82 | A Navigational Diversion
migration direction, south, when tested under planetarium skies. By con-
trast, those exposed as juveniles to a normal, rotating, planetarium sky
could pitter- patter towards the species’ naturally- preferred southerly
direction. A third group was exposed to an incorrect planetarium sky
in which the stars rotated about a fictitious axis. When tested during
the autumn, these birds took up the ‘correct’ migration direction rela-
tive to the stars’ new axis of rotation. These results suggest that the In-
digo Buntings use the stars to navigate, a process that has to be learnt
and is based on the axis of celestial rotation.
When the sky is cloudy, sun and stars may be of little use but the
earth’s magnetic field remains unaffected. Since the first demonstration
of the ability of birds to use this field in the 1960s, the ability has been
discovered in some 20 migrant species.^2 A typical test involves a bird
active in a cage similar to that described above. When the magnetic field
inside the cage is altered from the earth’s, the bird’s preferred direction
alters. If, for example, the field is reversed by 180°, a bird that was flut-
tering towards the north starts fluttering southwards.
Because birds seem to use smell less in their daily comings and goings
than mammals, there has been historical resistance to the idea that ol-
faction might help birds overcome navigational challenges. Moreover,
the landbird experiments, mostly with homing pigeons, have yielded con-
flicting results. That said, the balance of evidence is that smell is import-
ant if pigeons are to reach home successfully.^3 But, as we shall shortly
see, the evidence that smell may guide seabirds (as opposed to landbirds)
over substantial distances is accumulating.^4
While the sun, stars, and magnetic field can, at least in theory, pro-
vide the bird with positional information, even when it is beyond the
boundaries of the places it has ever visited, it is tricky to imagine how
that might apply to the olfactory landscape, or ‘smell- scape’. Knowledge
of that smell- scape can presumably accrue only piecemeal as the adven-
ture of life takes the animal beyond its current boundaries.
The same is true of landmarks. The closer a bird is to home, the more
likely it is to be familiar with local landmarks, lessening any reliance on
the various compasses just discussed. And the use birds make of such
local landmarks has become apparent as GPS- tracking has made it pos-
sible to plot the precise route followed by homing birds. For example,
different pigeons released repeatedly from the same locality near Ox-