UKBirds

Common Guillemot

Uria aalge

The Common Murre or Common Guillemot (Uria aalge) is a large auk. It is also known as the Thin-billed Murre in North America. It has a circumpolar distribution, occurring in low-Arctic and boreal waters in the North-Atlantic and North Pacific. It spends most of its time at sea, only coming to land to breed on rocky cliff shores or islands.

Common Murres have fast direct flight but are not very agile. They are more manoeuvrable underwater, typically diving to depths of 30-60 m (100-200 ft), and depths of up to 180 m (600 ft) have been recorded.

Common Murres breed in colonies at high densities, nesting pairs may be in bodily contact with their neighbours. They make no nest, their single egg is incubated on a bare rock ledge on a cliff face. Eggs hatch after ~30 days incubation. The chick is born downy, and can regulate its body temperature after 10 days. Some 20 days after hatching the chick leaves its nesting ledge and heads for the sea, unable to fly, but gliding for some distance with fluttering wings, accompanied by its male parent. Chicks are capable of diving as soon as they hit the water. The female stays at the nest site for some 14 days after the chick has left.

Both male and female Common Murres moult after breeding and become flightless for 1-2 months. In southern populations they occasionally return to the nest site throughout the winter. Northern populations spend the winter farther from their colonies.

The Common Murre is 38-46 cm (15-18 in) in length with a 61-73 cm (24-29 in) wingspan. Male and female are indistinguishable in the field and weight ranges between 945 g (2 lb) in the south of their range to 1044 g (2.3 lb) in the north. A weight range of 775-1,250 g (1.71-2.8 lb) has been reported. In breeding plumage, the nominate subspecies (U. a. aalge) is black on the head, back and wings, and has white underparts. It has thin dark pointed bill and a small rounded dark tail. After the pre-basic moult, the face is white with a dark spur behind the eye. Birds of the subspecies U. a. albionis are dark brown rather than black, most obviously so in colonies in southern Britain. Legs are grey and the bill is dark grey. Occasionally, adults are seen with yellow/grey legs. In May 2008, an aberrant adult was photographed with a bright yellow bill.

The plumage of first winter birds is the same as the adult basic plumage. However, the first pre-alternate moult occurs later in the year. The adult pre-alternate moult is December - February, (even starting as early as November in U. a. albionis). First year birds can be in basic plumage as late as May, and their alternate plumage can retain some white feathers around the throat.

Some individuals in the North Atlantic, known as "bridled guillemots", have a white ring around the eye extending back as a white line. This is not a distinct subspecies, but a polymorphism which becomes more common the farther north the birds breed - perhaps character displacement with the northerly Thick-billed Murre, which has a white bill-stripe but no bridled morph. The white is highly contrasting especially in the latter species and would provide an easy means for an individual bird to recognize conspecifics in densely-packed breeding colonies.

The chicks are downy with blackish feathers on top and white below. By 12 days old, contour feathers are well developed in areas except for the head. At 15 days, facial feathers show the dark eyestripe against the white throat and cheek.