Birds and Birding's Guide to:

Watching THE MIGRATIONS OF BIRDS

Theories on Bird Migration

This theory may afford an explanation for the migrations of birds that congregate in such colonies during the breeding season, but it should not be over- j looked that”survival of the fittest”may have been an equally important factor y in weeding out those individuals of such colonies that did not seek these secluded or isolated localities for breeding sites. These birds may at first have nested in scattered situations and have been driven by predatory animals or other causes to seek inaccessible locations, and seclusion and isolation may thus have been a resultant rather than a cause. It is also difficult to apply this theory to land birds. Take, for example, the Warblers of the genus Dendroica. Some species barely reach the United States during the nesting season; a few stop in the southern tier of states; others only reach to southern New England, while the bulk of the species press on from northern New England to Hudson Bay. If seclusion were the only point aimed at, it would seem that the Warblers which pass farthest north to breed could have found it in the mountains of the southern and middle states as some now do. Again, certain species, as the Cliff and Barn Swallows, Phcebe and Summer Warbler, seek the vicinity of human habitations during the nesting season, and, moreover, have greatly increased in numbers since the country became thickly settled.

The theory that is, perhaps, most naturally suggested, and the one that finds widest acceptance as explaining the facts, is that migration began in a search for j food. That is, the food supply becoming short in the vicinity of the home (a bird's home is thus assumed to be the place where it rears its young, and may therefore be quite different from the locality where it spends the remainder of the time) they wandered away in search of food, returning again and again to the home vicinity. These journeys were extended farther and farther, the birds returning each nesting season, undoubtedly oftener at first, to or near the locality where they were born. This process went on until their wandering became a fixed habit, and finally in the countless generations of birds that have come and gone, this habit has been crystallized into what we now call, for want of a better term, the instinct of migration.

This idea has been amplified and extended by Alfred Russel Wallace (Nature, X. 459). He supposed that”survival of the fittest”has probably exerted a powerful influence in weeding out certain individuals. He supposed further that breeding can only be safely accomplished as a rule in a given area, and that during a greater part of the rest of the year sufficient food cannot be obtained in that area.”It will follow that those birds which do not leave the breeding area at the proper season will suffer, and ultimately become extinct; which will also be the fate of those which do not leave the feeding area at the proper time.”His further argument is ingenious, and, it must be added, extremely plausible. He says: “Now, if we suppose that the two areas were (for some remote ancestor of the existing species) coincident, but by geological and climatic changes gradually diverted from each other, we can easily understand how the habit of incipient and partial migration at the proper seasons would at last become hereditary, and so fixed as to be what we term an instinct."

It will probably be found, however, if anything like a satisfactory explanation can be arrived at, that this habit or instinct has arisen in more than one way, but we may appropriately turn from a consideration of theories to a review of certain observed facts of migration.

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