Australian raven

Appearance

Measuring 46–53 cm in length with a 100 cm wingspan and weighing around the 650 g, the Australian raven is Australia's largest species of corvid. The adult Australian raven is an all black-bird with a black beak, mouth and tongue and sturdy black or grey-black legs and feet. The tibia is fully feathered and the tarsus is long, and the feet large and strong. It has white irises. The plumage is glossy with a blue-purple to a blue-green sheen, greenish over the ear coverts, depending on the light. The underparts are not glossy. The Australian raven has throat feathers that are lanceolate with rounded tips, while the other four species of Australian corvids have bifurcate tips, though this can be difficult to see in the field. The hackles are also longer than those of the other four species; when they are raised, they give the bird an unusual bearded appearance. The upper third of the upper mandible, including the nares and nasal groove, is covered with bristles, which can be up to 3 cm long. The heavy-set beak is tipped with a slight hook, and is longer than the bird's head. The wings are long and broad, with the longest of its ten primary feathers almost reaching the end of the tail when the bird is at rest. The tail is rounded or wedge-shaped. The Australian raven can be distinguished from the two species of crow occurring in Australia by the grey base of the feathers, which is white in the latter species. The demarcation between pale and black regions on the feather is gradual in the ravens and sharply delineated in the crows. Feather bases are not normally visible when observing birds in the field, but can sometimes be seen on a windy day if the feathers are ruffled. Unlike the other four species, the Australian raven has a bare patch of skin under and extending to beside, the bill. This can be hard to discern in the field. The three species of raven are more heavily set with a broader chest than the two crow species, with the forest raven the stockiest of all. Relative size of species is only useful when two species can be seen side by side, as the overlap in size is large and the difference in size small.

Juveniles resemble adults, but lack throat hackles, and sometimes have a pink fleshy gape. The bill is shorter and shallower; its base can be pinkish and the tip can be light grey. The plumage is more ruffled and softer in appearance, lacks the glossy highlights and often having a brown tinge. The bare skin on the throat is pink in birds that have recently left the nest. Eye colour varies with age, gradually lightening from juvenile to adult. Nestlings up to four months old have blue-grey irises, juveniles aged from four to fifteen months have dark brown irises, and immature birds have hazel irises with an inner blue rim around each pupil until age two years and ten months.Rowley and colleagues recorded iris colour changes of all five Australian corvid species raised in captivity. Immature birds older than one year develop hackles, while some pink remains in the gape until the bird is two or three years of age.

Naming

The Australian raven was first described by Nicholas Aylward Vigors and Thomas Horsfield in 1827, when they reported George Caley's early notes on the species from the Sydney district.

Distribution

The Australian raven is common throughout eastern Australia, and southern Western Australia, but it is rarer and more scattered in the north, with isolated sightings in Cape York at Coen, Windmill Creek and the Mitchell River, and becoming more common south of Rockhampton in central Queensland. It is found throughout New South Wales, though is uncommon in the northeast of the state. It is rare in the Australian Alps, being replaced there by the little raven. It occurs across Victoria and eastern South Australia, through the Eyre Peninsula and Nullarbor Plain into Western Australia, across the state north to the Wooramel River. It is found on some offshore islands such as Rottnest Island and Kangaroo Island. It is a rare vagrant to Lord Howe Island., though it was recorded as a vagrant in New Zealand in the 1870s, this has not been confirmed.

Behavior

Difficulties in distinguishing Australian corvids has hampered understanding of seasonal movements. The Australian raven is thought to be largely sedentary, with most movement of over 16 km due to flocks of non-breeding subadult birds. Juvenile birds leave their parents and join flocks when they are four or five months old. Smaller flocks of 8–30 birds stay within an area of around 260 square kilometres, while larger flocks of up to 300 birds may travel hundreds of kilometres seeking food.

