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DUCULA (GENUS)

Ducula is a major genus of the pigeon family Columbidae, collectively known as Imperial pigeons. They are large to very large pigeons with medium to long tails. They are arboreal, feed mainly on fruit and are related to the other genus of fruit doves, Ptilinopus. Both genera have distinctive brightly coloured markings, variously irridescent green, purple, orange and reds, in contrast to the dull browns and greys of the seed eating genera of Columbidae. Some Ducula have prominently swollen ceres. They have large gapes and swallow seeds whole, playing an important role in seed dispersal. Most species have little sexual dimorphism.

Imperial pigeons are found in forests of Asia, PNG, northern Australia and the Pacific islands. Many species are nomadic, travelling long distances to exploit seasonal fruit sources. Some undertake migrations and all are strong fliers. Because of habitat loss and predation, some species of Ducula are amongst of the most threatened avian species globally.

ETYMOLOGY

Generally there is no formal distinction between pigeons and doves, with pigeon usually describing the larger and dove the smaller species of Columba (Gibbs and david).

TAXONOMY AND SYSTEMATICS

The taxonomy of the Imperial Pigeons is confused, with number of species reported variably as 36 (Gibbs and David) 39 (web of life) and 44 (wikipedia). Some species are highly differentiated, especially those with widespread habitats, and some races are as distinct as other species. The treatment of D.bicolor and it’s close taxa is largely responsible for the variation in the number of species within Ducula. I will follow Gibbs classification here.

In 1893, Salvadori divided the genus into four distinct groups of species, a differentiation which is convenient to follow here (ref).

The first group is typified by D. bicolor which describes a large, white pigeon with black wingtips, found in PNG and northern Australia. D. bicolor and its variants are included as a single species with distinct races (Johnstone 1981), Sibley and Monroe in 1990 changed earlier classification and grouped the three as subspecies of D. bicolor. D. luctuosa was recognised as a separate species in ? (ref), so D. spilorrhoa and D. bicolor were also elevated to species or as five different species (Sibley and Monroe 1990).. Gibbs and David separate D. bicolor, D luctuosa and D. spilorrhoa into three separate species and this is the classification I shall follow here. A fourth species within this clade, D. subflavescens is included by Frith as a subspecies of D. spilorrhoa. see page 1606 Gibbs and david for more detail if needed. Also Christidis and Boles, page 74!

The difficulty in assigning or removing species rank is compounded by the lack of data on behaviour and breeding patterns of the taxa. Little is known about a large percentage of Ducula (Higgins, Goodwin), Similar species are allopatric, and there is no data on whether the species freely interbreed or not.

The confusion is not helped by English naming. D. bicolor is variously described as the nutmeg pigeon, the Torres Strait pigeon, and the pied imperial pigeon. D. spilorrhoa is also described as the nutmeg pigeon, the white nutmeg pigeon, the Australian pied pigeon, the Australian pied imperial pigeon, the Torres Strait pigeon or the Torresian imperial pigeon (Frith). Collectively, the four taxa are referred to as pied imperial pigeons.

A second grouping of imperial pigeons comprises four species found in Philippines and Sulawesi. They all have a distinct pale bank across the centre of the tail. D poliocephala, forsteni are allopatric and phenotypically similar species, and mindorensis and radiata – differ in size but more phenotypically similar. D. carola is sympatric to Phillipines birds, and though atypical as small and short tailed, it is grouped with theses species because of its resemblance to D. poliocephala and has a similar distribution (Gibbs 2010, Goodwin 1969)

Another group is found in the Solomons, Vanuatu and Fiji. Four species are more closely related – D. latrans, D. brencleyi, D. bakeri and D. goliath A further three species, D. melanochroa, D. pinon and mulleri are more distantly related, though the latter are sympatric (Gibbs and David). D. zoeae is distinct from but probably derived from the same stock as mullerii, so is included in this grouping.

The remaining species are more removed from the previous groups, and more closely related to each other. (p1443, gibbs and david). Six species have enlarged ceres, and live ???

Four Papua New Guinea species – D. rufigaster, D. basilica, D. finschii and D. chalconota are closely related to D. aenea are given species status by Gibbs and Goodwin. The four are grouped together within D. aenea by the Web of Life.

