Concluding Remarks

In conclusion, the navigational tools available to migrating birds include: (a) a celestial compass based on sun, skylight polarisation and star patterns; (b) a magnetic compass based on the earth's magnetic field; (c) an internal clock, recording diurnal (circadian) and longer-term time changes; and (d) an inherited mean migratory direction and time programme, which together ensure that the bird flies in an appropriate direction for an appropriate time. Some, if not all, birds also have a map sense used for homing to a previously experienced place. Moreover, the fact that birds can re-find places they have already visited implies a good spatial memory.

Equipped with these navigation aids, a bird could use at least four different route-finding strategies:

1. In guiding or 'follow the leader', some birds might complete their migration by following others which know the way, thereby learning the route. Providing the leaders were experienced migrants, travel routes could be passed from old to young by cultural transmission. This strategy is used by swans, geese and cranes, in which young migrate with their parents (see above). Any birds using this method would benefit from a back-up mechanism (such as clock-and-compass) in case they were left to migrate on their own.

2. In clock-and-compass (vector) navigation, birds aim to head in a constant migratory direction (which may change one or more times during a journey) for an innately determined amount of time controlled by the internal clock. By this mechanism birds could reach previously unknown but appropriate wintering areas. Theoretically, birds of all ages could use this orientation strategy, which has been demonstrated experimentally in young passerines and others. On this mechanism alone, birds are unable to determine their position and are therefore unable to correct for wind drift, directional mistakes, over-flight, or experimental displacement.

3. In bi-coordinate navigation, birds can sense at least two global coordinates forming a reliable grid through which they can determine their geographical position. Bi-coordinate navigation could provide continual positional feedback, enabling birds to correct for drift or directional mistakes. Theoretically, birds of all ages could use this strategy, but experimental evidence from several species suggests that it is used primarily by experienced birds returning to a known area.

4. In piloting, a migration route is retraced by using a sequence of learnt landmarks. This method would require birds to build a landmark-based map during a previous migratory journey which is retraced during each subsequent migration. Such landmarks could be visual, auditory, magnetic or olfactory. This is not a method for inexperienced migrants on a first journey, but could help birds returning to a known area.

Ideally, birds should also have emergency strategies for when things go wrong -for example, when over the sea, they might reverse direction and retrace the route or, if the wind is too strong to fly back, continue on the same heading or downwind until land appears. Migrants commonly experience wind drift during flight, and often compensate for this displacement at the time or during a later flight, showing that some course recording and correction is at work, even if they do not 'know' the coordinates of their migratory goal (Thorup & Rab0l 2001; for possible mechanism see Matthews 1968). The fact that juvenile (as well as adult) birds can correct for drift may seem at odds with some of the displacement experiments discussed above, in which adults corrected for displacement and headed towards their former wintering areas, whereas juveniles continued on their usual direction and made no correction. However, drifted juveniles experience the drift and can see the ground below them, whereas artificially displaced birds travel in a vehicle or airplane and are unable to see the outside world. The method of lining up successive landscape features and flying from one to another is one obvious way of maintaining a consistent course in a crosswind.

Perhaps the main message to emerge from this review is that migratory birds have a number of orientation and navigation mechanisms available to them, and are not restricted to just one. They also have an inherent ability to learn to make use of the more important navigational cues, re-assessing them and if necessary cross-checking them at points along the route. Some of the pressing questions in bird navigation currently centre on the role of the external signals from map-related factors in young birds (to supplement the clock-and-compass mechanism), and on the extent to which migrants can travel on great circle as opposed to rhumbline routes.

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