THE AUK' A O.•UARTERLY JOURNAL OF ORNITHOLOGY. VOL. X. APRIL, 893. NO. 2. THE GEOGRAPHICAL ORIGIN AND DISTRIBUTION OF NORTIt AMERICAN BIRDS, CONSIDERED IN RELATION TO I•'AUNAL AREAS OF NORTH AMERICA2 BY j, A. AI.LEN. TIlE PIlESENT paper consistsessentially of two parts. The first treats of the probable geographical origin aml present distribution of the genera of birds representediu North America, regardlessof any inferencesthat may be drawn from this general presentationof the factsof the case; the secondrelates to the sub- division of the continentinto faunal areas of various grades, with referenceto their relationships,classification and nomenclature. No very novelviews, nor many new factsare presented,the paper beingin greatmeasure a restatementof generallyknown facts in a new sequence,with a view to giving them greater emphasisin their bearingupon the specialsubject in hand. The leading ideas here embodiedhave already beenset forth by the writer in other connections,• but the evidenceis here for the first time presented • Read before the Tenth Congressof the American Ornilhologists' Union, held in Washington• D. C.• Nov. xS-x7• x892. '1See the author's recent paper,'The Geographical Distribution of North American Mammals' (Bull. Am. Mus. Nat. Hist.• Vol. IV, I892, pp. •99-243, pll. v-viii)• and the earlier papersthere cited. Tn•. A•I• VoL X. P•,AU'F. II[. i'•ORI'II AISlERICAN REGIoN--SHoV•IN(; •UBRE(;IONS AND Pg'OVINCES. Trn;Aur:, VoL. X. PLATEI;½. 2.O t80 1112 1GO t50 tqo 1•o lm,o I1O •o0 õo '•o 30 GO •0 &o •o 1. AppalachianSubprovince. 2. Austror•pavianSubp•,ovince. S. OampestrianSubprovince. Sonoran Subpeovince. 12.0 11o •UBPROVINCES OF Till'; V•'ARM TEMPERATE. 98 ALLEN,Ort•rt'll and D•lrlbullon of 2V.A. Bt'ra's. LAprilI'Auk in detail from the standpointof birds. As an introduction to Part I a f•w pagesare devotedto questionshaving a more or less direct bearing upon the general subject. I.--THE GEOGRAPHICAl. ORIGIN AND DISTRIBUTION OF NORTH AMERICAN BIRDS. It haslong been recognized by nearlyall •vriterson geograph- ical biologythat the two leadingfactors governing the distribution anddispersal of life overthe globe are the land connectionswhich now exist,or have fbrmerly existed,betweea the principal land masses,and clhnate; and that by far the most potent of the climatic inflnenccsis temperature. In consideringthe faunal relationsof North America to the Old World, it is important, therefore, to keep in mind the present slight separatiou of northern North America from Eurasia,and that, as currently believed by many geologists,the shallow basin now forming Bering Sea was at one time dry land, and thus formed a broad land connection between northwestern North America and north- easternAsia, duringat leasta portion of the Tertiary. As is well known,a largeproportion of the genera,and many of the species,of both animalsand plants occnrringin North America have a circumborealdistribution, even in many cases •vhere their present habitats do not extend quite to the Arctic regions. Furthermore,that manygenera, particularlyof birds, which are at presentlimited to the •varm temperateand tropical latitudes, are cnmmon to both the Old World and the two Ameri- cancontinents. It is, in certaincases, hard to seehow their pres- entdispersiou could have been brought about under thegeographic and climatic conditionsnow existing. Geology here comesto our assistance,furnishing evidence that in earlier times the climate of the globe•vas not only more uniform, but also much warmer over the regionsnow buried half the year under snow and ice. It is well knownthat in Miocenetimes a warm temperateflora pre- vailed over the presentArctic regions,and that subtropicalplants flourishedin Central Europe and in correspondinglatitudes in North America. Also that many typesof mammals,now repre- sentedonly in the tropics,formerly ranged over the greaterpart of the northernhemisphere, as shown by their fossilremains, long Vol.I893X] J ALL•N,Ort•[n a,d D/sir[butt'onof 2V. A. B[rdx. 99 sinceburied in the Tertiary depositsof both North America and Eurasia. Furthermore, there is abundant evidence of a consider- able interchangeof life betweenthe two northern continentsat a time when there was no climatic barrier, as no•v, to the north- ward extensionof subtroplctypes; and that Eurasia was in part populated by types originating in North America; and also that North America has derived a portion of its life fi'om the Old World. There is no reasonto supposethat birds were then any lessrestricted in their meansof dispersionthan no•v, in compari- soul for example, with mammals, insects, land mollusks and plants. The pasthistory of birds, while sodefective in comparisonwith that of someother groups. affordsproof of the former much wider dispersionof