BLACK-THROATED DIVER Gavia arctica

Black-throated Loon, Arctic Loon (N America)
L 58–73 cm, WS 110–130 cm.

IDENTIFICATION 

Intermediate in size and bulk between Red-throated and the largest divers; at times appearing distinctly elegant (recalling Red-throated) when neck erect, at times rather stocky (recalling Great Northern) when neck retracted. Bill less stout and head more rounded than in largest divers, but bill heavier and straighter, often longer, than in Redthroated. Head and neck often form an elegant S-shaped curve. (Note that appearance of head can alter: resting birds often erect forehead feathers, creating outline recalling Great Northern, and sometimes point head and bill somewhat upwards, recalling Redthroated.) At times a distinctive muscular bulge is visible on sides of neck (most obvious in birds seen head-on or facing away). 

Dives with smooth and fast action, associated with sharp backward jerk of head and neck. Adult summer distinguished by combination of pale grey head and hindneck, black patch on foreneck bordered on each side by black and white stripes, and conspicuous white markings on upperparts. Latter are more restricted in extent than in largest divers, mainly restricted to a solid ‘block’ on scapulars (a feature seen in large divers too, but those species also have smaller white spots fairly evenly scattered over rest of upperparts and flanks. Adult winter/Juvenile distinguished by contrast between lighter crown and hindneck (grey-brown in adult, brown in juvenile) and darker upperparts (normally vice versa in Great Northern), and by distinctive,
clear-cut white patch that generally shows above water line towards rear of flanks (absent or at least never as large and clean-cut in other divers). On buoyantly swimming bird this flank patch shows as extension of white belly. Also has different head/neck pattern: in side-on profile amounts of dark and white are about equal. Lacks pale indentations in collar and pale eye-ring of largest divers and shows more extensive dark area than Red-throated, with dark of crown reaching eye. 

Head pattern often enhanced by apparent darker line dividing white foreneck and lower face from grey-brown crown and hindneck, but this not always visible. Bill of adult is pale with dark tip and culmen, a pattern otherwise seen only in winter Great Northern. Juveniles show variable amount of dusky grey-brown on throat, lower face and foreneck and bill lacks strong pattern, so they are harder to tell from juvenile Red-throated: best separated by flank patch, paler nape and shape of head, neck and bill. At close range shows only inconspicuous pale edgings to scapulars instead of numerous pale buff feather edgings over most of upperparts. Birds moulting into adult summer plumage have head/neck pattern obscured by dark foreneck and chin patches, and grey of head and neck coming through, but such birds distinguished from Red-throated by presence of bold white ‘blocking’ on upperparts. In flight, in comparison with Redthroated, is distinctly heavier-looking, with more prominently projecting feet, less drooped neck, horizontally- held bill, apparently more centrally positioned wings, slower, stiffer wingbeats (with not such a high upstroke) and distinctly more black-and-white appearance with more obvious dark flank line and (in adult winter and older immatures) more extensive dark on head/neck. See also flight details for largest divers.

SEX/AGE 

Juvenile similar to adult winter, but has relatively indistinct pale scaling on upperparts (most obvious to middle of first winter). 

VOICE 

On breeding grounds makes loud, mournful  ‘kluiee-ku-kluiee-ku-kluiee-ku-kluiee’. Other calls include a repeated croaking snore, ‘knarr-knorrr, knarrknorrr’, and a yelping, gull-like ‘aah-oww’. Silent in flight.

STATUS/HABITAT 

Locally not uncommon. (In addition to mapped range, has bred Poland.) Breeds by larger lakes with deeper water than those typically favoured by Red-throated; rarely by marine inlets. At other times found in coastal waters; rarely on inland lakes except on migration.