Alternative Models Time And Energy Considerations

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While one idea assumes that wintering areas nearest the breeding areas are best, and that further migration results from competition, an alternative 'time allocation' model emphasises the benefits and costs of migration in influencing the optimal wintering area. Imagine that, within the potential wintering range, sites progressively further from the breeding range are more benign, so that day-to-day survival is higher there, but that migration costs increase with increasing distance from breeding areas (Greenberg 1980). The longer the period in each year that birds spend away from their breeding areas, the greater their survival benefits, compared to sites closer to the breeding area. Greenberg supposed that natural selection would lead to the non-breeding area being located wherever the benefits of improved survival most exceeded the costs of reduced survival imposed by migration to and from it. If migration costs were the same, regardless of date and length of journey, then longer movements should be favoured. The benefits should be especially great in high-latitude species which generally spend least time on breeding areas and most on wintering areas, and therefore gain the benefits of enhanced survival for the longest time each year. This could give another explanation for leapfrog migration in which the most northerly breeding populations winter furthest south (but does not eliminate a role for competition). The key questions centre on the relative costs of migration early and late in the season and of short and long journeys, and on what are the survival benefits in more benign climates.

A pattern of decreasing living costs with decreasing latitude has been found in arctic-nesting Sanderlings Calidris alba from measurements of energy consumption (using the doubly-labelled water technique) in free-living birds wintering in New Jersey, Texas, Panama and Peru respectively (Castro et al. 1992). The minimal living cost (observed in Panama) was equivalent to twice the basal (resting) metabolic rate (2XBMR), while the peak value (observed in New Jersey) was roughly twice this value (4XBMR), reflecting differences in winter temperatures, and hence in thermoregulatory costs at the two sites. These findings thus confirmed the large savings in body heat maintenance enjoyed by individuals migrating to the tropics.

Other studies of Bar-tailed Godwits Limosa lapponica and Red Knots Calidris canutus have measured the energy costs of both wintering and migration. In the Bar-tailed Godwit, the Eurasian-African populations occur in winter in a typical leapfrog pattern (Drent & Piersma 1990). The European population breeds in northern Europe and winters on the coasts of western Europe, especially the Wadden Sea. The Siberian-African population breeds further east and at higher latitude, and winters on West African coasts. The latter birds undertake two or more long-distance flights on their migrations, stopping at some of the same estuaries in Europe where the north European birds winter. They have to cover 8300 km one way, and spend much more time on migration than birds of the European population (2500 km one way). Energy costs per day in wintering areas were calculated at 3XBMR in Europe and at 2XBMR in Africa, the difference attributable to ambient temperatures. The energy cost of migration looms large in the annual energy budget of the long-distance migrants of the Siberian-African population (48% of annual expenditure) compared with the European population (22%). The Siberian-African birds experience peak energy demands during pre-migratory fuel deposition before the final leg of the spring migratory journey (the 4000 km separating the breeding area from the Wadden Sea), in the same places where the European birds sustained a high cost throughout their non-breeding season. In the Siberian-African birds, the costs of pre-migratory fattening in spring were so high that they offset the savings in thermoregulation costs during their stay in Africa. The overall annual energy costs did not differ greatly between the two populations, bearing in mind the greater migratory weight of the Siberian birds. We must therefore assume some advantage to wintering nearer the breeding area to explain why any godwits remain at that time in the seemingly inhospitable coastal region of Europe. Competition between the two populations may be the key factor involved, neither the European nor the African wintering areas being able to accommodate both populations for the whole winter.

However, another explanation is relevant to explaining the distribution of the two godwit populations. This comparison between them highlights the time constraints imposed by the lengthy process of accumulating the reserves required for long migratory journeys. The Siberian-African population expends the energy accumulated during 24 hours of spring feeding on the Banc d'Arguin in Mauritania in just over one hour of migratory flight. On the basis of these findings, Drent & Piersma (1990) suggested that:

'Africa is available as a wintering site only for populations nesting in the high arctic where the relatively late advent of conditions favourable for breeding provided the leeway necessary for fitting in the three months of preparatory staging. Conversely, the potential of the vast Siberian breeding range can only be realised because of the extensive capacity of the winter quarters in Africa, which in turn can only be reached thanks to the Wadden Sea situated as a stepping stone between the two.'

In contrast, the birds that winter in Europe migrate to the north of the continent, where spring comes earlier than in Siberia. They do not need such large body reserves, and can fatten in a shorter period.

Clearly, the evolutionary interpretation of migration patterns will not be solved by recourse to the role of competition alone, but must include assessment of the energy considerations related to different journey lengths, breeding and wintering areas (Drent & Piersma 1990). The main constraint on the numbers of godwits wintering in West Africa was the low levels of food available there. The West African tropics provided a low-cost/low-yield wintering option, while western Europe provided a high-cost/high-yield contrast. Because godwits can feed at night, as well as by day, they are not constrained by daylengths, only by tidal rhythms that are similar between areas.

The Red Knot Calidris canutus breeds in the high arctic, but spends its non-breeding season at specific localities spanning a wide range of latitude, from northern temperate through tropical to southern temperate regions (Figure 23.4). Although many individuals winter at British latitudes, the cold makes this location almost prohibitively expensive for them. During November-February, their daily energy needs amount to 4XBMR (Piersma et al. 1991). This is despite the fact that birds adopt all the energy-saving behaviours they can, such as feeding and roosting in dense packs, facing head to wind (to prevent the cold getting under their feathers), and reducing the blood supply to their exposed legs. At lower latitudes, the energy costs of keeping warm are much reduced. Largely because of the temperature difference, the maintenance costs of Red Knots in West Africa were about 40% less than in Europe (although the food supplies were also less good). In fact, in tropical areas Red Knots and other shorebirds may face the opposite problem to keeping warm - that of getting rid of surplus heat - which some species solve by resting on water when they are not feeding.

Interestingly, Red Knots wintering further from their breeding areas do not necessarily incur greater energy costs on migration (Piersma et al. 1991). This was shown in a comparison between two subspecies, one (C. c. canutus) breeding in Siberia and migrating via western Europe (the Wadden Sea) to West Africa (a round trip of 18 800 km), and the other (C. c. islandica) breeding in northeast Canada and Greenland, and migrating via Iceland to winter mainly in eastern England and the Wadden Sea (round trip of 9600 km) (Table 23.1). Most individuals performed these migrations in two non-stop flights separated by a single stopover. Although canutus knots migrated twice as far as islandica ones, the costs of migration were estimated to be of similar magnitude, apparently due to differences in tailwind availability en route. Both there and back, canutus birds flew parallel to the main weather systems and could always find following winds, sometimes up to 5 km above ground. Such winds could almost double the flight speed of canutus birds, giving them much shorter flight times and reduced travel costs.

These studies thus led to two conclusions: (1) for birds wintering further from their breeding areas on the same route, travel costs may be greater, but maintenance

Table 23.1 Travel distances and approximate annual average expenditure on long-distance flights by temperate-wintering Red Knots Calidris canutus of the subspecies islandica (Ellesmere Island to the Netherlands) and tropically-wintering Red Knots of the subspecies canutus (Siberia to West Africa). Cost factors for long-distance flights were calculated from estimated fat losses over the journey

Route Subspecies islandica canutus

Table 23.1 Travel distances and approximate annual average expenditure on long-distance flights by temperate-wintering Red Knots Calidris canutus of the subspecies islandica (Ellesmere Island to the Netherlands) and tropically-wintering Red Knots of the subspecies canutus (Siberia to West Africa). Cost factors for long-distance flights were calculated from estimated fat losses over the journey

West Africa-Wadden Sea (km)

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