American bison and cowbirds
Cowbirds are often observed perched on the backs of bison, picking at bugs as they fly by or get caught in the course hair found on the large animals' backs. The bison do not seem to mind the hitchhikers. Captured in Yellowstone National Park's Hayden Valley.
The Brown-headed Cowbird is a small brood parasitic icterid of temperate to subtropical North America. They are permanent residents in the southern parts of their range; northern birds migrate to the southern United States and Mexico in winter, returning to their summer habitat around March or April.