From time to time, birds may use yet other directional cues, including auditory and olfactory ones. For example, infrasound emanating from wind against mountain ridges or from waves breaking on shorelines can travel long distances, and could give strong directional clues to birds able to detect them (Keeton 1980, Wallraff 2003). At present, however, olfactory orientation is a debated hypothesis for birds, but it is well tested and accepted for some other animals, at least over short distances. Although they may not normally use this method, homing pigeons have been shown to be capable of olfactory navigation, again over relatively short distances, apparently on the basis of odours transported through the atmosphere by wind (Papi 1989). It is unknown, however, whether large-scale smell-gradients are sufficiently well developed over the globe to provide a useful long-distance navigation mechanism for birds. As humans, we are poorly equipped to appreciate any such gradients that might occur, but even with our impoverished senses we can detect aromatic differences (due largely to the changing vegetation) as we journey from Mediterranean through temperate to boreal regions.
Birds often behave in a way that suggests they can predict impending weather, for example by not setting off on migration when a storm is brewing. One way in which they might do this is by monitoring change in barometric pressure, and experiments have suggested that pigeons have this ability. Such a pressure sense could assist migrants in maintaining their flight altitude and in moving from areas of high to low pressure or vice versa. By this means, in arid areas, birds could move towards areas where rain is falling, a frequent behaviour which has so far defied explanation (Chapter 16). However, air pressure also changes with altitude, and homing pigeons can apparently detect barometric pressure changes equivalent to a startling 10-m altitude or less (Keeton 1980).
Other, as yet unappreciated, environmental cues could provide goal-orienting birds with an analogue of map information. Various possible means of navigation have been suggested, including inertial navigation, and different uses of the magnetic field, sun and stars, and use of the Coriolis force. But these proposed mechanisms have found no support so far, and some would require levels of sensory perception well beyond those known in birds (for review see Berthold 1993). Whatever the basis, however, a compass alone cannot reveal to a bird its current position, nor in which direction it should fly.
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