Imágenes de páginas

I. Species with protracted but irregular migrations.

Diomedeida. Procellariida.

Phaethontidæ (?) Fregatidæ (?)

II. Species with a migration range of 30° lat. or more.


Podicipida. Urinatorida (all ?) Stercorariidae. Laridæ (most). Sulida (most ?). Anatida. Gruidæ. Rallus virginianus. Porzana. Fulica americana. Phalaropodidæ. Recurvirostridæ. Scolopacidæ (most). Charadriida (most). Aphrizida. Columbidæ (most). Cathartidæ (?). Circus hudsonius. Accipiter velox. Falco (2 species). Ceryle alcyon. Sphyrapicus varius. Micropodida. Trochilus colubris.

Tyrannus (2 species). Myiarchus crinitus. Sayornis (2 species). Contopus (3 species). Empidonax (5 species). Dolichonyx oryzivorus. Icterus galbula. Calcarius. Zonotrichia (2 species). Spizella (2 species). Melospiza (2 species). Habia ludoviciana. Passerina cyanea. Spiza americana. Piranga (3 species). Hirundinida. Vireo (4 species). Mniotiltidæ (most). Motacillidæ (most). Galeoscoptes. Regulus (2 species). Turdus (3 species).

Now the species enumerated in Lists I and II migrate periodically through an area of 30° lat., or more, that is, a migration range of considerable extent, and, with a few exceptions to be considered later, all are sharply defined species, and even though the breeding areas of most are very broad, none of them have a tendency to split into geographical varieties. Accordingly there must be be some relation existing between the range of migration and the tendency to produce geographical races, for otherwise this coincidence could not be explained. So having found that those species undertaking long migrations do not, as a rule, tend to give rise to local varieties, we must conclude that the process of taking extensive migrations is a check upon the tendency to produce geographical varieties. But in order to round off further deductions, we must first determine whether species which do not migrate extensively have a greater tendency to geographical variation than those just considered ; and this assumption will be strengthened by a comparison of the species in the following List III with those in Lists I and II.

III. Species with a short or no migration range. Alcida.

Picidæ (most). Rhynchops.

Caprimulgidæ (most). Anhinga.

Trochilidæ (most). Phalacrocoracidæ (most).

Cotingidæ. Pelecanida.

Tyrannidæ (most). Phænicopterus.

Alaudidæ. Plataleidæ.

Corvida. Ibidida.

Icterida (most). Ciconiida.

Fringillidæ (most). Ardeida (most).

Euphonia. Aramida,

Piranga (most). Rallidæ (most).

Ampelida. Hæmatopodida.

Laniidæ. Tetraonida.

Vireonidæ (most). Phasianida.

Carebida. Cricida.

Mniotiltida (a few). Falconida (most).

Cinclidæ. Strix pratincola.

Troglodytidæ (most). Bubonidæ.

Certhiidae. Psittacidæ.

Paridæ. Cuculidae


Turdidæ (most).
Alcedinidæ (most).


It is at once apparent that almost all the species of North American birds which are divisible into geographical varieties are classed in this third list, that is, that those species evincing the greatest tendency to geographical variation, are also those which undertake migrations of the least extent. Thus, for instance, Melospiza fasciata is usually resident in most localities throughout the whole year, and has become differentiated into a number of geographical races, while Melospiza georgiana is migratory, and though it breeds in an area nearly equal in extent to that of fasciata, has not produced local varieties; the non-migratory Megascope asio shows great geographical variation, while the migratory Asio acciptrinus, though almost cosmopolitan in its breeding area, shows no tendency toward such variation. And, in fact, an examination and comparison of List III with Lists I and II, will lead to the conclusion, that given any two species of equally extensive breeding areas, the one with the smaller range of periodic migration will, as a rule, evince a greater tendency to produce geographical varieties than will the species with the greater range of migration. This conclusion may be concisely formulated as follows; it is the rule that the amount of geographical variation in species with more or less extensive breeding areas, stands in inverse ratio to the extent of its periodic migrations. Naturally, this law is only applicable to species with extended breeding areas, since diverse conditions in different sections of this area are necessary, according to the theory of Natural Selection, for the production of geographical subspecies or varieties; and in a limited breeding area, throughout which the conditions of the environment are similar, there could be no cause to produce geographical varieties, irrespective of the migratory or nonmigratory habits of the species.

