Common murre

The common murre flies with fast wing beats and has a flight speed of 80 km/h (50 mph).[20] Groups of birds are often seen flying together in a line just above the sea surface.[14] However, a high wing loading of 2 g/cm²[21] means that this species is not very agile and take-off is difficult.[22] Common murres become flightless for 45–60 days while moulting their primary feathers.[23] The sound of the wing beats of the murres are often described as similar to a helicopter.

Skeleton

The common murre is a pursuit-diver that forages for food by swimming underwater using its wings for propulsion. Dives usually last less than one minute, but the bird swims underwater for distances of over 30 m (100 ft) on a regular basis. Diving depths up to 180 m (590 ft) have been recorded[24] and birds can remain underwater for a couple of minutes.

The common murre can venture far from its breeding grounds to forage; distances of 100 km (60 mi) and more are often observed[31] though if sufficient food is available closer by, birds only travel much shorter distances. The common murre mainly eats small schooling forage fish 200 mm (8 in) long or less, such as polar cod, capelin, sand lances, sprats, sandeels, Atlantic cod and Atlantic herring. Capelin and sand lances are favourite food, but what the main prey is at any one time depends much on what is available in quantity.[31] It also eats some molluscs, marine worms, squid, and crustaceans such as amphipods. It consumes 20–32 g (11⁄16–1+1⁄8 oz) of food in a day on average. It is often seen carrying fish in its bill with the tail hanging out.[18]

The snake pipefish is occasionally eaten, but it has poor nutritional value. The amount of these fish is increasing in the common murre's diet. Since 2003, the snake pipefish has increased in numbers in the North-east Atlantic and North Sea and sandeel numbers have declined.[32]

The common murre has a variety of calls, including a soft purring noise.

Part of a U. a. californica colony, Farallon Islands, California

The common murre nests in densely packed colonies (known as "loomeries"), with up to twenty pairs occupying one square metre at peak season. Common murres do not make nests and lay their eggs on bare rock ledges, under rocks, or the ground. Despite the high density of murre breeding sites, sites may vary greatly in their quality over small spatial scales. Pairs breeding at those sites of highest quality are more likely to be occupied by a breeding pairs at all population sizes, and more likely to successfully fledge a chick.[33] They first breed at four to nine years old,[34] but most individuals recruit into the breeding population at ages six or seven,[34] although birds may disperse (permanently depart their natal colony) if space is limited. Annual survival probability for birds aged 6–15 is 0.895,[34] and average lifespan is about 20 years. Breeding success increases with age up to age 9–10 to 0.7 fledglings per pair, then declines in the oldest age birds, perhaps indicating reproductive senescence.[34]

High densities mean that birds are close contact with neighbouring breeders.[35] Common murres perform appeasement displays more often at high densities and more often than razorbills.[35] Allopreening is common both between mates and between neighbours. Allopreening helps to reduce parasites, and it may also have important social functions.[36] Frequency of allopreening a neighbour correlates well with current breeding success.[36] Allopreening may function as a stress-reducer; ledges with low levels of allopreening show increased levels of fighting and reduced breeding success.[36]

Courtship displays including bowing, billing and mutual preening. The male points its head vertically and makes croaking and growling noises to attract the females. The species is monogamous, but pairs may split if breeding is unsuccessful.[37][38]

Murre eggs

Common murre eggs are large (around 11% of female weight[30]), and are pointed at one end. The egg's pyriform shape is popularly ascribed the function of allowing the egg to spin on its axis or in an arc when disturbed, however there is no evidence to support this claim.[39] Various hypotheses have arisen to explain the egg's shape:

1. Pyriform eggs are more stable on a sloping surface than more elliptical eggs, such as that of the razorbill.[39]
2. The shape allows efficient heat transfer during incubation.[40]
3. As a compromise between large egg size and small cross-section. Large size allows quick development of the chick. Small cross-sectional area allows the adult bird to have a small cross-section and therefore reduce drag when swimming.[30]
4. Due to its pyriform shape, a higher proportion of the eggshell is in contact with the cliff minimising the effects of impact by neighbouring birds.
5. It helps to confine faecal contamination to the pointy end of the egg. The blunt end, where the embryo's head and air cell are located, is kept relatively free of debris, allowing gases to pass through the shell unimpeded;.[41][39]