D. aenea, the Green Imperial Pigeon has a widespread distribution and geographically with marked differentiation within the species. Gibbs includes ?? races within the species D. aenea, but

Because of the widespread and isolated island distribution of Ducula, there morphology is diverse, with variation amongst allopatric populations causing racial distinctions sufficient to be allocated to a species in other genera. Clade …

EVOLUTION

Australia has a poor representation of Ducula amongst its fauna, with only two species breeding on the mainland - the Torres strait pigeon (D. bicolor) and ?? Imperial pigeons are strong fliers, and are capable of flight over oceans, and have been successful in colonising islands and archipelagos. It would have been logical that Ducula had crossed the Torres Strait islands to reach mainland Australia. The genus Lopholaimus, comprising only one species may have been well represented on the Australian mainland before European settlement, thus excluding Ducula (Goodwin 1983).

DISTRIBUTION AND HABITAT

Imperial pigeons are only found in Asia Australia, PNG and the Pacific, covering a distribution from the Himalayas (Mountain Imperial Pigeon, D. badia) to Tahiti in the South Pacific (Polynesian Imperial Pigeon, D. aurorare). The greatest diversity of birds is in PNG. Only two species breed in mainland Asia, and only one (D. spilorrhoa) in Australia (Frith)

Imperial pigeons are arboreal, and found almost exclusively in forest and woodland habitats where there is availability of fruit bearing trees. About half of the species occupy mountainous forest to elevations of 2500m. Many are found on islands, and live in coastal forests, with a few species inhabiting forest edges or mangroves (Sibley and Monroe 1990).

The critically endangered (ref?) Marquesan imperial pigeon is the largest arboreal pigeon in the Columnidae family.

Ducula can be nomadic, and capable of undertaking long oversea flights (Gibbs and David).

Wallace line

DESCRIPTION

Tropical species of pigeons can display a rich variety of colour.

Most lack sexual dimorphism.

Imperial pigeons are large to very large pigeons – measurements. They have medium to long tails – measurements. and slow powerful flight. Most show contrast of light coloration of the head neck and belly, with darker black and blue wings and tails.

Some species have swollen operculum

Fat quills are present in D. bicolor (Berthold, 1967). Fat quills are modified feathers that produce a lipoid substance that is used in a similar way to the secretions of the preen gland. Fat quills are found around the rump D. bicolor and in other Columbidae species. (Abdulash 1966, Denon and Denon, Peters). Preening with the yellow coloured lipid causes variations in colour of the head and shoulders of D. bicolor, ranging from cream to yellow. Colouration differs between individuals and the stage of molt, deeper colouring found just prior to molt, leading to some confusion in identifying taxa. The function of the fat quills is debatable, with suggestions of a sexual signal (Dehley 2007), an antibacterial, or as an olfactory signal (Petres). Yellow lipoid substance can be expressed from the base of a broken rachis (Abdulali).

Preen glands absent in D. badia (Baptista)

BEHAVIOUR

There is a paucity of information on many species of Ducula, many of which are shy and live in remote areas making observation difficult. Though large and numerous, birds can be inconspicuous, feeding quietly in deep foliage (Frith). Breeding and nesting behaviour of many species has been poorly documented.

Ducula are highly mobile and can travel large distances to find fruit (Symes and Marsden 2007, Corlett 1998). They are strong fliers and able to fly between islands to exploit erratic food sources. Small islands whose flora would be unable to sustain a long term population can provide a temporary food source for nomadic birds(Gibbs and David).

Some live singly or in pairs, many are social, forming flocks of 30 to 50 birds. D. spilorrhoa forms large colonies on the Queenland coast, flying to fruit bearing forests during the day and roosting together at night. Pied imperial pigeons favour off shore islands or mangroves for breeding sites. The Low Isles off Port Douglas hosted 20,000 to 25,000 birds in the 1971-1973 breeding seasons (Crome). A volunteer based count in December 2014 reported a similar number of about 22,000 pied imperial pigeons (pipwatch.net). E.J. Banfield reported seeing tens of thousands of Torres Strait pigeons flying from the mainland to their roosting sites on Dunk Island in 1908 (Banfield).

In 2007 13,300 Island IP (D. pistrinaria) were counted flying from Tetapare Island in the Solomons, to nearby Hele Islands. This was during the peak nesting season in November, but thousands of pigeons fly daily in other months.

Suck water.

DIET

Imperial pigeons are arboreal, living in forests and mangroves that can supply seasonal fruit from tropical trees, palms, vines and bushes. Most birds clamber through twigs and branches of canopy, leaning or hanging upside down to reach fruit. Fruit is twisted off stems with bill and swallowed whole. They are able to use extended gapes, as much as 40mm in order to swallow large fruits (Holyoak 1979). D. galeata can extend its gape to 70 mm (ref)

Imperial Pigeons play an important role as seed dispersers (Gibbs and David).

Medium sized pigeons tend to feed on medium sized fruit, which larger pigeons feed on larger fruit (Diamond 1973, 1975).

