Letter-winged kite

Letter-winged kite
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Aves
Order:Accipitriformes
Family:Accipitridae
Genus:Elanus
Species: E. scriptus
Binomial name
Elanus scriptus
Gould, 1842
A letter-winged kite in flight. The underwing markings are visible.

The letter-winged kite (Elanus scriptus) is a small, rare and irruptive raptor that is found only in Australia. Measuring around 35 cm (14 in) in length with a wingspan of 84–100 cm (33–39 in), the adult letter-winged kite has predominantly pale grey and white plumage and prominent black rings around its red eyes. It gains its name from the highly distinctive black underwing pattern of a shallow ‘M’ or 'W' shape, seen when in flight. This distinguishes it from the otherwise similar black-shouldered kite.

The species begins breeding in response to rodent outbreaks, with pairs nesting in loose colonies of up to 50 birds each. Three or four eggs are laid and incubated for around thirty days, though the eggs may be abandoned if the food source disappears. Roosting during the day in well-foliaged trees and hunting at night, it is the world's only fully nocturnal Accipitriformes or Falconiformes raptor. Similarly to all the elanid kites, it is a specialist predator of rodents, which it hunts by hovering in mid-air above grasslands and fields. It is rated as near threatened on the International Union for Conservation of Nature (IUCN)'s Red List of Endangered species.

Taxonomy

The letter-winged kite was described by ornithologist John Gould in 1842 under its current binomial name Elanus scriptus.[2] The specific epithet is from the Latin word scriptum meaning "written" or "marked".[3] Charles Sturt wrote of seeing them on his travels in his 1849 book Narrative of an expedition into Central Australia.[4] The letter-winged kite is monotypic; no subspecies are recognized,[5] nor is there any recorded geographic variation.[6]

"Letter-winged kite" has been designated the official name by the International Ornithologists' Union (IOC).[5] In Central Australia, southwest of Alice Springs, the Pitjantjatjara term for the letter-winged kite is nyanyitjira.[7] It has been incorrectly called white-breasted sparrowhawk.[8]

Molecular evidence shows that the letter-winged kite and its relatives belong to a subfamily Elaninae that is an early offshoot within the raptor family Accipitridae.[9][10] There is some evidence they are more divergent from other raptors and better placed in their own family.[11][12]

Description

The adult letter-winged kite is around 35 cm (14 in) in length, with a wingspan of between 84 and 100 cm (33 and 39 in). The female is slightly heavier, weighing on average around 310 g (11 oz) compared to the male's average weight of 260 g (9.2 oz). The sexes have similar plumage. The adult male letter-winged kite has pale grey upperparts, wings and nape with a white head and white underparts. It has large deep red eyes, which are surrounded by a black eye patch. Its bill is black, with a dark grey-brown cere. Its wings are marked with a black shoulder patch above and a striking black line underneath running from the primary coverts to the body,[8] which resembles a letter 'M' or 'W' when flying.[13] The central retrices of the tail are pale grey, while the rest of the tail feathers are white. The legs and feet are fleshy pinkish-white or white.[8] The feet have three toes facing forwards and one toe facing backwards.[14] The female is similar but can be distinguished by a greyer crown, [13] and grey plumage is slightly darker all over.[15] Moulting has been recorded from all months except May and August, and is probably related to breeding..[6]

The juvenile has a white face, lower forehead, chin and throat, with a brownish orange band across the forehead, neck and breast. It as a similar dark eye patch to the adult. The hindneck is grey-brown, and upperpart feathers are grey-brown with orange tips. The rump and central tail retrices are pale grey tipped with orange. The eyes themselves are dark brown. The bill is black with a brownish-grey cere.[8] Birds in juvenile plumage are able to breed within their first year of age.[15]

The letter-winged kite soars with v-shaped up-curved wings, the primaries slightly spread and the tail fanned, giving it a square appearance. when flying actively, it beats its wings more slowly and deeply than the black-shouldered kite. The wing beats are interspersed with long glides on angled wings. It can also hover motionless facing into the wind and flapping its wings.[8] The 'M' or 'W' on the underside of its wing, and lack of black wing tips help distinguish it from the black-shouldered kite. Finally, the latter species is diurnal, not nocturnal.[13] At night, it could be mistaken for the eastern barn owl (Tyto javanica) or eastern grass owl (T. longimembris), but these species have large heads, longer, trailing legs, blunted wings and stockier bodies. The grey falcon (Falco hypoleucos) has somewhat similar coloration to the letter-winged kite but is bulkier and heavier overall and lacks the black markings.[16]

