because pigeons or birds for that matter sexcrete poo and pee from same lobang known as the cloaca. pigeons have no separate ducts and organs like in mammals to store and sexcrete pee (bladder) and poo (rectum). thus, mammals sexpel urine in the form of urea while birds sexpel urine in the form of guanine and uric acid. guanine and or uric acid reacts with water to produce the white gluey glob. because pee and poo cum out together from a pigeon, the white gluey glob covers the black sticky shit.