certain types than obtains at the present time. While for this class the geological record is so imperfect,it sufficesto show that what are no•v strictly tropical genera, for example,formerly reachedsouthern and central Europe. It also affordsevidence that birds in Tertiary times were not so very differentfrom the birds of to-day. The leading genera of the raptorial, gallinaceous,wading and swimming birds were much the same as now•; with them existed other generawhich have since become extinct, •vhile many of the now prevalent forms were absent, and have doubtless come later into existence. The • Among the existing genera of birds found in the Tertiary of Europe and North America are the following, most of which date back to the Miocene, and some to the Eocene. Struthio Olor Columba Colymbus Phcenicopterus Buteo Podiceps Leptoptilus Aquila Podilymbus Ibis Falco Uria Ardea MiNus Larus Grus Strix Sterna 'Railus Bubo Puffinus Fulica Psittacus Phaathon Phalaropus Picus Sula Scolopax Leptosoma Phalacroeorax Tringa Trogon Pelecanus Totanus Callocalia Merganser Numenius Corvus Anas Charadrius Scolecophagus Spatula Coturnix Passer Anser Phasianus Sitta Branta Meleagris Motacilla [00 ALLEN,Ort•o't'•t •t•td Dt'slr[bttl[ott of 2V.A. J•[•ds. [_April[-Auk fi'agiie nature of most bird skeletons,and the small size of the greater part of the species,and the fact that only thoseof more or less aquatic habits would be liable to rapid entombment under conditionsfavorable for their preservation, are circumstances which render hopelessany expectationof the future discoveryof the ancestrallines of the great bnlk of our presentarian fauna. Two hypotheses,however, may be hazarded regarding the presentlimited distributionof manygroups now restrictedwithin comparativelysmall areas. First, that they had not only a local origin, but that for somenot very obviousreason they havealways had a local distribution,as for examplethe Todies in the West Indies, and various South American and tropical Old World types. Second,that theyhave becomespecialized since the close of the Tertiary, with adaptationsto a tropical or semi-tropical environment. There is evidencethat towards the closeof the Tertiary a marked changein the earth'sclimate took place, culminating in the Glacial Period, during •vhich the whole northern half of the northern hemispherebecame covered xvith a heavy ice cap, lasting tbr possibly thousandsof centuries,and extending its chilling influencenearly to the northerntropic. The rise of the glacialperiod was of coursegradual, and the south•vardprogress of the great ice cap drovebefore it a}l formsof life capableof any considerablepower of locomotion,while those unable thus to escapemust haveperished fi'om cold. There washence a great crowdingtogether of exilesfi'om the north into the more favored regionsto the southward,leading to an intensestruggle for exist- ence, and a •veedingont on a grand scaleof forms least fitted for the contest. This period must thus have been one of great ac- tlvity in the evolutionof new types. Opportunitywas given fox' the gradual adaptationof manyforms to a lower temperature than that to which they had been accustomed,and to an enforced changeof food. The recessionof the ice fields•vas accompanied by the gradualextension northward of habitableland. A broader area becomingavailable in summer than in winter an annual migrationfor a greateror lessdistance on the part of the pioneer life became a necessity. Finally the ice recededto its present limits and the whole north, under radically altered climatic con- ditions, became again available fox' occupationby the more or lessmodified descendants of the pre-glacial exiles. To some of Vol.x893X] J ALLEN,Ot't•t'/• a•td DtSlribu/ion of 2V.A. Birds. IOI these a comparatively low temperature had doubtlessbecome congenial,and from thein may have been derived the distinctively arctic and subarctic birds of the present age. They belong mainly to our presentcircmnboreal genera, and are unrestricted by climatic conditionsin their dispersalthroughout the arctic and subarcticregions. Other forths proved less flexible, and remained in latitudes more nearly correspr)ndingto the climatic conditions of pre- glacial times. They had, however, before the beginning of the Glacial Period, becomebroadly dispersed,and now are found in widely disconnectedareas. \Ve have thus a reasonableexplana- tion of the disconnecteddistribution of congenericspecies in such groups as the Tree Ducks, Egrets and Herons, Spoonbills, Flamingoes, Snakebirds,Gannets, Gallinules, Barn and Pigmy O•vls, Kites (genus Elanus), Trogons, Barbers, Kingfishers, Swifts, Goatsuckers, Piculets, and a few Passerine birds. On the other hand, doubtlessm:•ny of the peculiar trtq,ical types of land birds were ah'eadyrestricted