28 The Geographical Distribution of Birds 



"it has been found in the case of mammals and birds that the effects of tempera- 

 ture, estimated numerically, are more than three times greater than the effects 

 of humidity upon genera, and many times greater upon the higher groups." 

 While there is some 1 difference of opinion as to the exact period during which 

 temperature exerts the greatest influence, "there can be but little doubt that for 

 both animals and plants it is the season of reproductive activity." 



There are various other factors, aside from those already mentioned, that are 

 known to exert a greater or less influence on geographical distribution. The 

 character of the soil, which carries with it an effect on the plant and insect life, 

 may be mentioned, as well as the mechanical purity of the atmosphere as evidenced 

 by the prevalence of fogs, etc. Deforestation, the usual mark of the advent of 

 civilization, has quite markedly affected distribution, and the extension of culti- 

 vated areas by means of irrigation over lands previously arid has increased the 

 habitable areas for some species, and has also resulted in displacing many indige- 

 nous forms. Mountain ranges have often been considered to be efficient barriers 

 against distribution, and that they have an effect is true, but it is mainly the effect 

 of altitude and temperature ; for if conditions are similar on opposite sides of a 

 range, they will usually be found inhabited by the same forms, which may have 

 reached these positions by passing around the extremities of the mountains or 

 by means of passes through them. The real barrier is climate and not mass. 

 Thus both sides of the Rocky Mountains as well as the high Sierra in California, 

 are found to be inhabited by the same species of birds, and, says Merriam: "The 

 great Himalaya has little or no influence in bringing about the really enormous 

 differences that exist between the faunas and floras of the plains on its two sides, 

 for these dissimilarities are due primarily to the great difference of temperature 

 resulting from unequal base level, the Thibetan plateau on the north being several 

 thousand feet higher than the plain on the south." 



Oceanic bodies of water have of course a powerful effect on distribution, 

 especially of land birds, but even here certain limitations must be borne in mind. 

 To purely terrestrial animals the presence of even a moderate width of open water 

 may prove an efficient barrier, but to birds, endowed as they are with the power of 

 flight, it is less so than might be supposed. The Galapagos Islands lying six 

 hundred miles off the coast of Peru have been stocked with an abundant fauna of 

 land birds, evidently of South American and West Indian crigin, and the Azores, 

 seven hundred miles distant from South Europe, have a fauna of one hundred 

 and twenty species and subspecies of birds, all Old World forms, the ranks of 

 which are being yearly augmented by fresh arrivals. Within the last fifty years 

 the White-eye (Zosterops ccemlescens}, a small passerine bird about the size of our 

 Parula Warbler, has crossed over the twelve hundred miles of open water sepa- 

 rating Australia from New Zealand, and has extensively and permanently colo- 

 nized the latter; the European Widgeon and Ruff have again and again been 

 found in the middle and western United States, and the American Catbird has 

 been taken in Italy. Examples of this erratic wandering, or apparently regular 

 journeying, might be continued almost indefinitely, but enough has been given 

 to show that the sea is not an insuperable barrier in all cases. 



