Guam crow
| Guam crow | ||||||||||||
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Drawing of a guam crow ( Corvus kubaryi ) |
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| Scientific name | ||||||||||||
| Corvus kubaryi | ||||||||||||
| Reichenow , 1885 |
The Mariana Crow ( Corvus kubaryi ) is a songbird art from the family of corvids (Corvidae). With a body length of 38 to 40 centimeters, it is one of the smaller species of ravens and crows ( Corvus ). Their distribution area is limited to the Mariana Islands of Guam and Rota . The habitat of the Guam Crow consists mainly of forests on limestone soils, but it also uses anthropogenic habitats. It feeds omnivor of arthropods , fruits, buds and small vertebrates . Guam crows move in family groups of three to five individuals, less often in small flocks. Their main breeding season coincides with the end of the dry season from August to February, but broods can occur throughout the year. The nest is built in the scrub or in the treetops; the clutch usually consists of two to three eggs, which are only incubated by the female.
The first valid description of the Guam crow by Anton Reichenow dates back to 1885. Its closest relative is the thick-billed crow ( C. macrorhynchos ). The population of the Guam crows has declined sharply since the second half of the 20th century. In addition to the introduction of the brown night tree snake ( Boiga irregularis ) and other neozoa in the Marianas, the cause is the disappearance of primary forest on the islands. A single-digit number of birds still lives on Guam, Rota is home to around 50 to 250 individuals. BirdLife International therefore classifies the species as critically endangered (“threatened with extinction”). Resettlements from Rota to Guam are discussed as protective measures for the Guam crows, protective devices against predators are being built and the restoration of primary forest is targeted.
features
Build and color
The Guam crow is a small representative of the ravens and crows ( Corvus ) with a body length of around 38–40 cm. It has relatively fine, loose plumage, which gives it a slightly messy appearance. This impression is reinforced in the moulting period from May to September, when feathers gradually fall out. The beak of the species is relatively long, straight and especially very narrow at the tip. The for corvids typical Nasalborsten are relatively short in the Mariana Crow, but cover the nostrils and the mouth basis. There is a slight dimorphism between the two sexes in terms of size, males are on average slightly larger than females. The weight of male guam crows is 231–270 g. Their wing length is 229–244 mm, the length of the tail is between 158 and 170 mm. The male beak is 51–57 mm long , the male barrel bone 49–52 mm long. The 205–260 g heavy females fall only slightly behind male Guam crows with 222–242 mm wing length and 143–166 mm tail length. The dimensions of the beak and running bones are usually only slightly below those of males at 47–53 mm and 46–54 mm, respectively.
The plumage of the Guam crow is coal black, with females being a little more brown than males. The feathers are grayish, in the neck area they become almost white, which makes the plumage appear a bit dirty. A faint, iridescent green shimmer can be seen on the head and back , whereas the tail has a bluish shimmer. The ventral side is almost dull black with an almost imperceptible green sheen. Legs and beak are colored black. The iris of the Guam crow is dark brown. Young birds differ only slightly from adults . Their plumage is more dull and their tail and wing feathers tend to be more brown.
Flight image and locomotion
In flight, the Guam crow is characterized by relatively short, wide wings and its slightly rounded tail. Sometimes it shows a flight pattern reminiscent of woodpeckers , in which phases of rapid wing flapping alternate with gliding flight with flared wings; the course of the flight becomes undulating. In the fast cross-country flight, Guam crows use the powerful and even wing beats typical of the Corvus genus .
Vocalizations
The vocalizations of the Guam crow are considered little explored. Adult birds communicate with a tall, croaking kah or hii . Both calls are often uttered in series of one to three syllables. Occasionally it pulls the Guam crow longer and intones it very nasally, a variant that is mainly used in communication between breeding partners. Possibly these monologues fulfill a similar function as the sub-song of other species of crows. A quick series of kah calls one after the other serves as an acoustic alarm. Individuals with whom there is a strong social bond often make complex series of calls at different volumes when they are in contact with one another. They can be extremely quiet but also very loud and consist of many different syllables. The calls of juveniles are similar to those of adult individuals, but are shorter. Her begging call is an aaa that rises in pitch and increases in intensity until fed.
distribution
Guam crows are resident birds and occur only on the Mariana Islands of Guam and Rota . It is possible that the range of the species once included the rest of the Mariana Islands or other parts of Micronesia . Genetic diversity was less in the Rota population than in Guam. This suggests that Rota was settled from the neighboring island to the south. Since Rota's vegetation and fauna suffered greatly from the fighting in the Pacific War , the low diversity may also be due to the fact that only a small remaining population survived the war. A high degree of correspondence in the DNA of the two island populations in a study from the 1990s suggests that Rota must have been colonized relatively recently (50–100 generations, as of 1999).
