RIO GRANDE GROUND-CHAT 585 



average 19.0 by 14.5 millimeters ; the eggs showing the four extremes 

 measure 19.7 by 14.5, 19.3 by 14.8, and 17.8 by 14.0 millimeters 

 (Harris). 



CHAMAETHLYPIS POLIOCEPHALA POLIOCEPHALA (Baird) 



RIO GRANDE GROUND-CHAT 



HABITS 



Wlien Baird (1865) described and named the Rio Grande ground- 

 chat he placed it in the genus Geothlypis and gave it full specific rank. 

 Since then allied races have been named in Mexico and Central Amer- 

 ica. The subject of this sketch belongs to the northern race, found 

 in northern and central Mexico, and is known to extend its range 

 across our border only in the valley of the lower Rio Grande and the 

 vicinity of Brownsville, Tex., where it seems to be rare. 



This genus seems to be intermediate between Geothlypis and Icteria. 

 Ridgway (1902) calls it "Ralph's ground-chat," and characterizes it 

 as "similar in general appearance to Geothlypis, but tail longer than 

 wing, graduated; bill very stout, with culmen strongly curved (much 

 as in Ictei-ia) ; tarsus nearly half as long as wing, or at least much 

 nearer one-half than one-third as long; no black on forehead nor 

 auriculars in adult males; sexes alike, or at least not very different 

 in color." From Icteria it "differs in its shorter and more rounded 

 wing, more graduated tail with pointed rectrices, longer tarsi, and 

 stouter feet." 



Nesting. — Very little seems to be known about the nesting habits of 

 the Rio Grande ground-chat. A nest and four eggs are in the Thayer 

 collection in Cambridge, collected by Gerald Thomas at Paisano, Tex., 

 on May 1, 1902. The nest was in a clump of coarse grass near a road. 

 The exterior was formed of dry grass and the interior with finer grass, 

 with an inner lining of horsehair. Externally it measures 3^4: inches 

 high by 3% in diameter; the inner cup is '^y^ inches deep and 1% 

 wide. 



Eggs. — The four eggs are ovate and slightly glossy. They are 

 creamy white, rather sparingly speckled, spotted and blotched with 

 "auburn," "bay," "chestnut," and "raw umber," and with undermark- 

 ings of "pale brownish drab" and "light brownish drab." Some of 

 them have small scrawls of very dark brown, almost black. The mark- 

 ings are somewhat concentrated at the large end, but none of the eggs 

 could be termed heavily marked. The measurements of 16 eggs aver- 

 age 17.5 by 13.8 millimeters; the eggs showing the four extremes 

 measure 18.5 by 13.7, 18.0 by 14.5, 16.9 by 13.2, and 17.8 by 12.7 milli- 

 meters (Harris). 



Plumages. — The sexes are nearly alike in all plumages. The nest- 

 ling plumage has apparently never been described. Ridgway (1902) 



