38 Field Museum of Natural History — Zoology, Vol. IX. 



*GROUP 1. Wing, 9.50 to II inches long. 



Back, with white spots; throat, white or whitish (immature or 

 winter ) . Gavia stellata. 



Red-throated Loon. 



See No. 9. 



Back, without distinct white spots; the feathers, edged with ashy; 

 throat, white or dusky (immature or winter). Gavia arctica. 



Black-throated Loon. 

 See No. 8. 



Head, ashy gray; throat, black (adult). Gavia arctica. 



Black-throated Loon. 



See No. 8. 



Throat, gray; front of neck, chestnut brown (adult). 



Gavia stellata. 

 Red-throated Loon. 



See No. 9. 



*GROUP 2. Wing, 11 to 15 inches long. 



SECTION 1. Depth of bill at base, more than .85. 



Head, black (adult). Gavia immer. 



Loon- 

 See No. 7. 



Top of head, grayish; throat, white (immature or winter plum- 

 age). Gavia immer. 



Loon. 

 See No. 7. 



SECTION 2. Depth of bill at base, less than .85. 



Head, ash gray; throat, black (adult). Gavia arctica. 



Black-throated Loon. 

 See No. 8. 



Throat, gray; front of neck, chestnut brown (adult ) . 



Gavia stellata. 



Red-throated Loon. 



See No. 9. 



♦For directions for measurement, see page 21. 