Rota is almost completely populated by the Guam crow. It is probably only missing there on the Taipingot peninsula in the southwest and the forest-free areas in the outer northwest, around the airport and Sinapalo as well as on the southern slope of Mount Manira . The range of the species on Guam shrank significantly in the 20th century. First it withdrew from the south of the island, then later also from its interior and in the early 1970s it was only to be found on the northern plateau of Guams. In the following decades this development continued, the species retreated further into the inaccessible, rugged north-west. The Guam crow can now only be found in the far north around Andersen Air Force Base . Since the purely male population is no longer able to reproduce there, the population on Guam has de facto died out.
habitat
The original habitat of the Guam crow is the primary forest vegetation of the Guams and Rotas limestone formations. These forests consist mainly of figs ( Ficus spp.) And ganiter trees ( Elaeocarpus spp.). For foraging, the animals also use the secondary limestone forests that have arisen since the first colonization of the Mariana Islands by the Chamorro (around 4000 BP ) and the island's parkland created by clearing. They can also be found in the coconut and screw tree plantations built in modern times . The birds prefer the primary forest, among the habitat types Guams and Rotas they probably only avoid the open savannah . While foraging for food, they mainly move in the crown and undergrowth area, less often they go down to the ground. For breeding, the species is apparently tied to the declining primary forest. The breeding habitat is usually at some distance from roads and similar forest cuttings and contains the native tree species Eugenia reinwardtiana , Neisosperma oppositifolia , Intsia bijuga or Guettarda speciosa .
Way of life
nutrition
The food spectrum of the Guam Crow is varied. It eats both animal food and plant material. The former consists of arthropods such as jumpers , crabs , caterpillars or wood-dwelling insects, as well as small vertebrates, especially lizards, but also rats, snakes and bird eggs. The plant-based diet of the species consists of the fruits of Cestrum diurnum and Premna obtusifolia , figs , the capsule fruits of the lime-leaved marshmallow ( Talipariti tiliaceum ) and the stone fruits of screw trees ( Pandanus spp.). In addition, the Guam crow also eats seeds and buds of various types of plants, unlike previously assumed, but apparently no leaves or bark.
The behavior of guam crows, working on loose bark with beak and feet, is due to the fact that they - like woodpeckers - try to pull larvae and other wood dwellers out from under the bark. The same applies to eggs and larvae of insects that develop in leaves. Vertebrates are usually brought out of crevices and tree holes. The scarce water on the karst soils of Guam and Rota is what the birds obtain from screw tree leaves, in which the rain collects. Guam crows eat most of their animal feed in the low canopy area (around 5 m high), where they pick up invertebrates from the leaves. The animals avoid the ground in open areas, but in the forest they are extremely active there and energetically stir up leaves to look for food underneath. Most of the studies and observations on the Guam crow's diet took place during the breeding season; the food spectrum and diet during the rest of the year has not been adequately researched.
Social and territorial behavior
Guam crows are considered to be moderately sociable birds. Unlike some other species of crows, they do not breed collectively, but defend the territory around the nest all year round. On Rota, these territories have a size of 12–37 ha. Breeding partners look after each other's plumage, and plumage care between parents and nestlings has also been observed. Usually the animals move in small family groups of three to five individuals. Individual individuals or flocks of up to 15 Guam crows have been observed less frequently. In earlier decades there were reports of roosts in Guam where 25–66 animals slept together. Guam crows have been observed chasing Mariana fruit pigeons ( Ptilinopus roseicapilla ); on the other hand, they are often hated by Carolinian starlings ( Aplonis opaca ), cinnamon-head pebbles ( Todiramphus cinnamominus ) and particularly aggressively by King Drongos ( Dicrurus macrocerus ) .
