How is migration coordinated with the seasons?
A coordinated sequence is apparent in the case of birds migrating from the northern Arctic regions to tropical winter regions; both life zones show broad fluctuations in productivity. In the Arctic, vegetation and animal production are very high during the summer. Ducks and shorebirds nest there in great numbers, exploiting the food resources. As winter comes, days shorten and food becomes scarce. The waterbirds migrate to southern climates where the rainy season has caused food production to increase to optimal levels. In winter, ducks and shorebirds concentrate in the most favorable areas and remain until spring when productivity there is lowest. By then, conditions at the breeding areas are again favorable for the birds. The life cycle of these birds is closely attuned with the productivity cycles in their breeding and winter habitats. The size of populations is controlled by the capacity of both habitats to sustain them.
