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aEstimated from measures of fat loss on migration of 0.8 kJ per km for southeast England to northern Norway; 0.7 kJ per km for northern Norway to Ellesmere Island; 0.3 kJ per km for West Africa to the Netherlands (Wadden Sea) and 0.5 kJ per km for Wadden Sea to Siberia (Taimyr). Observational evidence shows that these distances are normally covered in a single non-stop flight, the whole one-way journey being made up of two flights with a single stopover. From Piersma et al. (1991).

aEstimated from measures of fat loss on migration of 0.8 kJ per km for southeast England to northern Norway; 0.7 kJ per km for northern Norway to Ellesmere Island; 0.3 kJ per km for West Africa to the Netherlands (Wadden Sea) and 0.5 kJ per km for Wadden Sea to Siberia (Taimyr). Observational evidence shows that these distances are normally covered in a single non-stop flight, the whole one-way journey being made up of two flights with a single stopover. From Piersma et al. (1991).

costs in warmer wintering areas may be lower; but (2) for birds taking different routes (like the two races of Red Knots), travel costs are not necessarily greater on the longer migration, because they depend partly on wind conditions. This comparison was concerned only with the overall energy budget, however, and not with the mortality risks from periods of energy crisis, or from other causes, which may have differed greatly between the two populations. It is not the energy budget as such, but the costs in terms of mortality and reproduction that provide the selection pressures affecting the genetic make-up of populations and their particular migratory and wintering areas. Nevertheless, these calculations on the energy needs of migration question the assumptions on which some models of the evolution of migratory habits have been based. However, they do not eliminate the likelihood that, among closely related taxa, differences in winter range evolved partly in response to competition between individuals in different populations.

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