NIGHT HERON POOP. I posted here about the problems that night herons were causing for a mail box belonging to a neighbor of ours. The white-besmeared mailbox was right below a night heron nest. I was looking at THE BIRD WATCHING ANSWER BOOK by Laura Erickson and learned that: “Seabirds, cormorants, gulls, and other birds that take in a lot if water with their fishy diets…release large amounts of uric acid….Those big white splats can be valuable….”
Further, black-crowned night herons and similar birds have long been nicknamed “shitepokes”, a term first used in the 1770’s. The term refers to the fact that the birds shoot out “a noticeable stream of poop on takeoff”.
So the herons are not fouling their own nests. The takeoff behavior also explains the occasional large white areas that are not near a nest.