Dozens of species of birds are expert at soaring (list). Using rising pockets of air which are slightly warmer than the surrounding air (called thermals) many species can travel hundreds - or even thousands - of kilometres with very little need to flap their wings. They tend to circle around within the thermal they've found, then, when they reach a high enough altitude, they glide down to the next available one. This is called 'dynamic soaring'.

The question arises - how do they know where the thermals are?

One straightforward way is simply to randomly fly in roughly the required direction until upward movement is detected, and then start to circle. If the bird senses that it's dropping, the circle is tightened.

But the superb efficiency which some birds (e.g. albatrosses and terns) have for dynamic soaring have led some to suggest that there could be another system at work.

One theory is that they may be able to detect infra-sound (as many species of birds can) caused by the air currents of rising thermals (ref.) To date, there has been no experimental confirmation of this theory.

What is known is that some species (e.g. vultures) take clues simply by watching the circling behaviour of other birds nearby. See Journal of The Royal Society Interface, Volume: 15, Issue: 148,

Further reading (technical) : Experimental verification of dynamic soaring in albatrosses Journal of Experimental Biology, volume 216, issue 22.