Page:Popular Science Monthly Volume 42.djvu/476

458 plains and begin to straggle eastward after the cattle were introduced? An allied form in the West, the yellow-headed blackbird, has similar habits, trooping among the cattle and horses on the plains, though it is a good householder, unlike the loose, vagabond cowbird.

The bobolink may have been a denizen of the river marshes of the East long before the discoverer first set foot upon these shores, though from its wide range toward the West, breeding on the plains of the Saskatchewan, we might infer that it had come eastward with the opening of the country. Similar conclusions might be adduced concerning the red-winged blackbird from its life and distribution, but it is a bird more of marshy land than of upland fields. Certain shore birds seem also to have taken advantage of the clearing of the country, as the killdeer and the grass plover, both being frequenters of plowed and fallow land.

Several characteristic prairie birds have at times by some accident found their way East, notably in the case of the lark finch, a beautiful Western form, and the yellow-headed blackbird above mentioned, both of which have wandered east as far as Massachusetts and Pennsylvania. This fact at least shows the capability of a bird to wander far from its original home, the regular phenomena of migration being still another proof.

Birds, owing to their superior organization and power of flight, have, more than any other forms of life, a constant tendency to widen their ranges and to occupy adjacent territory whenever the proper physical conditions are presented. This has been very clearly shown in the case of certain species along the Mexican border occupying the lands on which chaparral has lately grown up as a result of the invasion by cattle. We can picture to ourselves a few prairie stragglers finding their way into the newly cleared lands of the settlers and gradually establishing themselves in the Eastern fields. By what route they came is a matter of conjecture—probably from the southwest in the northward-setting tide of the spring migration, or possibly by way of the Great Lakes and St. Lawrence Valley.

Thus has man in his history of progress and discovery unconsciously affected the distribution of other living beings. It is a very small fragment in the history of a country, but one of especial interest as showing how remotely and by what strange means causes and effects operate. Man appears in a new land, clears its face of timber, and erects his home. By and by the swift forsakes the hollow tree to build in the settler's chimney, and the swallow leaves the overhanging tree trunk or rocky ledge for the