Crows and ravens corvidae

Дата канвертавання18.04.2016
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The two species of crow and three ravens that occur in Aust are similar in appearance, with black glossy plumage, white eyes in adults and overlapping in size and behaviour, so there are some difficulties for the observer. Range will clearly eliminate some (only place where more than three occur is New England); areas where only one occurs are Tasmania, Flinders I, Otway Ranges and Wilson's Promontory, Vie, (all Forest Raven), and Cape York Peninsula, Top End and Kimberley, where all or nearly all are Torresian Crow. Where overlap occurs (see maps), identification should ideally be based on average appearance and behaviour and should encompass a variety of characteristics. First task is to separate crows from ravens. Crows have (a) white bases to neck feathers visible when ruffled by wind; (b) short hackles on neck (raised when calling in Torresian); (c) nasal quality to most-often-heard calls; (d) mostly western and northern in range. Ravens have (a) grey bases to neck feathers, less obvious when ruffled by wind; (b) medium-length (Little, Forest) to long (Australian) specialised hackles on throat, often distended when calling; (c) commonest calls lacking nasal quality; (d) mostly eastern and southern in range.

Each species has a wide variety of calls with different meanings; to learn them all requires an expert ear and much experience. However, some commonly uttered calls are diagnostic. Basically, Forest Raven is bass with gravelly quality, Little Raven and Little Crow are baritones, latter with nasal quality, Australian Raven and Torresian Crow are tenors, latter with nasal quality. Best for diagnostic purposes is territorial call, uttered with throat hackles fanned from prominent perches by Forest, Australian and Torresian, and uttered with throat hackles unfanned from variety of perches (including ground) by Little Raven and Little Crow:

Forest: rich, deep, slow 'korr-korr-korr-korrrr', last note often drawn out.

Australian: far-carrying, high-pitched wailing 'aah-aah-aah-aaaahh' with last note drawn out.

Little Raven: guttural 'kar-kar-kar-kar' or 'ark-ark-ark-ark', uttered from less prominent perches or on ground, wings flicked up at each note.

Little Crow: similar to Little Raven but with hoarse quality 'nark-nark-nark-nark', doesn't flick wings.

Torresian: notes usually shorter than Little Crow, nasal high-pitched clipped 'uk-uk-uk-uk-uk' or 'ok-ok-ok-ok-ok', sometimes finishing with longer notes.

Forest: wings rounded, tail short in southern form; wings rounder, tail broader than Australian in northern NSW form, soars, noisy wingbeat.

Australian: wings tapered, long slender rounded tail, noisy wingbeat.

Little Raven: wings tapered, more agile, rapid flight than Australian and Forest, often in large cohesive flocks, occasional aerobatics.

Little Crow: wings rounded, tail rounded, rapid agile flight, often in large cohesive flocks, performs spectacular tumbling aerobatics, soars.

Torresian: wings rounded more than Australian Raven but less than Little Crow (useful only in direct comparison), tail squarer than little Crow, in pairs or small (usually) flocks. Torresian often looks more compact in flight than Forest and Australian.
Diagnostic characteristics (found in one species only)

(a) Extensive loose skin under beak, characteristic of Australian; injuv noticeably pink. Juv Forest has pink edges to underside of beak, visible in field.

(b) 'Wing-quivering' greeting ceremony by Torresian: wings rapidly fluttered repeatedly above back.

(c) 'Wing-shuffling': when alighting, corvids shuffle wings to settle feathers; Torresian has exaggerated shuffling, often giving two or three shuffles.

(d) Bill length in Little Crow shorter than head length; equal in others. Tail shorter than folded wing in southern form of Forest, longer than folded wing in Little Crow, equal in others. Legs longest in Forest, shortest in Little Crow.
Other characteristics

(a) 'Currawong flight' or 'missing a beat': performed in flight by both crows, wings closed for space of one beat, calling at same time; performed by Torresian with single call-note and with throat hackles extended, less commonly by Little Crow, with double note.

(b) 'Winnowing flight' or 'reduced amplitude flight': flight display given in territory when one bird of pair is 'returning home': wings are 'winnowed' like typical kestrel flight with quick shallow wingbeats and descending call; Australian — high-pitched, tremulous wail; Forest - deeper and harsher; Little Raven - similar to Forest with creaky quality; Little Crow - similar to Little Raven; Torresian - more nasal than Australian not tremulous, sharply cut off.

(c) Throat hackles: most developed in Australian, with pointed tips; less developed in Forest and Little Raven with double tips; minimal development in either crow, look more like normal feathers. Erectile in Australian, Forest and Torresian.

(d) Nests are large, made of sticks; Australian, Forest and Torresian tend to nest in high situations, Little Raven and Little Crow more flexible and will nest in lower situations. Eggs of all species are similar, blue to blue-green with olive-brown blotches and spots, Little Crow smallest. (e) Adult corvids are glossy black with white eyes; juveniles are less glossy, with eyes at first blue-grey turning brown before or shortly after leaving nest; immatures have eyes hazel (mottled brown and white), eyes become white at about 3 years in Forest and Australian, about 2 years in others.
Source: Slater P, Slater P, Slater R The Slater Field Guide to Australian Birds. Landsdowne, 1995

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