REDUCED BASAL METABOLIC-RATE OF MIGRATORY WADERS WINTERING IN COASTALAFRICA

We measured Basal Metabolic Rate (EMR) of 16 wader species (order Char adriiformes) on their wintering grounds in Africa. The allometric regr ession equation relating BMR to body mass: BMR (W) = 4.02 x M (kg)(0.7 24) runs parallel to that of waders in temperate areas, but at a 20% l ower elevation. Since waders wintering in the tropics have a reduced b ody 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-di stance 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 a rrival in the tropics. We hypothesize that this reduction is due to so matic factors under control of hormonal factors, including a reduction of organ sizes and probably suppressed thyroid activity induced by th e high ambient temperatures in the tropics. The involvement of thyroid hormones in the regulation of BMR would be consistent with the fact t hat waders in the tropics regulate their body mass at a lower level th an birds in temperate areas and may explain why the rate of premigrato ry fattening is slower in the tropics than further north. The mechanis m, 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 fr om the breeding grounds.