Therazorbill (Alca torda) is a North Atlantic colonialseabird and the only extant member of thegenusAlca of the familyAlcidae, theauks. It is the closest living relative of the extinctgreat auk (Pinguinus impennis).[4] Historically, it has also been known as "auk",[5] "razor-billed auk"[6] and "lesser auk".[7]
Razorbills are primarily black with a white underside. The male and female are identical in plumage; however, males are generally larger than females. This agile bird, which is capable of both flight and diving, has a predominantly aquatic lifestyle and only comes to land in order to breed. It ismonogamous, choosing one partner for life. Females lay one egg per year. Razorbills nest along coastal cliffs in enclosed or slightly exposed crevices. The parents spend equal amounts of time incubating, and once the chick has hatched, they take turns foraging for their young.
Presently, this species faces major threats, including the destruction of breeding sites,[8] oil spills,[9] and deterioration of food quality. The IUCN records the population of the species as fluctuating,[1] causing its status to interchange. It has been recorded that the population had increased from 2008 to 2015,[10] decreased from 2015 to 2021,[11] and appears to be increasing or stable at the present.[1] It is estimated that the current global razorbill population lies between 838,000 and 1,600,000 individuals.[1] In 1918, the razorbill was protected in the United States by theMigratory Bird Treaty Act.
The razorbill (Alca torda) is now the sole species in the genusAlca,[17] though its close relative, thegreat auk (Pinguinus impennis), which became extinct in the mid-19th century, was also formerly included in the genusAlca.[12] Razorbills and great auks are part of thetribe Alcini, which also includes thecommon murre or common guillemot (Uria aalge), thethick-billed murre or Brünnich's guillemot (Uria lomvia), and thelittle auk (Alle alle).[18]
There are two subspecies of razorbill accepted by the IOC:[17]
The two subspecies differ slightly in size and bill measurements, withA. t. islandica, which occurs in warmer waters, being slightly smaller.[14] A third subspeciesAlca torda pica Linnaeus, 1766 (originally described as a speciesAlca pica from the Arctic,[19] and reduced to subspecies bySalomonsen in 1944[20]), is no longer accepted because the distinguishing characteristic, an additional furrow in the upper mandible, is now known to be age-related.[14]
The razorbill has a white belly and a black head, neck, back, and feet during the breeding season. A thin white line also extends from the eyes to the end of the bill. Its head is darker than that of acommon murre. During the non-breeding season, the throat and face behind the eye become white, and the white line on the face and bill becomes less prominent.[21] The bill is black, deep and laterally compressed, with a blunt end. It has several vertical grooves or furrows near the curved tip, one of them adorned with a white, broken vertical line. The bill is thinner and the grooves are less marked during the non-breeding season. It is a large and thick-set bird for analcid, and its mean weight ranges from 505 to 890 g (17+3⁄4 to31+1⁄2 oz).[22] The female and male adults are very much alike, having only small differences such as wing length. It is 37–39 cm in body length, the wing length of adult males ranges from201–216 mm (7+15⁄16–8+1⁄2 in) while that of females ranges from201 to 213 mm (7+15⁄16 to8+3⁄8 in).[23] During incubation, this species has a horizontal stance and the tail feathers are slightly longer in the center in comparison to other alcids. This makes the razorbill have a distinctly long tail which is not common for an auk. In-flight, the feet do not protrude beyond the tail.
Their mating system is female-enforced monogamy; the razorbill pairs for life. It nests in open or hidden crevices among cliffs and boulders. It is acolonial breeder and only comes to land to breed. The annual survival rate of the razorbill is between 89 and 95%.[24] Though the razorbill's average lifespan is roughly 13 years, a bird ringed in the UK in 1967 survived for at least 41 years—a record for the species.[25]
Razorbills are distributed across the North Atlantic; the world population of razorbills is estimated to be at less than 1,000,000 breeding pairs (Chapdelaineet al. 2001). Approximately half of the breeding pairs occur in Iceland. Razorbills thrive at water surface temperatures below 15 °C. They are often seen with the two largerauks, thick-billed murre and common murre. However, unlike other auks, they commonly move into largerestuaries with lowersalinity levels to feed. These birds are distributed across sub-arctic and boreal waters of theAtlantic. Their breeding habitat is islands, rocky shores, and cliffs on northernAtlantic coasts, in easternNorth America as far south asMaine, and in westernEurope from northwesternRussia to northernFrance. North American birdsmigrate offshore and south, ranging from theLabrador Sea south to theGrand Banks ofNewfoundland toNew England. Eurasian birds also winter at sea, with multiple populations aggregating in the North Sea[26] and some moving south as far as the westernMediterranean. Approximately 60 to 70% of the entire razorbill population breeds inIceland.[27]
The life-history traits of the razorbill are similar to that of thecommon murre. However, razorbills are slightly more agile. In North America it is a largely migratory seabird, as during the colder months, it leaves land and spends the entire winter in the waters of the Atlantic Ocean,[34] though western European birds often remain close to their breeding sites.
During breeding, both males and females protect the nest. Females select their mate and will often encourage competition between males before choosing a partner.[35] Once a male is chosen, the pair will stay together for life.