I have not meant to imply, in the preceding pages, that species with migration ranges of 30° lat., or more, are all sharply definable, i. e., that such species are never divisible into geographical varieties; but, on the contrary, that this tendency to produce geographical races is less in the species with extensive migrations, than in those with shorter ranges of migration. For it is usual, even in species with extensive migrations, whose breeding areas are extraordinarily great, so as to include the whole of the arctic region, or northern America together with northern Eurasia, for them to subdivide into two geographical varieties, occupying respectively the eastern and western hemispheres. Thus, the eurasiatic Colymbus nigricollis is represented by a variety (californicus) in western North America ; and to give other examples where an eurasiatic form, which undertakes long periodic migrations, is represented by a geographical variety in North America, may be mentioned one species of Fratercula, 1 Uria, 1 Larus, 1 Hydrochelidon, 2 Aythya, 1 Glaucionetta, 1 Somateria, 1 Anser, 1 Tringa, 1 Limosa, 1 Charadrius, 2 Falco, 1 Pandion, and others. But no species with extensive migration ranges shows any tendency to geographical variation, unless its breeding areas are also very large in extent. And the species with the least demonstrable tendency to produce local races, are those in which the wing power is greater, and the range of migration more extensive, than in any other species of birds, namely, those enumerated in List I. Further, we find it to be the rule, that in those avian families most of the species of which undertake long migrations, if species are present which are divisible into geographical varieties, that these latter are more restricted in their migrations than the former; examples are Uria troile, Rissa tridactyla, Fulmarus glacialis (only North American species of the family presenting geographical varieties), Rallus longirostrus, Porzana jamaicensis, Aegialitis wilsonia and Ae, meloda, and others. After the consideration of these facts it is certainly permissible to conclude that, as a rule, species which undertake annual migrations of comparatively great extent, through distances of 30° lat., or more, evince no tendency to give rise to geographical varieties, unless their breeding areas are very extensive; and, conversely, that species which do not undertake extensive migrations, owing to insufficient wing power or to some other cause, and which occupy broad breeding areas, have the tendency to produce geographical varieties. Consequently, also, extended migration acts as a check upon the production of varieties; and the extent of the range of migration will, therefore, stand in inverse ratio to the amount of geographical variation evinced. Thus the postulate of Darwin, that wide-ranging species vary most, must be modified after a consideration of the facts given here. But to pass over to certain apparent exceptions to the rule. Falco columbarius, breeding chiefly north of the United States, and migrating in winter as far as South America, has a variety (suckleyi) on the Pacific coast from Sitka to California ; Helminthophila ruficapilla, breeding as far north as Hudson's Bay, and migrating in winter aş far as Guatemala, has a variety (gutturalis) from the Rocky Mts. to the Pacific coast, in winter to Mexico; and a number of similar cases could be mentioned, where the species, although it has a wide range of migration, and a breeding area which is not extraordinarily extensive, has, nevertheless, the tendency to geographical variation. But such apparent exceptions to the rule are, in fact, not valid objections, since in these cases the geographical variety is much more restricted in the range of its migration than the type species, or vice versa. And in any of these cases, the species, including the variety, is to be regarded as a number of individuals, some of which undertake extensive migrations, while others migrate not at all or through much shorter distances. Therefore, these are not true exceptions to the law, that the extent of the migration stands in inverse ratio to the amount of the tendency to produce geographical varieties; since a number of the individuals do not undertake extensive migrations. Real exceptions may, however, be found in such cases where the individuals of the type species as well as its varieties make prolonged periodic migrations; and after a careful examination of all the North American species and their varieties, I have found only four species which represent such exceptions to the rule: Dendroica æstiva, with its variety morcomii, Sciurus noveboracensis, with the subspecies notabilis, Sylvania pusilla, with the variety pileolata, and Turdus ustulatus, with its eastern variety swainsonii. These four species represent cases where, with not very extensive breeding area, both races of the species possess extensive migration ranges. But I think that the importance of these cases as exceptions to the rule is diminished, when we consider that in each case the migration route of the variety is different from

« AnteriorContinuar »