Vocalisations

The letter-winged kite is generally silent when alone, but often noisy when roosting communally at night or breeding, often beginning at the rising of the moon.[17] Its calls have been likened to chicken-like chirping or a repeated loud kacking,[14] and at times resemble those of the barn owl or balck-shouldered kite. A rasping call, or scrape, composed of six or seven half-second long notes is the main contact call between a pair, often used by the female in answer to a whistle by her mate, when a bird alights at the nest, or—loudly—in response to an intruder. The male can utter a loud whistle in flight, which can serve as an alarm call. Mated pairs chatter to one another at night in the colony.[17]

Distribution and habitat

The usual habitat of the letter-winged kite is arid and semi-arid open, shrubby or grassy country, across the arid interior of the continent, particularly western Queensland, the southern Northern Territory, particularly the Barkly Tableland, and northeastern South Australia. Its abundance or even presence in any given area is heavily dependent on availability of food; spells of significant rainfall inland lead to surges in rodent numbers, which in turn leads to irruptions of letter-winged kites.[16] Nesting and raising multiple broods in succession, the kite population may increase ten-fold.[18] Major irruptions have taken place in 1951–53, 1969–70, and 1976–77.[16] Eventually dry conditions lead to a fall in rodent numbers and dispersal of birds, which often starve.[16]

Its range may spread south with rodent plagues.[19] The species is generally rare in New South Wales,[16] and has been recorded in the vicinity of Broken Hill in far western New South Wales,[20] and a dead bird recorded in a street in Inverell in the north of the state in 1965 and another spotted there a year later.[21] In South Australia it may reach the Eyre Peninsula and southeastern corner on occasion. In Queensland it is relatively common in western areas south of 20° south, and has been recorded as far afield as Townsville and Stradbroke Island. It is rare in Western Australia.[16]

Behaviour

The letter-winged kite is the only fully nocturnal member of its family,[13] typically hunting at night, with daytime foraging taking place in areas of superabundant or scarce prey.[16] By day, birds roost in leafy trees with plenty of cover, in colonies of up to 400 individuals, and becoming active at dusk.[16] Their social behaviour is poorly known on account of their nocturnal habits and shy nature, being difficult to approach when roosting.[17]

Breeding

On a nest in foliage

Within its range, the letter-winged kite mostly breeds an area covering the Diamantina and Lake Eyre drainage basins, Sturt Stony Desert, eastern Simpson Desert and Barkly Tableland, to Richmond, Queensland and Banka Banka Station in the north and Boolkarie Creek, South Australia in the south. Nesting has also been recorded in Exmouth Gulf and southwest Western Australia, the southwest of the Northern Territory, and the Clarence River district and northwest of New South Wales.[16]

It is not known if breeding pairs remain bonded between breeding seasons.[22] Aerial courtship displays involve mutual flight high above the nest, with the male flying much higher than the female and holding its wings high with rapidly fluttering wingtips. This individual then drops near its mate, who responds by holding her wings in a similar manner. The two then chatter while circling each other. Copulation often follows.[22]

The letter-winged kite nests in colonies of up to 50 pairs, and have more than one nest and brood at once. At times their nests are near to those of spotted harriers, black kites, whistling kites, brown falcons and black falcons. [17]