Individuals only breed at 3–5 years old. As pairs grow older they will occasionally skip a year of breeding. A mating pair will court several times during breeding periods to strengthen their bond.[22]Courtship displays include touching bills and following one another in elaborate flight patterns. Once the pre-laying period begins, males will constantly guard their mates by knocking other males away with their bills.[22] The pair will mate up to 80 times in a 30-day period to ensure fertilisation.[36] Females will sometimes encourage other males to engage in copulation to guarantee successful fecundity.[35]
Throughout the pre-laying period, razorbills will gather in large numbers. Two types of social behaviour occur; large groups dive and swim together in circles repeatedly and all rise up to the surface, heads first and bills open; secondly, large groups swim in a line weaving across each other in the same direction.
Nest site choice is very important for these birds to ensure the protection of the young from predators. Unlikemurres, nest sites are not immediately alongside the sea on open cliff ledges but at least 10 cm (4 in) away, in crevices on cliffs or among boulders. Nests are usually confined among the rocks or slightly more open. Some sites are along ledges, however, crevice sites seem to be more successful due to reduced predation.[37]
The mating pair will often reuse the same site every year.[38][39] Since chicks cannot fly, nests close to the sea provide easy access when leaving the colony. Generally, razorbills do not build a nest; however, some pairs may use their bills to drag material upon which to lay their egg.[40] Nest under a boulder, rarely on an exposed ledge, may use Puffin or rabbit burrow.[41] Although gregarious in breeding colonies, nests are not contiguous, but some metres apart, resulting in less aggression than in Guillemot colonies.[41]
Females lay a single egg per year, usually from late April to May. The egg is anovoid-pyramidal shape, creamy white to pale brown with has dark brown blotches.Incubation starts generally 48 hours after laying the egg. Females and males take turns incubating the egg several times daily for a total of approximately 35 days before hatching occurs. Razorbill chicks are semi-precocial.[42] During the first two days after hatching, the chick will spend the majority of its time under the parent's wing. There is always one parent at the nest site while the other goes to sea to collect food for the chick. The hatchling develops a complete sheath 10 days after hatching. After 17–23 days, the chick leaves the nest by jumping from a cliff, closely followed by the male parent, who will accompany the chick to sea. During this time, the male parent will dive more than the female parent.[43]
Razorbills dive deep into the sea using their semi-folded wings and their streamlined bodies to propel themselves toward their prey. They keep their feet spread. While diving, they rarely stay in groups but rather spread out to feed. The majority of their feeding occurs at a depth of 25 m (80 ft) but they have the ability to dive up to 120 m (395 ft) below the surface.[44] During a single dive, an individual can capture and swallow many schooling fish, depending on their size. Razorbills spend approximately 44% of their time foraging at sea.[45]
When feeding their young, they generally deliver small loads. Adults will mainly feed only one fish to their chick with high feeding deliveries at dawn and decreased feeding 4 hours before dark.[22] Females will generally feed their chicks more frequently than males.[45] They may fly more than 100 km (60 mi) out to sea to feed when during egg incubation, but when provisioning the young, they forage closer to the nesting grounds, some12 km (7+1⁄2 mi) away, and often in shallower water.[28]
The diet of razorbill is very similar to that of a common murre or common guillemot. It consists generally of mid-water schooling fish such ascapelin,sandeels, juvenilecod,sprats, andherring. It may also includecrustaceans andpolychaetes. A recent study suggests the diet is affected by local and regional environmental conditions in the marine environment.[39]
The adult razorbill has several predators which includegreat black-backed gulls,peregrine falcons,ravens,crows, andjackdaws. The general predators of their eggs aregulls andravens. The best chance for adult razorbill to avoid predation is by diving.Arctic foxes andpolar bears can also predate significant numbers of adults, eggs, and chicks in some years in the north of the species' range.[46]
Razorbill eggs were collected until the late 1920s inScotland's remoteSt Kilda islands by their men scaling the cliffs. The eggs were buried in St Kildapeat ash to be eaten through the cold, northern winters. The eggs were considered to taste likeduck eggs in taste and nourishment.[47]
In the early 20th century, razorbills were harvested for eggs, meat, and feathers. This greatly decreased the global population. In the USA, they were finally protected by the 1917 Migratory Bird Treaty Act which reduced hunting.[22] Other threatening interactions include oil pollution which can damage breeding sites. Any damage to breeding sites can reduce possible nest sites and affect the reproduction of the species.Commercial fishing affects populations because razorbills can become tangled in nets. Overfishing also decreases the abundance of razorbill prey and thus affects their survival.
While the razorbill is the only living species, the genusAlca had a much higher diversity in thePliocene. Some ornithologists also feel it is appropriate to retain thegreat auk in the genusAlca, instead ofPinguinus.[48] A number of fossil forms have been found:
Alca "antiqua" (Late Miocene/Early Pliocene of Lee Creek Mine, US)[citation needed]
Alca sp. (Late Miocene/Early Pliocene of Lee Creek Mine, US) - possiblyA. stewarti
Alca stewarti (Kattendijk Sands Early Pliocene of Belgium)
Alca ausonia (Yorktown Early Pliocene of Lee Creek Mine, US - Middle Pliocene of Italy)
Alca sp. (Puerto de Mazarrón Pliocene of El Alamillo, Spain) - may beA. antiqua orA. ausonia
As far as is known, the genusAlca seems to have evolved in the western North Atlantic or the present-dayCaribbean like most otherAlcini. Its ancestors would have reached these waters through the still-openIsthmus of Panama during theMiocene.[49]
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