A female king eider, sometimes known as a queen eider, appeared close inshore, near the boating club in Scalloway this week.
The king eider is a very scarce migrant from the Arctic. Although this species has been recorded in all months in Shetland, the majority are seen from January to March.
Most UK records are from northern parts of Britain during the winter months.
King eiders breed on the coastal tundra in Alaska, Canada, Greenland and Russia. They nest near freshwater lakes and pools, sometimes moving further inland.
When the chicks hatch they are led to expanses of brackish or salt water. After the breeding season, the king eider spends most of the time at sea, migrating to ice-free waters in the Bering Sea, southern Greenland, Iceland, eastern Canada, north east regions of the USA, eastern Russia and northern Scandinavia.
Their diet varies with the season, eating freshwater insects, other invertebrates and some plant material when breeding. During winter, they switch to a marine diet, diving up to depths of 50m to take molluscs, crustaceans, worms and other invertebrates from the seabed.
The king eider has a specialised bill which can efficiently remove prey from soft sediments and rocky sea floors.
Adaptations to the extreme conditions in the Arctic and sub-Arctic include thick insulating plumage along with a layer of subcutaneous fat.
Their ability to make deep dives allows king eiders to access prey which is unavailable to some other species.
The male king eider is a very striking bird with a black and white body, pink breast and bold head pattern. The squarish blue-grey head has greenish cheek patches, a black-rimmed yellow frontal shield and a red bill with a white tip.
The queen eider's brown plumage acts as camouflage when she is sitting on the nest.
The warm, reddish brown colour is mottled with black spots and chevrons and there are two narrow white wing bars.
The lighter brown head and neck contrast with the body.
As we watched, the eider turned so that her profile was visible. The bill, unlike that of a female eider, is black, smaller and less wedge-shaped.
Local rarities reported this week included white-billed diver, red-necked grebe, great white egret, ring-necked duck, American wigeon and firecrest. Sea watching from the Sumburgh Head lookout produced a high count of 744 little auks heading south in three hours; a shorter earlier watch also counted 66 of these tiny seabirds.
There have been several sightings of humpback whales, also heading south, with at least 10 off the South Mainland one day this week. Humpbacks are identified by their surfacing profile.
First, they produce a mushroom-shaped blow, then the broad dark-coloured head breaks the surface followed by a long back. The short dorsal fin is about two thirds of the way along.
When the humpback dives, the back arches to form a humped shape. Before a deep dive, the ragged-edged tail flukes appear above the surface and the long white pectoral flippers may be visible.
An individual humpback can be identified from photographs of the underside of the tail flukes.
Each has a unique white pattern of pigmentation and scarring which, combined with the size and shape of these flukes, provides identification. Humpbacks are baleen whales, trapping shoaling fish and invertebrates in the bristles of the baleen plates which hang down inside the lower jaw. The whale takes in large quantities of sea water through its mouth.
As the jaws close, the grooves on the throat contract, forcing the water through the baleen and filtering out the prey, before the water is expelled.
These large whales (11-15m) are migrating from the Arctic Circle, where they have been feeding during the summer. They are now travelling to warmer waters near the equator where they will give birth and mate.
They have to move away from the cold waters as their newborn calves do not have an insulating layer of blubber.
During these global migrations, humpbacks can travel up to 25,000km (15,530 miles) annually.
Sightings of humpback whales in UK waters have increased in recent years which may be a sign of recovering populations and/or changes in the marine ecosystem.