Divers/Loons

Genus Gavia

Five species:
  • White-billed Diver or Yellow-billed Loon Gavia adamsii,
  • Great Northern Diver or Common Loon Gavia immer,
  • Black-throated Diver or Arctic Loon Gavia arctica, two subspecies: 
    • G. a. arctica,
    • G. a. viridigularis
  • Pacific Diver or Pacific Loon Gavia pacifica
  • Red-throated Diver or Red-throated Loon Gavia stellata

Distribution

The group of the Divers or Loons, as they call them in North America, is formed by five species exclusively from the Northern Hemisphere. All of them are rather large birds, breeding in the arctic and boreal zone of Eurasia and North America. Although ranges overlap a great deal, identification is pretty straightforward. The bills are so distinctive that it is easy to tell them apart, with the exeption of the Pacific and Arctic Divers which are rather similar.
The Great Northern and the White-billed Diver, both the larger of this group, are more or less similar in appearance by their plumages, black headed and intensively spotted white on the backs in summer and greyish in their juvenile and winter plumage. The main structural difference are their large bills which are straight and black in the Great Northern and yellowish white and somewhat 'upturned' in the White-billed. The White-billed occurs in the high North of North America and Siberia, moving south in winter. The Great Northern is a bird from northern North America, moves southward and to the European coast in winter.
Until recently the smaller Black-throated and Pacific Diver were considered subspecies of a single grey-headed and black-throated circumpolar species, but are now separated. Both have straight dark bills, in the slightly smaller Pacific Diver a little bit shorter.
The Pacific Diver lives in eastern Siberia, Alaska, Canada to Baffin I. Both Arctic Diver subspecies are found in northern Europe east to Lena River and European coast and Mediterranean in winter (arctica), and from Lena River eastward into Siberia (viridigularis).
The Red-throated Diver is the smallest of all, but also a grey-headed with - as the name says - a red throat in summer. Its bill is less heavy, but the main difference is its upturned appearance. Winter and juvenile plumages of the latter species are also greyish. It occurs circumpolar in the high North, moving south in winter.

Behaviour

Divers are purely aquatic birds that forage mainly on fish. On land they are very clumsy as a result of their adaptations to foot propelled diving. The legs are positioned at the rear of the body and are not suited for walking. Divers can reach depths up to 60 meters but mostly no deeper than ca 10 m. (Johnsgard 1987). Divers are flap-flyers that can cruise at speeds of about 75 km/h.


White-billed Diver Gavia adamsii. Terschelling, The Netherlands
Culmen: 91.8 mm; total: 185 mm; adult female

Great Northern Diver Gavia immer, Hampton, VA, USA
Culmen: 74.4 mm; total: 159 mm; adult female. Summer bill

Black-throated Diver Gavia arctica arctica. Schleswig-Holstein, Germany
Culmen: 61.0 mm, Total: 137.8 mm, adult. Courtesey of N. Guse, Universität Kiel, D

Red-throated Diver Gavia stellata The Netherlands
Culmen: 57.0 mm, Total: 126.3 mm, adult male

Osteology

Skulls of all divers have very pronounced depressions for the nasal glands above the eyecases and strongly developed supraorbital ridges. Like in the auks this ridge becomes thicker with age. In all species bills of juvenile and first winter birds are not as heavy as in adult birds and the lachrimal bones are not fused to the nasal bones yet.
  • Skull
  • Flight apparatus
  • Pelvis and legs
  • Vertebrae and ribs

References