A single breeding pair and their brood can occupy a territory of up to around 120 hectares and remains there year-round, though groups of ravens may enter this area to forage. Australian ravens will defend their territory by chasing, dive-bombing and occasionally striking the backs of birds of prey, foxes or even people. They generally mate for life, though occasionally one male has been found to be mated with two females in adjacent territories. If the female dies, the male Australian raven maintains the territory and finds another mate, while if the male bird is lost, the female abandons the territory. No courtship behaviour has been observed, and species that mate for life often lack elaborate courting displays. Once they begin breeding at three years of age, they live another four to five years on average. During this time they produce two surviving young each year on average. The longest-lived Australian raven recorded is an adult that was banded and recaptured alive 12 years and 5 months later.

Australian ravens generally walk when moving around on the ground, though do hop when hurrying. They preen themselves frequently, particularly when roosting in the middle of the day. They also engage in allopreening, where birds will preen each other's head and neck. This takes place particularly in autumn, winter and spring, and is important in pair bonding. Either member may initiate it, generally by landing near the other bird, shuffling next to its mate, then bending its head forward and presenting its nape.

Habitat

The Australian raven can be found in a wide range of natural and modified habitats. It requires available water and trees to roost in or perch on. Preferred habitats include eucalypt-dominated sclerophyll forest, and farmland adjacent to trees. It is also found in heath and mangroves. In areas where it occurs with the little raven, namely over much of central New South Wales, Victoria and into South Australia, the Australian raven is restricted to more forested areas while the latter species prefers more open areas. Similarly, in inland Australia it can share a range with the little crow, as the two do not appear to compete. However, the ranges of similar-sized forest raven and Torresian crow only narrowly overlap with the Australian raven as all three compete with each other. In central and western regions, Australian ravens and Torresian crows vie for the scattered uncommon trees and outcrops, and only one or the other are found there. It co-occurs with the forest raven in northeastern New South Wales from Port Stephens northwards. The Australian raven has adapted very well to human habitation in some cities and is the most common corvid in Canberra, Sydney and Perth; in Melbourne and Adelaide it is replaced by the little raven, and by the Torresian crow in Brisbane. Its large range, abundance and increasing population mean it is classified as "Least Concern" on the IUCN Red List.

Reproduction

Australian ravens begin breeding once they are three years old. Breeding season is from July to September, with no substantial difference in timing across its range around the country despite it inhabiting a range of diverse climates and habitats across 19 degrees of latitude. Rowley has pointed out this is unusual for a bird species with a wide range and has postulated that breeding is initiated by day length. Rarely, breeding can take place in May, June or October. Australian ravens generally nest in tall trees, never near to the ground as some species do. The nest also functions as a lookout post and so tall or emergent trees are selected. The ravens occasionally nest on buildings, telegraph poles, or tall windmills which allow the species to occupy areas lacking in tall trees. Windmills may have assisted the spread of the species in North Queensland and the Northern Territory. The highest recorded corvid nest in Australia was found atop the AWA Tower in Sydney.

Nests are generally large and untidy, consisting of a bowl or platform of sticks lined with grasses, barks, and feathers that can be up to 5 cm thick. As they are relatively heavy, they are built on larger forks in trees rather than out in the canopy. Building the nest is often time-consuming initially as the birds try to wedge sticks, which are 30–60 cm long and 0.6–1.2 cm thick, into the tree fork to make a platform. Thinner sticks and rootlets are used to make the bowl before the bowl is lined with feathers. Both birds build the nest, with the female taking over the lining of the nest while the male brings her material. New nests are built each year generally, as the re-use of old ones might spread disease or parasites—nests become caked with faeces as the nestlings grow and the parents cannot keep up with its removal. Furthermore, old nests often disintegrate within twelve months due to their exposed locations. The female develops a brood patch—a patch of bare skin on the bird's underparts that reddens and becomes much more extensive from around three weeks before the first egg is laid. The skin itself is oedematous and wrinkled, and does not get re-feathered until December after the breeding season has finished.

