Raso lark

Raso lark
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Passeriformes
Family: Alaudidae
Genus: Alauda
Species: A. razae
Binomial name
Alauda razae
Alexander, 1898

The Raso lark (Alauda razae) is a small passerine bird with a highly restricted range, being found only on Raso islet in the Cape Verde Islands. This critically endangered member of the Alaudidae lives in highly arid terrain, and is considered one of the least known birds in the Western Palaearctic region, due to its remoteness and the lack of much ornithological study on the archipelago as a whole.

Description

The Raso lark is restricted to one small island in the Cape Verde group, although historically it is believed to have ranged over two other islands, Branco and São Vicente Island; all three of these islands were joined in the last Ice Age. Branco island itself has no permanent water and has never been inhabited by people, a fact that has probably saved the lark from extinction until now.

The Raso lark feeds by digging in the ground for insect larvae, and bulbs of the nutsedge. Both sexes also feed on grass seeds, and insects such as butterflies, moths and grasshoppers. It has an enlarged bill, but it is not used for foraging, but for dominance displays among males.[2] The female lays one to three eggs after the rains. Courtship behaviour is like that of the skylark. The incubation time is thought to be 15 days.

Identification

Raso larks (Alauda razae) are relatively small in size. The bird can range from sizes between 14–18 cm. The adult population is mostly made up of males who are larger than the females. The birds are thick-based. The body has black and brown short streaks and paler around the chest with and an erectile crest. The bill is thick and robust, longer for the males. The Raso lark is considered an adult when develop a reddish-brown town between the ears and tail, with a pattern on the crown and back. Since the Raso lark is confined to a small islands-Cape Verde Islands, during times of drought and little rain, they survive by gathering and eating subterranean bulbs of the nutsedges. The females, even though smaller in size still provide by gathering bulbs, while the males stay in the burrows and vigorously watch over, protecting from outside intruders. During times of little food, the females are the ones to die from starvation because they have to find bulbs for the male and incubate the eggs. While in flight they display a short tail with short broad wings. The tail and edge of the wings are white. Raso lark breeding starts after a rain shower. The male courts the female by signing softly, raising its chest, and hopping up and down while the wings are spread open. Once done mating, both the male and female collect dried grass in order to build a nest. The male defends the nesting site while the female finds a safe location to build a nest. The female can lay up to three eggs per clutch, usually a day apart. During incubation the female sits on the eggs for ten-minute intervals and then goes off to find food. The Cape Verde Giant Gecko eats most chicks and eggs before they are hatched and fledged.

Threats

The tiny population size, last thought to be ~150 birds, coupled with the highly skewed sex ratio (around two males to each female) make this species one of real concern. The reproductive success of the birds is very low, probably due to predation by the near-endemic Cape Verde giant gecko. The neglected common kestrel Falco tinnunculus neglectus is a predator of Razo larks. Although the island is currently free of mammalian predators such as rats or feral cats, and is a closed reserve, the likelihood of a single unauthorized visit causing massive damage remains high. It has been suggested that establishing a second population on the gecko-free island of Santa Luzia, which historically might also have had the lark, should be a conservation priority.

Geographic range

The Raso lark, geographically, is located on a small island off of the western tip of Africa, named the island of Raso; about the same latitude as Senegal. This island is part of the Cape Verde archipelago which consists of nine inhabited islands and five uninhabited islands. Currently the Raso lark inhabits only the island of Raso but evidence from subfossil bone deposits reveal that the Raso lark once existed on the islands of Santa Luzia, São Vicente, Santo Antão, and possibly Branco during the glacial times, also part of the Cape Verde archipelago, before human colonization in the 15th century. (IUCN 2012) Unfortunately the colonization was followed by a rapid extinction.

The island of Raso has remained uninhabited favouring preservation of the bird but suitable breeding grounds cover less than half of the island. (IUCN, 2012) The island mostly consists of rocky desert with sandy parts in the west. (Ratcliffe et al. 1999) The barren island suffers from drought often which create dry plains forcing the Raso larks to move amongst the island in search for food. Their most common areas of residence would be around dry river beds. But due to the vast scarcity, only patches of vegetation remain for Raso larks to reside in. Their nesting and feeding grounds consist of those same areas where vegetation is available. The Raso larks sensitivity to drought has kept it endangered for quite some time due to the dry lands of the Cape Verde islands.

Ecology

Most Raso larks are found on level plains with volcanic soil and are associated with small vegetated patches along dry stream beds in which it feeds and breeds (Ratcliffe et al. 1999). A number of desert-dwelling larks have evolved long bills, apparently to aid digging for food in a sandy environment. Flocks have also been observed feeding among rocks close to the sea, and the birds excavate holes in sand soil to extract the small bulbs of nets edges, which are perennial weeds in the sedge family and superficially resemble grasses.

Breeding is very unpredictable with the Raso lark, simply because it is governed by the slight and irregular rains. Their population changes rapidly in response to rain; a prerequisite for breeding, and has fallen to extremely low levels during droughts (Ratcliffe et al. 1999, Donald et al. 2003, Donald and Brooke 2006). However, the Raso lark isn't very fit. Fitness is defined not as strength, but as reproductive success, and in the case of the Raso lark, their reproductive success is very low. Most of their population are males. The adult sex ratio is a key parameter of the demography of animal populations and the Raso lark is very male aggressive, hence their future chances of reproductive success are also endangered.

Within their environment, the geckos weigh roughly 100 g and include birds in their diet. It also has other predators which make it very difficult for them to survive, such as: cats, dogs, rats, and other birds (Falco neglectus, Corvus ruficollis, and Tyto detorta). Many aspects of the species’ ecology and behaviour closely resemble that of the kylark, suggesting that the species’ current placing in the genus Alauda is correct.

Diet

Food and water for the Raso lark is scarce during the drought seasons. These necessities are provided by eating Cyperus bulbosus or Cyperus cadamosti which are bulbs of nutsedges. Males and females have different sized bills, which is why it was thought that males fed more on bulbs than females do. To get to a bulb the Raso lark must use its beak to dig burrows in the sandy soil. New studies are showing that males consume more bulbs than females not because of the difference in bill size. Males bills are 20% larger than females bills (Donald 2007). Males consume more bulbs because males control the territories (Donald 2007). Females can dig the burrows just as well as the males. Males dig 0.32 bulbs per minute and females dig 0.36 bulbs per minute (Donald 2007). Males dig for 61.8 seconds while females dig for 58.0 seconds (Donald 2007). Females are just as adequate at digging as males, but they only spend about 1/3 as much time digging as males do. Because of this, females spend significantly more time feeding from the surface (Donald 2007). The males protect these territories, which usually have more than one burrow that provides them with food, even from the females of their species (Donald 2007). Females are dying at a much faster rate because of this and because their only source of food without the bulbs are insects and grass seeds. Females spend a larger portion of their time foraging which means less time looking out for predators and they also have a higher food stress which both can lead to their deaths (Donald 2007).

Gallery

References

  1. ↑ BirdLife International (2013). "Alauda razae". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
  2. ↑ Kingdon, Jonathan (1989). Island Africa: The Evolution of Africa's Rare Plants and Animals. Princeton, New Jersey: Princeton University Press. p. 39. ISBN 0-691-08560-9.

External links

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