Reproduction and breeding
The breeding season of the Guam crow usually falls in the dry season between August and February, but broods can also occur outside of this. Nest-building on Rota begins in July at the earliest, the last young usually fly out there in May. The breeding season on Guam was recently shortened (October to mid-April), possibly as a reaction to brown night tree snakes ( Boiga irregularis ) that ate the clutches there. The nest is built by males and females together and is usually placed 4–10 m high in the undergrowth or in the treetops. On Guam, the nests were mostly on the outside of the crowns, while on Rota they are increasingly being built in the crowns of smaller, dense trees. These differences probably result from different nest security requirements on the two islands. While the main danger for clutches on Guam comes from the brown night tree snake, the Guam crow on the neighboring island possibly evades the pressure of competition from King Drongos. Nest building takes about seven days. First, a platform made of banago branches ( Jasminum marianum ) is erected, which is then draped with an intermediate layer of fig boxes, roots and leaf fibers. Then the inside of the nest is lined with fine plant fibers, which on Guam mostly come from the Indian whip leaf ( Flagellaria gigantea ). The finished nest is misshapen and has a diameter of 24–53 cm. Its walls are around 15 cm thick.
A Guam crow clutch usually consists of 1–4, on average 2.3 eggs, which are mainly hatched by the female. They are light blue, mottled brown and about 30–35 × 20–25 mm in size. It takes 21–23 days for the offspring to hatch. The young are fed by both parents before they fly out after 36–39 days. Even after that, they remain dependent on their parents for a relatively long time and are fed by them for a further 99-537 days. Only after the offspring have gained independence do the parents begin to breed again. The breeding success of the species is apparently low: Most nests contain only one chick and only around 50% of all nestlings fledge. If a clutch fails, Guam crows immediately start a replacement brood; some breeding pairs bring it up to seven breeding attempts per year. This behavior and the relatively short breeding season is probably due to the fact that the Marianas are regularly devastated by typhoons , which can sometimes defoliate entire islands.
Predators, diseases and other causes of mortality
As one of the largest birds of the Mariana Islands, the Guam crow probably had no predators before the human colonization of the Mariana Islands. It was not until the Chamorro that the Asian house rat ( Rattus tanezumi ) and the Pacific monitor ( Varanus indicus ) introduced animal species that could endanger the crows and their nests. Domestic cats ( Felis silvestris ) followed in the course of the European colonization of the Marianas . In the 1960s, another predator appeared on Guam with the brown night tree snake, which mainly eats nestlings and eggs and also reaches nests that are inaccessible to other species. Deaths from pesticides and kills, as they happened up until the 1980s, are no longer known from recent times. Illnesses are also not considered a major cause of mortality among Guam crows. From time to time, however, typhoons lead to increased deaths. The animals are hosts of the mallophage species Myrsidea bakeri and Rallicola insulana , whose genus otherwise only infects Rallen (Rallidae spp.). There is little knowledge of the lifespan of Guam crows. In a 2010 study, the chance of survival in the first year of life was around 40%; in the early 1990s it was around 70%. Adult birds had survival rates of 82%, a decrease of four percentage points from 1990 .
Systematics and history of development
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| External classification of the guam crow according to Jønsson et al. 2012. The species is part of the thick-billed crow complex ( Corvus macrorhynchos s. L. ) And probably had a direct ancestor in Southeast Asia . |
The Mariana Crow was in 1858 for the first time scientifically as "Corvus solitary" described . Since Heinrich von Kittlitz , the author of this first description, did not provide an external description, it is not valid. "Corvus solitarius" is not just a noun nudum , as early as 1858 the name was no longer available due to Paul von Württemberg's invalid description of the palm crow ( Corvus palmarum ) from 1852. A valid description is therefore Anton Reichenow's oral report on the Guam Crow, which he put on record in 1885 at a meeting of the General German Ornithological Society . His holotype came from Johann Stanislaus Kubary , who claimed to have shot him on the Palau Islands and honored the Reichenow with the species name Corvus kubaryi . In fact, the bird probably came from nearby Guam.
