The reason why migratory birds travel in a certain direction is intuitively obvious, but what determines the distance they travel?
The answer can be understood in terms of a trade-off between the costs of travelling a certain distance, and the benefits that accrue when an individual arrives. If there were no costs to migration, evolution would favour ever greater extension of migration to the location providing the maximum probability of survival until the beginning of the following breeding period.
However there is a cost for every unit of distance travelled away from the breeding site, not only in terms of survival of a hazardous journey, but also in terms of breeding success once the migrant arrives back in its breeding area. Costs begin even before the journey itself, as most migrants have to build up reserves for the journey in the form of fat deposits, which means investing more effort in foraging than required for simple maintenance. Once the journey begins a migrant encounters numerous hazards. It leaves its familiar area, therefore becomes more prone to accidents and more at risk from predators. It runs the risk of becoming exhausted in an area that cannot support recovery, such as over the sea or a desert, or even simply in an area of unsuitable habitat. A migrant is also more exposed to poor weather conditions, and runs the risk of becoming lost or blown off course. Finally, a migrant also runs the risk of being 'gazumped' if non-migrants have the pick of breeding sites by virtue of remaining nearby all year.
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All of these costs continue to increase so long as a bird extends its migratory journey. However the benefits will also inrease if the bird is travelling along a gradient of increasing food availability, improving habitat or more favourable climatic conditions. The bird should therefore stop as soon as the additional cost of travelling further outweighs the benefit, as illustrated in the plot to the right.
If a bird travels 1000 km from its breeding site, it will almost certainly survive the 'winter' or non-breeding season. However, its probability of surviving migration is only 0.6: It will survive a 1000 km migration six times out of ten. The best option, therefore, is to migrate 600 km, where survival of migration multiplied by over winter survival is maximised at about 0.65. |
Does this mean that every single bird in the population should travel exactly the same distance from the breeding area? The answer is no, because the more birds settle at a particular site, the fewer resources there are to go round, so survival declines as the population increases. In the jargon of population biology, survival is 'density dependent'.
This means that it becomes equally viable to settle at sites slightly closer or further away from the breeding site with lower population density, and the more birds settle, the wider the non-breeding range becomes. How wide depends on the shape of the survival curve:
If the survival curve is narrow, so is the 'width' of the non-breeding range. If the survival curve is broad in shape, the non-breeding range will be broad in extent. Compare, for instance, two European warbler species with similar-sized breeding ranges and populations:
The Subalpine Warbler winters in a narrow stretch of territory known as the 'Sahel' on the southern fringe of the Sahara Desert, whereas the Melodious Warbler occupies practically all of the territory between the desert and the sea.
The Melodious Warbler also breeds further north, on average, in Europe than the Subalpine Warbler, so this represents an example of inter-specific 'Leap-frog' migration, which is a very common pattern among migratory birds. The causes of leap-frog migration are considered in the following page..