CHESTNUT-BACKED BLUEBIRD 263 



from southern Utah and southern Colorado south to Sonora and 

 Zacatecas." 



For a clear understanding of the characters separating the races of 

 Sialia mexicana, as well as the individual variation within each of the 

 subspecies, the reader is referred to an extensive paper on the subject 

 by Robert Ridgway (1894). Later Ridgway (1907) described the 

 chestnut-backed bluebird more concisely as — 



similar to S. m. occidentalis, but adult male with whole back and scapulars uniform 

 chestnut, producing a large and conspicuous dorsal patch; cinnamon-rufous of 

 under parts more extended, always extending broadly across chest, sometimes 

 covering whole breast; adult female with upper parts browner than in S. m. 

 occidentalis, the back and scapulars hair brown to between sepia aDd prouts 

 brown, usually in strong and abrupt contrast with the mouse gray or hair brown 

 of pileum and hindneck; young much darker and browner than those of S. m. 

 occidentalis or S. m. anabelse, with under parts more heavily streaked or squamated 

 and the streaked areas more or less strongly suffused with pale fulvous or rusty 

 brownish. Decidedly larger than S. m. occidentalis, with smaller bill. 



There is, of course, intergradation vith the adjacent races and con- 

 siderable individual variation in the distribution of the blue and 

 chestnut areas. 



Aiken and Warren (1914) write of its haunts and habits in El Paso 

 County, Colo.: 



While this species is common almost everywhere on migration, though probably 

 never ranging quite as high as the next species, it breeds mainly in the yellow 

 pine region between 7,000 and 8,000 feet, where it outnumbers the Mountain 

 Bluebird. July 17, 1899, on the Divide north of Peyton, Aiken saw 20 Chestnut- 

 backed to 5 of the Mountain Bluebirds, and it is probably more numerous on the 

 Divide than anywhere else in the County. The two species are sometimes found 

 in mixed flocks in the spring, especially when the weather is stormy. The ap- 

 pearance on the plains of this Bluebird during the spring migration is but for a 

 short time, as it goes into the mountains and onto the Divide by the first of 

 April, but the storms which usually come early in May drive the birds down in 

 small flocks which remain until the weather clears and the snow melts. At these 

 times the birds often become much emaciated and some die from starvation, 

 being unable to obtain food while the snow is on the ground. 



Harry S. Swarth (1904) says of its haunts in the Huachuca Moun- 

 tains, Ariz.: 



During February and the early part of March I found the Chestnut-backed 

 Bluebirds quite numerous in the lower foothills, and on the plains immediately 

 near the mountains, being entirely absent from the higher parts of the range, 

 where the snow still lay deep on the ground; but about the middle of March they 

 began to move upward, and by the first of April there were none to be seen except 

 in the higher pine regions, their breeding grounds. Here they remained through 

 the summer in the greatest abundance, none being seen below 8,000 feet, and 

 being most numerous along the divide of the mountain. About the middle of 

 August they began, to some extent, to move down to a lower altitude once more, 