The relationships of the species within their genus have long been unclear. As the only representative of the ravens and crows ( Corvus ) in Micronesia , the Guam crows separate several hundred kilometers from their closest conspecifics. Molecular genetic studies of the genus Corvus classify it as the sister species of the big-billed crow ( C. macrorhynchos ) to the exclusion of Corvus (macrorhynchos) philippinus . The latter forms the sister clade of the guam and thick-billed crows. According to the molecular clock , both emerged in the late Pliocene (about 2.5 mya ago ) from a common ancestor who colonized the Marianas from Southeast Asia. There is not much genetic variation between the populations on Guam and Rota. Investigations of the respective mtDNA - haplotypes found significant differences between the deposits, which does not provide a sufficient basis for taxonomic distinction only two nucleotides of the sequenced sections.
Inventory and status
| year | Guam | rota |
|---|---|---|
| 1981 | 357 | - |
| 1982 | - | 1,318 |
| 1985 | 100 | - |
| 1988 | - | 600-1,000 |
| 1991 | 41 | 2.132 |
| 1992 | 57 | 447-931 |
| 1993 | 51 | 800 |
| 1994 | 40-50 | 336-454 |
| 1995 | 26th | 592 |
| 1998 | - | 234 |
| 1999 | 7th | - |
| 2001 | 16 | - |
| 2003 | 0 | - |
| 2006 | 10 | - |
| 2007 | - | 150 |
| 2008 | 2 | 75-374 |
| 2009 | 4th | - |
| 2011 | 1 | - |
| 2012 | 0? | - |
| 2017 | - | 46–54 pairs (178 ind.) |
The population of the Guam crow is declining on both Rota and Guam. While the species was still considered an agricultural nuisance on Guam in the 1950s, its population declined rapidly after the arrival of the brown night snake. In 1981 the local population was around 350 birds, by 1999 it had fallen to seven. In response to this, some adult animals were relocated from Rota to Guam in 2001, but this did not prevent the species from becoming extinct there in 2003. In 2006 another ten Guam crows were relocated from Rota. Of these, a maximum of two males are still alive. The last sighting was in 2011. The species may have already become extinct on Guam. On Rota, populations hold up better in the absence of night tree snakes, but they are also falling. In 1982 the population there was estimated at around 1,300 individuals, in 2008 it was probably 75–374. A few birds are kept in captivity. Since 1996 the total has probably decreased by 80%. In addition to the brown night snake, the reasons for the population decline are the nesting site competition by the King Drongo, the progressive decline in the primary forest on Guam and Rota and the destruction of nests on Rota by the Pacific monitor. Especially on Rota, however, there is no clear picture of the causes, the connections are considered to have been insufficiently researched.
After the Hawaiian crow ( Corvus hawaiiensis ), the Guam crow is one of the most endangered corvidae species. BirdLife International classifies the population as critically endangered (threatened with extinction) and assumes that the species will have disappeared from the wild in 75 years at the latest as development progresses. The cost of a successful reintroduction of the species on Guam and long-term measures against the population decline are over 600 million US dollars estimated. Concrete measures include research into the causes of the population decline, securing nesting trees against brown snakes, their long-term eradication and the restoration of breeding habitats. Other predators and competitors that have been introduced, such as drongos , cats, monitor lizards and rats, should also be contained or eliminated. Another pillar is to educate the population of the Mariana Islands. As things stand at present, the necessary duration of a successful protection program is estimated at 50 years.
swell
literature
- Rollin H. Baker: The Avifauna of Micronesia, Its Origin, Evolution, and Distribution. In: University of Kansas Publications, Museum of Natural History 3, pp. 1–359.
- MA Carriker: On a Collection of Mallophaga from Guam, Marianas Islands. In: Proceedings of the United States National Museum 100 (3254), 1949. pp. 1-24. ( Full text ; PDF; 6.8 MB)
- W. Donald Duckworth, Steven R. Beissinger, Scott R. Derrickson, Thomas H. Fritts, Susan M. Haig, Frances C. James, John M. Marzluff , Bruce A. Rideout: The Scientific Bases for Preservation of the Mariana Crow. National Academy Press, Washington, DC 1997. ISBN 0-309-05581-4 .
- Derek Goodwin : Crows of the World. 2nd Edition. The British Museum (Natural History) , London 1986, ISBN 0-565-00979-6 .
