This subspecies of the Brown-headed Cowbird is uncommon in Coahuila. Amadon and Phillips (1947:579) took an adult male and a juvenile female _M. a. ater_ at Las Delicias on August 15, both of which were considered early migrants. Dickerman obtained No. 31513 from a flock of eight cowbirds. Although the measurements of this specimen agree equally well with those of _M. a. ater_ and _M. a. artemisiae_ (Grinnell, 1909:275-281), the shape of the bill most closely resembles that of _ater_. Grinnell (1909:278) said that "_ater_ has a tumid bill, broad and high at [the] base with [a] conspicuously arched culmen" whereas "_artemisiae_ has a longer and relatively much slenderer bill, vertically shallow at [the] base and laterally compressed, with the culmen in its greater portion straight or even slightly depressed." The size of the ovary (8×4 mm.) of No. 31513 and the date (May 3) on which it was obtained suggest that this individual was a late migrant, still south of the breeding range of _M. a. ater_.
_Molothrus ater artemisiae_ Grinnell.--This subspecies of the Brown-headed Cowbird is an uncommon migrant in Coahuila. Amadon and Phillips (1947:579) obtained, at Las Delicias, a juvenile male on August 15 and an adult male on August 17.
*_Molothrus ater obscurus_ (Gmelin).--_Specimens examined:_ total 18: [Male] [Male] 32112-32115 from 12 mi. N, 12 mi. W Jiménez, 850 ft., June 18 and 19, 1952; [Male] [Male] 32108-32111, [Male] 32116, and [Female] [Female] 32117-32120 from 2 mi. W Jiménez, 850 ft., June 20, 1952; [Female] 32491 from Las Margaritas, August 4, 1955; [Male] 31511 and [Female] 31510 from 16 mi. E, 18 mi. N Ocampo, May 8 and 7, 1952; and [Male] [Male] 35409-35410 (skeletons only) from 4 mi. N San Isidro, May 11, 1954.
This subspecies of the Brown-headed Cowbird is common in Coahuila and breeds there. Amadon and Phillips (1947:579) suggested that _M. a. obscurus_ breeds at Las Delicias. Burleigh and Lowery (1942:206) saw the Brown-headed Cowbird in "small numbers on the outskirts of Saltillo...."
Measurements of the adult males that I have examined are: wing, 101.1 mm. (97-106); tail, 66.5 mm. (62-69); tarsus, 25.6 mm. (24.5-28); culmen, 17.3 mm. (16-18.5). Measurements of the adult females that I have examined are: wing, 92.3 mm. (90-97); tail, 60.1 mm. (56.5-62.5); tarsus, 23.5 mm. (22.5-24); culmen, 14.4 mm. (14-15). The sizes of the testes of three of the males (6-7 mm. long) and of the largest ova of four of the females (6-9 mm. in diameter) indicate breeding by this subspecies in Coahuila, as does the small size of one of the juvenile males (tail, 33.5 mm. long).