Lonesome Dove, Volume 2: Comanche Moon by Larry McMurtry

BUFFALO HUMP had taught Blue Duck that the safest time to attack a white man, and a Texas Ranger particularly, was while the man was squatting to do his morning business. The whites were foolish in their choice of clothes; they wore tight trousers that slowed their movements when they squatted to shit. Blue Duck, like most braves, only wore leggins-even those he discarded unless it was bitter cold. The leggins didn't interfere with his movements, if he had to rise quickly. But a squatting white man was like a hobbled horse: you could put an arrow in him or even jump on him and cut his throat before he could get his pants up and run.

Blue Duck knew they had killed a ranger the morning before. He had seen the other men hacking out a shallow grave with their bowie knives. They even spent a long time gathering rocks and piling them on the grave, to protect the corpse; a foolish labor. Then they sang a death song of some kind over the rocks, and rode off.