There does not appear to be a set breeding season, instead the species forms nesting colonies in response to rodent plagues. Birds produce broods for as long as the rodents are abundant, and stop when their food source declines.[17] Often smaller trees as nesting sites are chosen over larger ones, with some preference given to the beefwood (Grevillea striata). Other species used include waddy (Acacia peuce), coolibah (Eucalyptus microtheca) and sheoaks (Casuarina spp.). Generally there is one nest per tree,[23] though there may be multiple nests in single trees in cases of rodent plagues and hence abundance of food. The nest is a large untidy shallow cup of sticks usually in the foliage near the top of trees, some five metres (15 ft) or higher off the ground.[19] On average it is about 50 cm (20 in) wide and 34 cm (13 in) high, with a 20 cm (7.9 in) diameter cup-shaped depression within.[23] It is lined with green leaves and other material such as regurgitated pellets. The clutch consists of three to four, or rarely five or even six, dull white eggs measuring 44 x 32 mm with red-brown blotches and tapered oval in shape. The markings are often heavier around the larger end of the egg.[19] The female incubates the eggs for 30 days,[13] though this has been difficult to confirm due to unpredictable breeding. The young are born semi-altricial, covered in white down with black beaks and feet and dark brown eyes. By a week old, they have pale tan down on their back and brown eyes. They are fully feathered by 3–4 weeks of age and can fly at 7 weeks. During this time they are brooded by the female, while the male brings food at night.[15] He calls as he approaches, at which the female flies out to meet him and receive the food which she then conveys to the young. At times the male brings food to the female on the nest but has not been known to feed the young himself. As the brood grows, the female joins the male in catching food, and may even begin a second brood and leave the male to feed the older brood.[17]

Nestlings fledge at around 32 days,[13] although have been known to be abandoned if the food supply suddenly disappears.[15]

Food and hunting

The letter-winged kite hunts mainly in the first two hours after sunset. It flies at a height of 10 to 20 m (33 to 66 ft), moving in wide circles. It hovers at a height of up to 30 m (100 ft). When a mouse or other prey is spotted, the kite drops silently onto it, feet-first with wings raised high.[22]

The letter-winged kite's principal prey is the long-haired rat (Rattus villosissimus). When population numbers of this rodent build up, following good rainfall, the kites are able to breed continuously and colonially so that their numbers increase in parallel. When the rodent populations decline, the now superabundant kites may disperse and appear in coastal areas far from their normal range in which, though they may occasionally breed, they do not persist and eventually disappear.[13] One central Australian study over two and a half years found the kites had relocated to an area within six months of a rodent outbreak starting.[24] Across Central Australia, it shares its habitat with another nocturnal rodent hunter, the eastern barn owl, with the latter species preferring larger rodents such as the plains rat (Pseudomys australis) whereas the kite took all species, including the sandy inland mouse (Pseudomys hermannsburgensis) and spinifex hopping mouse (Notomys alexis), on availability.[25] Overall, letter-winged kites average one rodent consumed a day. Other predators sharing its habitat and prey include the dingo, feral cat and fox.[24] Letter-winged kites have also been recorded hunting the introduced house mouse (Mus musculus) in north-eastern South Australia.[26]

Other animals recorded as prey include rabbit, fat-tailed dunnart (Sminthopsis crassicaudata), stripe-faced dunnart (Sminthopsis macroura), Forrest's mouse (Leggadina forresti), beetles and spur‐throated locust (Nomadacris guttulosa).[22]

Predation

Black falcons have been reported hunting adult letter-winged kites, while black kites have taken nestlings..[15]

Conservation status

The letter-winged kite's fluctuations in abundance make its conservation status difficult to assess.[1] It also rarely comes into contact with people in most parts of its range.[16] It is rated as near threatened on the International Union for Conservation of Nature (IUCN)'s Red List of Endangered species,[1] as its population may number as low as 1000 individuals between irruptions.[18] It is unknown to what extent competition for food with the red fox or feral cat might impact on the letter-winged kite's breeding success,[24] or if feral cats prey on nestlings.

