Page:Condor7(1).djvu/9

8 THE CONDOR VOL. VII An Untenable Theory of Bird Migration BY WELLS W. COOKE OME years ago Palmen advanced his theories of bird migration, which have not received much support among American ornithologists. One phase of his belief was favorably commented upon by a scientist in this country and has lately been brought so prominently into notice that a few words in regard to it at this time seem advisable, before the error makes further headway. Palmen's theory has been stated as follows: "The annual migration route of a species indicates the way by which it originally immigrated into its present breed- ing home. "? A few months ago Mr. C. C. Adams, of the University of Michigan, used this theory to explain the migration route of the Kirtland warbler (IDendroica 1'irt- landi.) b In conversation with Mr. Adams I told him that I did not believe that theory was correct and he said he hoped I would write out its refutation. At the outset one is met by the fact that several species have different migra- tion routes spring and fall. The Connecticut warbler (Geot,?13,pis a?rilis) is one of the best known examples among land birds and the golden plover (Cha- radrius dominicus) among the water birds. Evidently both routes cannot be the original path of immigration and the theory will not hold for such species. The species Mr. Adams selects, the prothonotary warbler (?rotonotaria citrea), is probably as good as any one that could be chosen to show the strong points in the belief. According to the theory, the prothonotary warblers of the Mississippi Valley spread northward following the retreating ice of the Glacial Period, and gradually worked up the river bottom, following the lead of the swampy bottom lands that form their natural home. Now year by year they follow back and forth over these river courses that marked their original entry into the country. However plausible this may seem to one who looks at the map of the Mississippi Valley and notes how the whole great river system seems especially adapted for a natural highway of bird migration, yet the argument fails when it attempts to answer the question: how did the birds originally get to the mouth of the Miss- issippi River to begin their extension up its watershed? It happens to be known that the prothonotary warblers of the Mississippi Valley pass neither to the west along the coast of Texas, nor to the east through Florida, but on arriving at the coast they make a flight across the Gulf of Mexico, here nearly at the widest. To my mind it seems an impossibility that any land bird should voluntarily take a flight across water for an unseen shore, unless it had previously learned the route by a gradual extension from a shorter flight, or was in company with some bird who had so learned it. Two suppositions are possible. First, that for- merly a chain of islands extended across the Gulf of Mexico, and that the birds having learned the way from southern Mexico to the United States, by way of these islands, continue to travel the same route after the islands have disappeared. Against this supposition is the fact that the Gulf of Mexico off the mouth of the Mississippi River is a vast abyss, with no indication that any of its central portion has been above water since bird life appeared on the earth. This first supposi- tion then may be considered not available as an explanation of the manner in which the birds learned their course across the Gulf of Mexico. Recourse must be a Stejneger, American Naturalist, XXXlII, 1899, p. 68. b Bull. Mich. Orn. Clnb, V, x9o4, pp.