For most migrating birds, their journey is two-way and the need for navigational information is obvious. Sedentary birds that do not migrate do of course make short, localised trips - for example, to and from their nests. Nonetheless, these short trips form the basis from which the longer return movements that characterise migration can develop.
Although short trips may be facilitated by learning landscape features in the local area, travelling over long distances requires something more. Recall from Section 1 the young ospreys leaving Martha's Vineyard with no experience and no help from adult birds. Detailed landscape features would be of no help.
From what you have read, what gross landscape features probably guide the young ospreys?
If they distinguish between land and sea and only fly south over land, they will reach suitable feeding areas.
What other cues might be available to birds to help them navigate?
Possible navigation cues might be:
- celestial cues derived from the Sun, Moon and stars
- magnetic cues from the Earth's magnetic field
- the plane of polarisation of light
- chemical signals detectable as smells.
To navigate effectively, birds need an internal map and a compass. The internal map provides a bird with the general location of where it is relative to its homing or migration goal and the internal compass is required to guide its flight and keep it on course.
Birds are believed to use two kinds of maps for goal-directed orientation. The first of these is an olfactory map, suggested to be used by homing pigeons. This theory is still controversial.
The second hypothesis is that birds use a magnetic map to locate themselves in space. In fact, it is now believed that birds can use the Earth's magnetic field as a map and as a compass based on variation in the strength of the field and its inclination relative to the Earth's surface, respectively.
There are three types of compass that birds can use. The magnetic one has already been mentioned.
What two other compass cues might be available to birds?
The stars and the Sun both provide compass information that birds could use.