Why Don’t the Birds Stay South?
The result of all this is that the south-land is crowded with birds of all kinds and the food supply is enough for all. But soon in following the laws of nature in birds, as in other living things, comes the time for breeding. The south-land is warm enough for nesting and hatching, but it is so crowded that there wouldn’t be enough food for all the old birds and the little ones too and so the birds begin to scatter again. Just think of what would happen in the south-land if all the birds that stay there in the winter built their nests there and brought up a new family. A bird family will average four young birds, so that if all the bird families were born and raised in the south the bird population would quickly multiply itself by three and there would be the same old necessity of traveling away to look for food. To avoid this the birds begin to scatter to their old homes before the breeding season begins.