a navigational diversion | 81These experiments from yesteryear suggested that the shearwaters
possess some map of the world, and a compass that utilises the sun, pos-
sibly in conjunction with other cues. Since then, the study of bird navi-
gation has hugely advanced. However most of the new information has
derived not from seabirds but from landbirds, not least because the lat-
ter are so much easier to keep in captivity, and there subject to experi-
mentation. It is therefore timely to detour briefly into what is known
about landbird navigation, before pondering whether the findings can
be extended to seabirds.
A sun compass is likely widespread among birds. A common experi-
ment involves shifting the internal clock of a captive bird by altering
the light regime within its cage such that ‘dawn’ and ‘dusk’ occur, say, six
hours later than in the outside world. When the bird is displaced some
distance from home, and released on a sunny day, it will, if using a sun
compass for navigation, assess the position of the sun. Now comes the
problem. If the northern-hemisphere bird is released at a time its inter-
nal clock considers midday, and it needs to fly south to reach home, it
will fly towards the sun. Alas for the bird, the real time is 6 pm in the
late afternoon. Flying towards the sun takes the bird west, a 90 degree
error. By means of such clock- shift experiments, the importance of a sun
compass to birds has been repeatedly confirmed.
Night falls and the sun dips below the horizon. But, in the absence of
cloud, the stars shine bright in positions that differ according to the
position of the observer – or bird – on the ground below. Experiments
have confirmed the importance of the starry heavens in determining the
direction in which migrating birds head.
Many such experiments exploit the fact that, during the migration
seasons, small nocturnally- migrating birds are restless at night, typically
fluttering in their cage while pointing in the direction they would mi-
grate if unrestrained. This preference can be recorded if the circular
cage floor is inked. The concentration of inky footprints in one sector of
the floor indicates the birds’ preferred direction. In a classic 1970 study
of a widespread summer migrant to eastern North America, Steve Emlen
of Cornell University hand- raised three groups of Indigo Buntings vari-
ously isolated from the stars.^1 Those prevented from viewing the night
sky prior to the autumn migration season could not select the normal