of answers!
Anyway, most bird pooh isn’t actually faeces but the bird equivalent of urine (uric acid).
There are some birds like grouse and the ptarmigan that produce pooh that is quite fibrous as thy eat a lot of plant material (heather etc) or roughage as we sometimes call it but these still have wee bits of white pooh it in (uric acid)
Owl pellets are not ‘poohed’ out but bones and hair that have been puked up. Quite a lot of other birds also do this.
All vertebrates (animals with a backbone) need to get rid of nitrogenous waste (from breakdown of protein). We do it in the form of urine. This needs water to dissolve it and allow us to get rid of it. Reptiles and birds didn’t have the advantage of lots of water to get rid of their nitrogenous waste as they evolved to fill niches and habitats on land. The white semi-solid sludge is uric acid.This is insoluble and birds and reptiles don’t therefore waste water getting rid of it.
Another interesting explanation is that as birds and reptiles are egg-laying, any water soluble toxins like urine would build up in the egg and poison the developing youngster. As uric acid is insoluble it will not poison the young.
Last theory is Prestonpans seagulls simply enjoy the domino-look of white marks on my black car.