ICJO Ridgway, Geographical Variation in Sialia mexicana. TApril 



partially correlated with locality, its absence or very slight 

 development is the rule among the birds of this species resident 

 on the high mountains of Lower California ; that it is never ( ?) 

 wholly absent though sometimes very slightly developed among 

 those from the Pacific coast district north of Lower California, 

 from which area most specimens have the chestnut on the back 

 moderately developed, that is, almost always more or less inter- 

 rupted (often widely) by blue along the median line; and that 

 among those from the interior plateau region the back is, as a 

 rule, entirely chestnut. 



The series of resident Mexican birds is much too small to 

 admit of even a guess as to which style of plumage prevails 

 among them. 



I have taken advantage of the opportunity offered by the large 

 series of specimens brought together as the basis of this paper, 

 to prepare descriptions of typical specimens of the several forms, 

 representing the two sexes in these seasonal adult plumages, 

 which are presented herewith, together with their principal 

 synonymy and various critical observations. 



The preparation of a 'key' for the ready identification of speci- 

 mens has proven a matter of extreme difficulty, for the reason 

 that no single character will serve for the definition of any form, 

 the nearest approach to constancy of a single character being, 

 so far as the material examined indicates, the much darker, 

 more purplish, blue of the resident birds of southern Mexico 

 and the wholly chestnut back of the birds from the plateau 

 region of the United States. 



I am aware the following attempt at a key is very unsatis- 

 factory ; but on account of the fact that our series of true S. 

 mexicana is very inadequate, and not wholly sufficient as to the 

 other forms (the proportion of specimens obtained during the 

 breeding season being too small), it is the best I have been able 

 to devise. 



A. Chestnut color of back extremely variable in amount, sometimes 

 entirely absent, but in the latter case chestnut of breast continuous 

 anteriorly. 



a. Back usually (?) with little if any chestnut; blue of upper parts 

 of a rich deep 'smalt' hue; wing in adult male averaging 4.38. 

 (Highlands of southern Mexico.) 1. S. mexicana Swains. 



