A year later I again visited the same preserve and {119} found great lamentation. More than five thousand Pheasants had been swept away by disease within a few weeks. Is it going too far to say that the gunmen and trappers had overdone their work? So few Hawks or Owls or foxes had been left to capture the birds first afflicted, that these had been permitted to associate with their kind and to pass on weakness and disease to their offspring until the general health tone of the whole Pheasant community had become lowered. In the end five-sixths of the birds had succumbed to the devastations of disease. All birds have their part to play in the great economy of the earth, and it is a dangerous experiment to upset the balance of Nature. {120} CHAPTER VII CIVILIZATION'S EFFECT ON THE BIRD SUPPLY Twelve hundred kinds of wild birds have been positively identified in North America. About one-third of this number are called sub-species, or climatic varieties. To illustrate the meaning of "sub-species," it may be stated that in Texas the plumage of the Bob-White is lighter in colour than the plumage of the typical eastern Bob-White, which was first described to science; therefore, the Texas bird is known as a sub-species of the type. Distributed through North America are nineteen sub-species of the eastern Song Sparrow. These vary from the typical bird by differences in size and shades of marking. In a similar way there are nine climatic variations of Screech Owls, six Long-billed Marsh Wrens, and fourteen Horned Larks. It is {121} difficult to explain why this variation in colour and size is so pronounced in some species and yet is totally absent in others of equally wide range. The Mourning Dove breeds in many localities from the southern tier of Canadian Provinces southward throughout the United States and Mexico, and yet everywhere over this vast range the birds are the same in size and colour. Nowhere do the individuals exhibit any markings suggestive of climatic influences. Some birds are very rare and are admitted to the list of North American species because of the fact that during the years a few stragglers from other parts of the world have been found on our continent. Thus the Scarlet Ibis from South America, and the Kestrel and Rook from western Europe, are known to come to our shores only as rare wanderers who had lost their way, or were blown hither by storms. Eighty-five species of the birds now listed for North America are of this extra-limital class. Among those naturally inhabiting the country, some are, of course, much more abundant than others, thus every one {122} knows that Bald Eagles are comparatively rare, and that Robins and Chipping Sparrows exist by millions.