SPECIES

Yellow-billed Loon Gavia adamsii Scientific name definitions

Brian D. Uher-Koch, Michael R. North, and Joel A. Schmutz
Version: 1.0 — Published March 4, 2020

Identification

Field Identification

The Yellow-billed Loon is most notable for and recognized by a yellow or ivory bill that is proportionally larger and deeper than that of the Common Loon (Gavia immer), though the 2 species are similar in body mass (27). Females weigh 3,700–5,800 g, males 4,800–6,400 g (28, BDU). Length (based on limited data): males 838–920 mm; females 774–831 mm. In summer, adult Yellow-billed Loon is black above with extensive white spotting, white below, with bill yellow or ivory. Adults in winter and immature birds have white underparts, gray-brown upperparts (immatures paler above with barring on back), and outer half of bill entirely pale.

Similar Species

Can be distinguished during breeding from the world’s other four loon species by bill color. Very similar to Common Loon in size, appearance, behavior, and calls. Can be distinguished from Common Loon by number of white lines in necklace in mid-neck patch (< 12 in Yellow-billed Loon; >12 in Common Loon) and number of white stripes in patch in upper foreneck (4–8 in Yellow-billed Loon; 6–10 in Common Loon). White stripes of necklace, patch in upper foreneck, and chest are broader in Yellow-billed Loon and black parts of chin, throat, foreneck, and sides of neck posterior to necklace have a strong purple gloss, in Common Loon a strong green gloss (29). Black hindneck, however, has a greenish gloss in Yellow-billed Loon and a purple gloss in Common Loon (29). The Yellow-billed Loon has fewer but larger white spots on back, scapulars, wing coverts, sides, flanks and lower rump; scapular white spots about 18 mm long in Yellow-billed Loon, 10 mm long in Common Loon (29). White spots absent on the upper tail coverts and absent or few on upper rump in Yellow-billed Loon (29).

Winter and immature birds are nearly identical to Common Loon. Can be distinguished by discrete auricular patch, lacking in Common Loon. Auricular patch about 12–13 mm in diameter about 25 mm behind eye and visible in the field, often at a distance, but may be obscured or appear as a dirty smudge in preserved skins (29). Sides of neck, face (extending onto lower lores, over the eye, and onto ear coverts), and posterior auricular region are paler than in Common Loon, thus giving a broad blending between white underparts and dark upperparts; in Common Loon this transition is more distinct (29, 30). Head and neck of juvenile Yellow-billed Loon said to be strikingly paler than all except most heavily bleached Common Loons in spring (31). Most Common Loons have a dark band around the front of neck that is lacking in Yellow-billed Loons (29). Juvenile Yellow-billed Loon have strikingly scalloped pattern on back (31).

Only characteristic diagnostic in all plumages and ages is color of culmen (ridge of upper mandible). In Common Loon in winter plumage this ridge and about 5 mm on either side is black (in breeding plumage the entire bill is black), but in Yellow-billed Loon at least the distal half and usually distal two-thirds is whitish-yellow (29, 30). Color of 7 outer primary shafts provide another useful characteristic with birds in hand (32, 29): generally pale in Yellow-billed Loon, dark brown in Common Loon; Binford and Remsen (29), Burn and Mather (30), and Phillips (33) described specific differences in detail. Also, 20 rectrices in Common Loon, 18 in Yellow-billed Loon (34).

Seven bill characteristics related to shape used together can distinguish adult Common and Yellow-billed Loons at close range (from 29): (1) swelling near base of culmen is slightly more pronounced in Yellow-billed Loons, (2) distal 15–37 mm of culmen not as strongly decurved as in Common Loons, (3) remainder of culmen straight or slightly recurved, not decurved as in Common Loon, (4) tomia from center of nostrils to within 5–13 mm of tip differs (faintly recurved in Yellow-billed Loon, faintly decurved in Common Loon), (5) tomia from center of nostrils to base is strongly recurved, producing appearance of a “smile”; in Common Loons this part of tomia only slightly recurved, (6) ramus from gonydeal angle towards base always recurved but in Common Loons usually decurved, and (7) gonydeal angle more often pointed (less often rounded) in Yellow-billed Loon, but amount of overlap with Common Loon makes this characteristic the least useful. The often-cited characteristic of upturned bill in adult Yellow-billed Loon has too much range overlap with Common Loon to be useful; bill shape in juvenile Yellow-billed Loons differs significantly from that of adults, and is more similar to that of Common Loon (29). Chin feathering below the two halves of the lower mandible usually extends farther and more acutely towards gonys in Yellow-billed Loon than in Common Loon (30). Cross-sectional shape of bill differs between the species, as does extent of maxillary feathering above nostril, which never extends beyond nasal tubercle in Common Loon and always extends beyond nasal tubercle in Yellow-billed Loon (30). Binford and Remsen (29) found a few exceptions to the latter trait, however.

Also, bill longer and higher, neck thicker, chin feathers extend farther anteriorly along lower mandible, but eye smaller in Yellow-billed Loon than Common Loon (29). Yellow-billed Loon usually carries head and bill in uptilted position, whereas Common Loon does not (29, 30). Forehead bump more pronounced and neck usually thicker looking in Yellow-billed Loon than in Common Loon (31), but forehead bump may apply only to male Yellow-billed Loons (MRN). Palmer (35), Burn and Mather (30), Binford and Remsen (29), and Phillips (33) described characteristics that distinguish the 2 species. Additional empirical contrasts in morphology between these 2 species are cited in Evers et al. (27).

Recommended Citation

Uher-Koch, B. D., M. R. North, and J. A. Schmutz (2020). Yellow-billed Loon (Gavia adamsii), version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.yebloo.01