They might be using both! Studies on homing pigeons [1] have found that they use atmospheric volatiles to navigate until they're in the vicinity of their home r

They might be using both! Studies on homing pigeons [1] have found that they use atmospheric volatiles to navigate until they're in the vicinity of their home region, at which point they use landmarks (like roads or field boundaries) to navigate. So it's possible that vultures have learned that the air on this side of the river smells better than the air on that side of the river, and therefore know to turn around when they wander across. [1]: http://theconversation.com/explainer-how-do-homing-pigeons-navigate-25633 http://theconversation.com/explainer-how-do-homing-pigeons-n...