YELLOW-BILLED DIVER Gavia adamsii

Yellow-billed Loon, White-billed Diver
L 76–91 cm, WS 137–152 cm.

IDENTIFICATION 

The largest diver (averaging larger and bulkier than Great Northern and often looking even thicker-necked). Distinctive pale bill is typically a little longer than bill of Great Northern with a straight (adult) or slightly convex (juvenile) culmen (obviously convex in Great Northerns of all ages) and distinctly up-angled lower mandible (less obviously so in juvenile), giving bill a noticeably upturned appearance. This effect added to by habit of pointing head and bill at a markedly upwards angle. (Recalls Redthroated in this respect, but overall bulk much greater with large head and thick neck.) Adult summer distinguished by pale straw-yellow bill (black or mainly black in Great Northern) with distinctive shape that contrasts with dark head. In close view, shows fewer and broader white stripes in neck patches, which taper towards nape (unlike Great Northern) and slightly larger and fewer white markings on back than Great Northern. Adult winter/Juvenile likely to be confused only with Great Northern. 

Distinguished by ivory-white bill (typically pale grey with blackish culmen,cutting edges to mandibles and  often tip in Great Northern, but note that Yellow-billed can have dusky greyish bill base) with distinctive shape (less obvious in juvenile), paler overall coloration (especially on crown and hindneck, which are paler than upperparts) and less sharply defined and subtly different head neck pattern. Latter is much less obviously dark-andwhite, with dusky brownish-grey invading sides of neck, foreneck and lower face. Pale indentation and dark area at base of neck both less obvious than in Great Northern, although latter feature is often even more extensive, sometimes forming almost complete collar that contrasts with white breast. Yellow-billed often shows a characteristic but rather ill-defined oval dark patch at rear of ear-coverts. At very close range, shafts of primaries are whitish (brownish in Great Northern). 

See also difference in typical body outline described under Great Northern. (Beware young juvenile, or bleached adult winter-plumaged Great Northern in spring or summer, which can be confusingly pale above.) Birds moulting into adult summer plumage are very similar to Great Northern and best distinguished by bill colour and shape (but beware Great Northern at similar stage, which can show quite extensive whitish at tip of bill). In flight looks elongated
and bulky like Great Northern, with well projecting feet, but wingbeats rather slower and neck more uniformly thick. Head/neck pattern of adult winter/immature indistinct, without clear white indentation and solid dark half-collar of adult winter/older immature Great Northern, and often with dark eye clearly visible against pale background.

SEX/AGE 

Juvenile broadly resembles adult winter, but has conspicuous pale scaling on upperparts (most obvious to middle of first winter) and is paler on head and neck with less obvious dark area at base of neck. By first summer shows more contrasting head/neck pattern and has rather variegated upperparts, with a mixture of dark new feathers and pale-edged juvenile feathers. By second winter much as adult winter, but lacks white spotting on upperwing coverts (difficult to see in the field); in summer of third calendar year often inseparable from adult winter, but some show elements of adult summer plumage on upperparts and have very sooty head and neck. Adult summer plumage finally acquired in spring of fourth calendar year.

VOICE 

Resembles Great Northern but loud, repeated, haunting yodelling lower-pitched and tremulous scream higher-pitched.

STATUS/HABITAT 

Generally rare, although local concentrations occur (e.g. Varangerfjord). (Map note: may not breed regularly in Murmansk area.) Breeds by lakes of various sizes, not necessarily with deeper water. At other times found in coastal waters.