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Energy expenditure of terrestrial locomotion on a linear treadmill was measured in five wader species: Turnstone Arenaria interpres, Knot Calidris canutus, Grey Plover Pluvialis squatarola, Oystercatcher Haematopus ostralegus and Bar-tailed Godwit Limosa lapponica. Additional data on Redshank Tringa totanus were taken from the literature. The cost of running in these waders, measured as the slope of the regression line of energy expenditure against speed of locomotion, is significantly less than an allometrically calculated slope for all bird species (Taylor et al. 1982). It is also less than in grouse species which, like waders, must walk to gather their food. Cost of running for a 100 g wader is 22% below the cost of a grouse, and 68% below the cost of a hypothetical penguin of similar mass. Intraindividual cost of running in relation to body mass of a Turnstone and interindividual cost of running in Knots reveal much stronger increases of running costs with increasing body mass than interspecific allometric relations would predict, and this elevated activity cost probably importantly influences the set point for body mass regulation in birds.

We measured Basal Metabolic Rate (EMR) of 16 wader species (order Charadriiformes) on their wintering grounds in Africa. The allometric regression equation relating BMR to body mass: BMR (W) = 4.02 x M (kg)(0.724) runs parallel to that of waders in temperate areas, but at a 20% lower elevation. Since waders wintering in the tropics have a reduced body mass, the difference in BMR between members of the same species at tropical and temperate latitudes was even larger and amounted to 31%. BMR did nor differ between families nor did it differ between long-distance migrants that breed in the Arctic and species that spend their entire life in the tropics. As migrant waders have a higher BMR during autumn migration in the north, we suspect that they reduce BMR upon arrival in the tropics. We hypothesize that this reduction is due to somatic factors under control of hormonal factors, including a reduction of organ sizes and probably suppressed thyroid activity induced by the high ambient temperatures in the tropics. The involvement of thyroid hormones in the regulation of BMR would be consistent with the fact that waders in the tropics regulate their body mass at a lower level than birds in temperate areas and may explain why the rate of premigratory fattening is slower in the tropics than further north. The mechanism, a dual control system including somatic and hormonal factors acting in concert, rapidly adjusts BMR of migratory waders to the different ecological conditions encountered during their long journeys to and from the breeding grounds.