Their lofty locations makes monitoring of Australian raven nests difficult. A clutch can comprise up to six eggs, though usually four or five are laid, with five being the most common number. Measuring 45 by 30 mm, eggs are pale green or bluish-green and splotched with darker olive, brown and blackish markings. Eggs are quite variable, and thus which Australian corvid laid them cannot be reliably identified. Incubation of the eggs is done solely by the female over roughly 20 days. Incubation is intermittent initially, becoming constant by the time the third or fourth egg is laid. Only one brood is raised per year, though a second clutch may be laid if the first clutch is lost early in the season. Late clutches have poor survival rates, possibly due to chicks getting dehydrated on hot days as the year progresses or being eaten by wedge-tailed eagles. The chicks are altricial and nidicolous; that is, they are born helpless, naked and blind, and remain in the nest for an extended period. They have pink skin until 5 days of age, when feathers under the skin turn it grey. They lose their egg tooth at the same time. Their eyes begin opening at 5 to 6 days of age and are fully open by 11 to 12 days, by which time their feathers begin emerging. At 14 days, their primary feathers begin emerging, and they are fully feathered by 35–36 days old. They leave the nest at 40–45 days of age, and stay with the parents for three to four months after that. They follow their parents and beg for food for the first month outside the nest but are feeding themselves by the third month. Young birds are often attacked when they enter neighbouring territories, and melees ensue as their parents try to defend them and herd them back.

Food

The Australian raven is omnivorous, though eats more meat than smaller corvids. Its diet in summer contains a high proportion of insects, while more plant items are eaten in autumn. Flesh makes up over half its diet in winter. Invertebrates commonly eaten include spiders, millipedes, centipedes, grasshoppers, cicadas and caterpillars, which are important in feeding nestlings. Australian ravens sometimes eat yabbies from the edges of dams. Unusually for a ground-feeding omnivore, earthworms are rarely eaten. Australian ravens have been reported killing birds of such size as young galahs and starlings. Most mammals are eaten as carrion, as many species are too large for the raven to kill, though young rabbits are a frequent prey item. Australian ravens drink water frequently, up to ten times a day in hot weather. Birds have been observed dunking pieces of meat in water before eating them, as well as doing the same with hard biscuits to make them soggy and soft.

Australian ravens are intelligent birds, and like many other corvids have innovative methods of seeking out food. Foraging takes place in the early morning or late afternoon; birds rest in the hotter part of the day. Food is taken mainly from the ground, birds either finding objects while flying overhead or by walking along and looking. However, they occasionally feed in trees—Australian ravens forage eucalypt foliage for Christmas beetles, and devote a substantial amount of time to look for nests and eggs to eat. They have also been known to take golf balls from fairways, possibly mistaking them for eggs. Ravens use their bill rather than their feet to explore or turn items on the ground over or hold or snatch food while flying. They have also been recorded using fence posts as anvils to bash snails against before eating them. Australian ravens most often eat food where they find it unless taking food back for nestlings. Occasionally they have been observed caching carrion or a killed animal in a hole nearby to store it. They can pack shredded meat in their mouth under their tongue. Australian ravens have adapted well to eating food scraps in urban areas, such as school playgrounds, rubbish tips, bins outside supermarkets or restaurants, abattoirs, piggeries and farmyards. In one isolated study, they were observed feeding on nectar from eucalypt flowers. Australian ravens sometimes forage in mixed-species flocks with any of the other four species of Australian corvids. Sometimes they are aggressive with little ravens if both are at a food source and drive them off, though not if the smaller species greatly outnumber the larger.

Predators

A circovirus—given the name raven circovirus or RaCV—was isolated from an Australian raven suffering from feather lesions in 2006. It has affinities with canary circovirus and pigeon circovirus. Its clinical significance is unknown. A species of "Isospora"—given the name of "I. coronoideae" was isolated from this species, its only known host. Tick infestation is rare in the Australian raven, with "Ixodes holocyclus" and "Amblyomma triguttatum" recorded. Lice and hippoboscid flies have been recorded yet little-researched, and an infestation by the fly "Passeromyia longicornis" was recorded in one nest.

The wedge-tailed eagle preys on adult, nestling, and fledgling Australian ravens, while the little eagle also takes nestlings, and powerful owl has been recorded killing adults; other birds of prey are seen as threats, yet there is no evidence they have successfully preyed on the ravens. The introduced red fox competes with the Australian raven for carrion and can drive it off. It may also kill young birds that it catches on the ground. The channel-billed cuckoo has been recorded as a brood parasite.