In contrast to seed eating pigeons, Ducula have thin-walled gizzards and short wide guts, allowing large seeds to pass through (Meehan et al 2003, Corlett 1998, Lambert 1989). After digesting the fleshy parts, they void the seeds intact. Corlett’s 19?? Study on seed dispersal looked at the seed and fruit eating behaviour of Columbidae in ???. He found that species of pigeons differed widely in the proportion of fleshy fruit eaten and the fate of the seeds they consumed, finding Ducula eats almost exclusively fleshy fruits.

Other genera of Columbidae tended to have grinding gizzards which either destroyed seeds, or regurgitated larger seeds. Some large seeded fruits are eaten by both Nicobar pigeons (Colaeneas nicobarica) and Imperial pigeons, with the former destroying seeds and the latter excreting them intact (Corlett 19??, Goodwin 1983).

Imperial pigeons are amongst the largest frugivores in the forest, and can play a part in the dispersal of seeds from fruit too large for most other species to handle. Fruits size about 30 mm would exclude all vertebrates other than hornbills and Ducula (Corlett,19??)They are thus critical in the management of forests throughout their habitat. Pacific Pigeons (D. pacifica) are shown to be excellent seed dispersers in Tonga (McConkey et al, 2004)

McNab (2000) measured basal metabolic rates in large land based and island based fruit-eating pigeons of the South Pacifice. Lower rates were found in those pigeons living on islands, perhaps facilitating their survival on islands where food resources and weather are less reliable than on mainland habitats.

REPRODUCTION

Imperial pigeons construct somewhat flimsy nests, comprising loosely woven twigs placed over a branch of a tree. Species that roost in mangroves construct more substantial nests from mangrove shoots (Frith). They generally lay a single egg with a relatively short incubation period. Ducula, as in all pigeons produce a nutritiously rich crop milk which allows the chicks to rapidly fledge and leave the nest, reducing the period of vulnerability The breeding cycle is short, allowing several broods to be reared in succession (Gibbs and David). .

Crop milk is unique among birds of the Columbidae family, and promotes a high growth rate in squabs. The thick cheesy substance is derived from squamous cells sloughed off from the crop of both male and female pigeons. While studies of the composition of crop milk in domestic pigeons (Columba livia) (Brarathi), no studies specific to Ducula were found. The crop milk of C. livia contains lipids, proteins, enzymes, and can transfer maternal antibodies to squabs, as in mammals (Kocianova). The hypertrophy of crop tissue and production of crop milk is determined by the hormone prolactin, also as in mammals (Brarathi).

VOICE

Variable between species. Typical coo and coowoo of other pigeon families, with a resonant whoop (Higgins)

MIGRATION

Columbidae are generally strong fliers, having larger wings, strong flight muscles and lower wing loadings than other families (Baptista 2009). They are effective colonisers, being able to make across ocean flights and making use of seasonal fruit supplies (ref?).

ECOLOGY AND CONSERVATION

About one third of extant pigeons in the family Columnidae are endangered to a greater or lesser degree, making the family one of the most threatened in the world. Asia, Australasia and Oceania are home to nearly two-thirds of extant species, and three-quarters of threatened species. (Gibbs and David,, Walker 2007)

Many threatened species have restricted ranges or live on isolated islands. Globally island bird species comprise 93% of known recent extinctions (reference needed(. Island species are often vulnerable as highly specialised populations, ill prepared for introduced predators such as cats, rats and mice.

Large breeding colonies require unexploited forest to provide fruit bearing trees for daily feeding. Large flocks are mobile.

In addition they are exposed hunting, habitat loss and degradation, disease and possible competition from introduced species (Thorsen) Many species of Ducula live on forested islands in the Indian ocean, SE Asian archipelagos and in the Pacific and Australasian islands. Several are critically endangered.

One of the most threatened species of Ducual is the Marquesan Imperial Pigeon (D. galeata) whose numbers have been reduced from 250 individuals in 1998 to fewer than 100 birds in 2000 (Thorsen). A translocation programme has been initiated, increasing the number of islands in which D. galeata has a breeding colony (Thorsen).

An abundance of fruit is important to the breeding success of frugivorous birds (Thorsen). Frugivorous pigeons play an important role in seed dispersal and population loss may have a detrimental effect on fauna of a regions, especially when deforestation by logging leaves large cleared areas needing rehabilitation ((Read).

IUCN Redlist www.iucnredlist.org

Ducula and humans Trichomonas in gut

PHOTOGRAPHS

SPECIES LIST

FOOTNOTES

REFERENCES

EXTERNAL LINKS