Eggs are laid between May and July for the Atlantic populations and March to July for those in the Pacific. The female spends less time ashore during the two weeks before laying. When laying, she assumes a "phoenix-like" posture: her body raised upright on vertical tarsi; wings half outstretched. The egg emerges point first and laying usually takes 5–10 minutes.[42]

The eggs vary in colour and pattern to help the parents recognize them, each egg's pattern being unique.[43] Colours include white, green, blue or brown with spots or speckles in black or lilac. After laying, the female will look at the egg before starting the first incubation shift.[42] Both parents incubate the egg using a single, centrally located brood patch for the 28 to 34 days to hatching in shifts of 1–38 hours.[30]

Eggs can be lost due to predation or carelessness. Crows and gulls are opportunist egg thieves. Eggs are also knocked from ledges during fights. If the first egg is lost, the female may lay a second egg. This egg is usually lighter than the first, with a lighter yolk. Chicks from second eggs grow quicker than those from first eggs. However this rapid growth comes at a cost, first chicks have larger fat reserves and can withstand temporary shortages of food.

Chicks occupy an intermediate position between the precocial chicks of genus Synthliboramphus and the semi-precocial chicks of the Atlantic puffin.[44] They are born downy and by 10 days old they are able to regulate their own temperature.[19] Except in times of food shortage there is at least one parent present at all times, and both parents are present 10–30% of the time.[45] Both parents alternate between brooding the chick or foraging for food.

Provisioning is usually divided equally between each parent, but unequal provisioning effort can lead to divorce.[38] Common murres are single-prey loaders, this means that they carry one fish at time. The fish is held lengthways in the adult's bill, with the fish's tail hanging from the end of the beak. The returning adult will form its wings into a 'tent' to protect the chick. The adult points its head downwards and the chick swallows the fish head first.

Alloparenting behaviour is frequently observed. Non-breeding and failed breeders show great interest in other chicks, and will attempt to brood or feed them. This activity is more common as the chicks get older and begin to explore their ledge. There has also been a record of a pair managing to raise two chicks.[46] Adults that have lost chicks or eggs will sometimes bring fish to the nest site and try to feed their imaginary chick.

At time of extreme food stress, the social activity of the breeding ledge can break down. On the Isle of May colony in 2007, food availability was low. Adults spent more of their time-budget foraging for their chicks and had to leave them unattended at times. Unattended chicks were attacked by breeding neighbour which often led to their deaths. Non-breeding and failed breeders continued to show alloparental care.[47]

Chick, Alaska

The chicks will leave the nest after 16 to 30 days (average 20–22 days),[15] and glide down into the sea, slowing their fall by fluttering as they are not yet able to fly. Chicks glide from heights as high as 457 m (1,499 ft) to the water below. Once the young chick has left the nest, the male is in close attendance for up to two months. The chicks are able to fly roughly two weeks after fledging. Up until then the male feeds and cares for the chick at sea. In its migration south the chick swims about 1,000 km (600 mi). The female remains at the nest site for up to 36 days after the chick has fledged (average 16 days).[48]

Major oil spills double the winter mortality of breeding adults but appear to have little effect on birds less than three years old.[49] This loss of breeding birds can be compensated by increased recruitment of 4–6 year olds to breeding colonies.[49]

Nesting common murres are prone to two main sources of recreational disturbance: rock-climbing and birdwatching. Sea cliffs are a paradise for climbers as well as birds; a small island like Lundy has over 1000 described climbing routes.[50] To minimise disturbance, some cliffs are subject to seasonal climbing bans.[50]

Birdwatching has conflicting effects on common murres. Birdwatchers petitioned the UK government to introduce the Sea Birds Preservation Act 1869. This act was designed to reduce the effects of shooting and egg collecting during the breeding season.[51] Current concerns include managing the effect of visitor numbers at wildlife reserves. Common murres have been shown to be sensitive to visitor numbers.[52]

When common murres are feeding their young, they return with one fish at a time. The provisioning time relates to the distance of the feeding areas from the colony and the numbers of available fish. There is a strong non-linear relationship between fish density and colony attendance during chick-rearing.[53]

In areas such as Newfoundland and Labrador, the birds, along with the related thick-billed murre, are referred to as 'turrs' or 'tuirs', and are consumed. The meat is dark and quite oily, due to the birds' diet of fish. Eggs have also been harvested. Eggers from San Francisco took almost half a million eggs a year from the Farallon Islands in the mid-19th century to feed the growing city.[54]