Page:The Migration of Birds - Thomas A Coward - 1912.pdf/45

Rh north, and to birds of the same species which winter in La Plata, arriving from supposed breeding places to the south when the northern birds leave. Captain R. Crawshay, author of "The Birds of Tierra del Fuego," found it in this little known land, but speaks somewhat doubtfully of its identity; we shall probably learn that the southern form is sub-specifically distinct from the northern. There are other wide-ranging waders which we suspected of having a southern nesting area, but we still await proof.

The lack of sufficient or suitable food in the winter home during our northern summer may also cause the exodus, but this is a difficult point to prove when it is remembered that the winter home of every bird is not the patched tropical [and or the waterless desert. From some zones removal must he a necessity, but in others there is food for all, so far as man can tell.

Dr J. A. Allen, a severe but discriminating critic of migration theorists, says—"Migration is the only manner in which a zoological vacuum in a country whose life-supporting capacity is a regular fluctuating quantity, can be filled by non-hibernating animals" (51). When in the early days of migration this periodically-supplied northern zoological vacuum was filled to overflowing by the increased numbers of avian inhabitants at the close of the