- James C. Ha, Alyssa Butler, Renee Robinette Ha: Reduction of First-year Survival Threatens the Viability of the Mariana Crow Corvus kubaryi Population on Rota, CNMI. In: Bird Conservation International 20, 2012. doi : 10.1017 / S0959270909990281 , pp. 335-342.
- Renee Robinette Ha, John M. Morton, James C. Ha, Lainie Berry, Sheldon Plentovich: Nest Site Selection and Consequences for Reproductive Success of the Endangered Mariana Crow (Corvus kubaryi). In: The Wilson Journal of Ornithology 123 (2), 2011. doi : 10.1676 / 10-027.1 , pp. 236-242.
- J. Mark Jenkins: The Native Forest Birds of Guam. In: Ornithological Monographs 31, 1987. ( Full text ; PDF; 3.8 MB)
- Knud A Jønsson, Pierre-Henri Fabre, Martin Irestedt: Brains, Tools, Innovation and Biogeography in Crows and Ravens. In: BMC Evolutionary Biology 12 (72), 2012. doi : 10.1186 / 1471-2148-12-72 .
- Steve Madge , John Marzluff: Family Corvidae (Crows and Allies). In: Joseph del Hoyo, Andrew Elliot, David Christie (Eds.): Handbook of the Birds of the World. Volume 14: Bush-shrikes To Old World Sparrows. Lynx Edicions, Barcelona 2009, ISBN 978-84-96553-50-7 , pp. 494-640.
- Paul Matschie : Report on the January meeting. In: Journal für Ornithologie 33 (169), 1885. pp. 108-110. ( Full text )
- Gary A. Michael: Notes on the Breeding Biology and Ecology of the Mariana or Guam Crow Corvus kubaryi. In: Avicultural Magazine 93, 1987. pp. 73-82.
- CL Tarr, RC Fleischer: Population boundaries and genetic diversity in the endangered Mariana crow (Corvus kubaryi). In: Molecular ecology 8, 1999. pp. 941-949.
- Diana F. Tomback: Observations on the Behavior and Ecology of the Mariana Crow. In: The Condor 88, 1986. pp. 398-401.
- Andria Kroner, Renee Robinette Ha: An update of the breeding population status of the critically endangered Mariana Crow Corvus kubaryi on Rota, Northern Mariana Islands 2013–2014. In: Bird Conservation International 27, 2017. doi : 10.1017 / S0959270917000053 .
Web links
- Alan Saunders, Tina de Cruz, John Morton, Gordon Rodda, Rick Camp, Jaan Lepson, G. Wiles: Mariana Crow Corvus kubaryi. BirdLife International, www.birdlife.org, 2012.
Individual evidence
- ↑ a b Matschie 1885, p. 110.
- ↑ Goodwin 1986, p. 108.
- ↑ a b Baker 1951, p. 245.
- ↑ Madge & Burn 1994, p. 141.
- ↑ Tomback 1986, p. 399.
- ↑ Tomback 1986, p. 400.
- ↑ Duckworth et al. 1997, p. 5.
- ↑ a b Tarr & Fleischer 1999, p. 946.
- ↑ Plentovich et al. 2005, p. 214.
- ↑ Duckworth et al. 1997, p. 34.
- ↑ a b c d e f Saunders et al. 2012. Retrieved August 18, 2012.
- ↑ a b c Michael 1987, p. 75.
- ↑ Ha et al. 2011, p. 240.
- ↑ Jenkins 1983, pp. 26-32.
- ↑ Jenkins 1983, p. 32.
- ↑ Duckworth et al. 1997, p. 20.
- ↑ a b Jenkins 1983, p. 26.
- ↑ Madge & Marzluff 2009, p. 620.
- ↑ Madge & Marzluff 2009, pp. 620–621.
- ↑ Michael 1987, p. 80.
- ↑ a b c d Madge & Marzluff 2009, p. 621.
- ↑ Duckworth et al. 1997, pp. 27-29.
- ↑ Carriker 1949, pp. 3-24.
- ↑ Ha et al. 2010, p. 339.
- ↑ a b Jønsson et al. 2012, pp. 21-22.
- ↑ a b Baker 1951, p. 245.
- ↑ Baker 1951, p. 247.
- ↑ Duckworth et al. 1997, p. 7.
- ↑ Kroner & Ha 2017.