References

  1. 1 2 3 BirdLife International (2016). "Elanus scriptus". IUCN Red List of Threatened Species. International Union for Conservation of Nature. Retrieved 15 January 2018.
  2. ↑ Gould, John (1842). "Elanus scriptus". Proceedings of the Zoological Society of London. Part 10: 80.
  3. ↑ Simpson, D.P. (1979). Cassell's Latin Dictionary (5th ed.). London: Cassell Ltd. p. 539. ISBN 0-304-52257-0.
  4. ↑ Sturt, Charles (1849). Narrative of an Expedition Into Central Australia, Performed Under the Authority of Her Majesty's Government, During the Years 1844, 5, and 6: Together with a Notice of the Province of South Australia, in 1847, Volume 2. London: T. and W. Boone. p. 16.
  5. 1 2 Gill, Frank; Donsker, David, eds. (2017). "New World vultures, Secretarybird, kites, hawks & eagles". World Bird List Version 7.1. International Ornithologists' Union. Retrieved 9 December 2017.
  6. 1 2 Marchant & Higgins 1993, p. 52.
  7. ↑ Goddard, Cliff (1992). Pitjantjatjara/Yankunytjatjara To English Dictionary (2nd ed.). Alice Springs: Institute for Aboriginal Development. p. 95. ISBN 0-949659-64-9.
  8. 1 2 3 4 5 Marchant & Higgins 1993, p. 45.
  9. ↑ Lerner, Heather R. L.; Mindell, David P. (November 2005). "Phylogeny of eagles, Old World vultures, and other Accipitridae based on nuclear and mitochondrial DNA" (PDF). Molecular Phylogenetics and Evolution. 37 (2): 327–46. doi:10.1016/j.ympev.2005.04.010. PMID 15925523.
  10. ↑ Griffiths, Carole S.; Barrowclough, George F.; Groth, Jeff G.; Mertz, Lisa A. (2007). "Phylogeny, diversity, and classification of the Accipitridae based on DNA sequences of the RAG-1 exon". Journal of Avian Biology. 38 (5): 587–602. doi:10.1111/j.2007.0908-8857.03971.x.
  11. ↑ Wink, M.; Sauer-Gürth, H. (2004). "Phylogenetic relationships in diurnal raptors based on nucleotide sequences of mitochondrial and nuclear marker genes". In Chancelor, R. D.; Meyburg, B.-U. Raptors Worldwide (PDF). Berlin: WWGBP. pp. 483–98.
  12. ↑ Debus, Stephen (2004). "Australian raptors: the big picture" (PDF). Boobook. The Australian Raptor Association. 22 (1): 4–5.
  13. 1 2 3 4 5 6 7 "Letter-winged Kite". Birds in Backyards. Birds Australia, Australian Museum. 11 November 2009. Retrieved 10 May 2010.
  14. 1 2 Slater, Peter (1970). A Field Guide to Australian Birds:Non-passerines. Adelaide: Rigby. p. 239. ISBN 0-85179-102-6.
  15. 1 2 3 4 5 Marchant & Higgins 1993, p. 51.
  16. 1 2 3 4 5 6 7 8 9 10 Marchant & Higgins 1993, p. 46.
  17. 1 2 3 4 5 6 Marchant & Higgins 1993, p. 49.
  18. 1 2 Garnett, Stephen; Szabo, Judit; Dutson, Guy (2011). The Action Plan for Australian Birds 2010. Collingwood, Vic: CSIRO. pp. 141–42. ISBN 978-0-643-10368-9.
  19. 1 2 3 Beruldsen, Gordon (2003). Australian Birds: Their Nests and Eggs. Kenmore Hills, Queensland: self. p. 199. ISBN 0-646-42798-9.
  20. ↑ "Broken Hill Complex – biodiversity". Office of Environment and Heritage. State of New South Wales and Office of Environment and Heritage. 26 April 2016. Retrieved 8 October 2018.
  21. ↑ Baldwin, Merle (1975). "Birds of Inverell District". Emu. 75 (2): 113–20. doi:10.1071/MU9750113.
  22. 1 2 3 4 Marchant & Higgins 1993, p. 48.
  23. 1 2 Marchant & Higgins 1993, p. 50.
  24. 1 2 3 Pavey, Chris R.; Eldridge, Stephen R.; Heywood, Mike (2008). "Population dynamics and prey selection of native and introduced predators during a rodent outbreak in arid Australia". Journal of Mammalogy. 89 (3): 674–83. doi:10.1644/07-MAMM-A-168R.1.
  25. ↑ Pavey, Chris R.; Gorman, J.; Heywood, Mike (2008). "Dietary overlap between the nocturnal letter-winged kite Elanus scriptus and barn owl Tyto alba during a rodent outbreak in arid Australia". Journal of Arid Environments. 72 (12): 2282–86. doi:10.1016/j.jaridenv.2008.07.013.
  26. ↑ Baker-Gabb, DJ; Pettigrew, J.D. (1982). "Non-breeding diet of the Letter-winged Kite in north-eastern South Australia". Corella. 6: 19–20.

Cited text

  • Marchant, Stephen; Higgins, Peter Jeffrey, eds. (1993). Handbook of Australian, New Zealand and Antarctic Birds. Vol. 2: Raptors to Lapwings. Melbourne, Victoria: Oxford University Press. ISBN 0-19-553069-1.
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