Closely -allied Varieties of Birds Interbreeding. 181 



winter in Patagonia. From what has been stated, it will be 

 observed that, as a general rule, the migratory birds of eastern 

 and western America pursue a northern and southern course 

 in their respective provinces. In accordance with these 

 facts, we find the two great regions characterized by 

 distinct forms or types ; even in cases of species common to 

 both, there" are characters which distinguish them from one 

 another. As an example, the well-known yellow woodpecker 

 {Colapates auraius) extends from the Atlantic across the con- 

 tinent to the upper Missouri, where it is replaced by a very 

 closely allied species, differing from it in having the shafts of 

 the quills red instead of yellow. Now, whether these two birds 

 are distinct species or only permanent varieties of one species, 

 we have it on the excellent authority of Professor Baird that 

 at the line of junction of the two grand ornithic provinces, in- 

 dividuals are met with showing a combination of the charac- 

 ters of both, which, in conjunction with the following examples, 

 would seem to indicate that the eastern and western forms 

 interbreed. The well-known snow bird (F. kyemalis), also 

 the long-crested, and S teller's jays, present characters common 

 to both on these boundary lines between the eastern and 

 western provinces, whilst the seaboard specimens are at once 

 characterized by very permanent and distinct colourings."* Let 

 us see what indications are presented of similar crossing of 

 allied species among the Old World avifauna. In Egypt the 

 common chimney swallow is distinguished from the European 

 bird by the intense rufous on the belly, with spots of the same 

 colouring on the tail. These, as is well known, are pale rufous 

 or dirty white in the true Hirundo rustica. Now the latter 

 passes through Egypt in spring and autumn during its annual 

 migrations ; the result is that, now and then individuals of the 

 two are to be met with, showing intermediate grades, which 

 may be owing to interbreeding. Again, in Malta, we find 

 specimens of the Spanish sparrow (par excellence the domestic 

 * " Birds of United States/' Pacific Raiiway Report. 



