Friday, February 1, 2013

ID Feature: Corellas

Corellas in the south-west (particularly the Perth area) must be some of the most commonly misidentified birds in WA, particularly by visiting birders. Indeed a quick Google Image Search for Western Corella will turn up photos of all three species labelled as Western. A large part of the problem is caused by confusion over the distribution of the three corella species in the south-west; many field guides do not show introduced populations of Little and (Eastern) Long-billed Corellas, and/or show Western Corella as regularly occurring in Perth. Even the original Thomas & Thomas’ Complete Guide to Finding the Birds of Australia (widely regarded as THE where-to-find guide for Australia) lists Western Corella as a key species at Lake Monger! Fortunately, this has been corrected in the recently published new edition. The reality is that the vast majority of corellas in the Perth metropolitan area are Little or Long-billed Corellas, with very few Western Corellas occurring.


Western Corellas of the northern race derbyi at New Norcia. Note the long bill, relatively long crest (though not fully extended), and strong red lores not extending over eye.



Western Corellas (Cacatua pastinator) occur in two distinct populations north and south of the Perth area, each of which have subspecific status. The nominate race pastinator (Muir’s Corella) has a restricted distribution in the south-west, centred around the Lake Muir and Rocky Gully areas, where it is locally common. The more widespread northern subspecies derbyi (synonym butleri) occurs predominantly in the wheatbelt north-east of Perth and can usually be found quite easily in farmland or around country towns such as Moora and Wongan Hills (though Little Corella can also occur so caution is required).

Western Corellas of the nominate race pastinator near Unicup. Note the tall crest and long upper mandible.

Western Corella (race derbyi) at Wongan Hills. Note the long upper mandible, long crest, and obvious red on the lores which does not extend over the eye.

Little Corellas (C. sanguinea) occur in the greater Perth metropolitan area and are also established around some other large towns further south along the coast (notably Busselton). They also occur in the wheatbelt north and east of Perth, sometimes forming mixed flocks with Western Corellas.

Little Corella at Jackadder Lake in the Perth suburbs. Note the short upper mandible, relatively restricted red on the lores, and absence of red on the throat.

Long-billed Corellas (C. tenuirostris) are restricted to the greater Perth area, having established relatively recently from escaped aviary birds, though small populations may also occur around some other larger southern towns like Busselton. Roughly speaking in the Perth area, they extend east into the Darling Scarp, at sites such as Bungendore Park, north of Perth to the Joondalup area, and south to the Mandurah area.

(Eastern) Long-billed Corella photographed in the Perth area. Note the relatively short crest, long upper mandible, obvious red 'cut-throat' and extensive red on the lores extending over the eye.

Key Features
The three features most useful in separating the three corella species are the bill morphology, the size of the crest (only possible if the crest is raised), and the extent of red colouration on the lores and throat. Each of these features is dealt with in turn below. Additionally, information on voice is included as, with experience, this can also be a useful indicator, particularly for identifying Long-billed Corellas.

Bill
The bill morphology is a key difference between the three corella species, especially between Little Corella and the two other species. Most noticeably, there is variation in the length of the upper mandible. Little Corella has a noticeably short upper mandible compared to Western and Long-billed Corellas, although observers should be aware that wear can result in some individual variation in upper mandible length within a species, so that ‘long-billed’ Littles may approach ‘short-billed’ Westerns in appearance.

There is little difference in upper mandible length between Western and Long-billed Corella (though Westerns average shorter, especially northern derbyi birds), but fortunately these two species are fairly easily separable based on the extent of red on the lores and throat, and crest length.

Long-billed Corella at Herdsman Lake. Western Corellas can have similarly long bills, but note the short crest, extensive red 'cut-throat', and extensive red on the lores extending over the eye.

Crest
Long-billed Corellas have relatively short crests compared to Little and Western Corellas. The difference in crest size between Little and Western Corellas is not as noticeable, but Western’s crests are usually slightly longer. Note that these differences are obviously only noticeable when the birds have their crests erect.

Comparative photos showing crest length in the three species, (L-R) Long-billed, Little and Western . Long-billeds have quite short crests compared to the other two species. There is much less difference in crest size between Little and Western but Westerns are typically slightly longer. Note also the difference in upper mandible lengths and the extent of red colouration.

Red colouration
There is variation between the three species in the extent of red colouration on the lores and breast. Long-billed Corellas have the most extensive red colouration around the lores and a readily visible, extensive red ‘cut throat’. By contrast, both Little and Western Corellas have less extensive red lores and throat feathers. Both species do have red bases to the throat feathers (slightly more extensive in Western than Little) but these are often not readily visible, particularly in Littles, unless the throat feathers are ruffled (e.g. by wind).

Comparison photos of the heads of the three corella species, (L-R) Little, Western (derbyi), Western (pastinator), and Long-billed. Note the differing upper mandible lengths and varying amounts of red on the throat and lores.

Little Corella at Jackadder Lake. Note the absence of red cut throat, and relatively limited red lores (also rather pale red in this individual). Little Corellas have the least amount of red colouration on the lores, though there is variation and the extent of red overlaps significantly with Western Corellas (especially the northern race derbyi) - look for bill length to help separate them.
 
Western Corella (race derbyi) at Wongan Hills. Note the amount of red on the lores is within the range of Little which can also occur at this location - the long upper mandible confirms this bird as a Western.
 
Long-billed Corella at Herdsman Lake. Note the long bill and extensive red colouration on lores and throat. Long-bills are the only one of the three species to show much red on the throat - Western can show some red some situations (feather bases reportedly red), but it is never this extensive
 

Call
The calls of the three species are relatively similar, however there are differences which can be useful particularly in separating Long-billed Corella from the other species. The Long-billed’s call is typically more noticeably disyllabic than that of the Little, which can be useful for identifying distant or unseen birds in the Perth area. The Western Corella’s call resembles that of the Little, so separation between these two species based on call is probably not possible in the field with any confidence. Observers should be aware that caution is needed when attempting to identify birds based on call. In particular, this discussion refers only to the basic contact call of the three species (refer to the audio below - note that the Long-billed recording also contains Little calls in the background) – all three species also make other screeching calls that we consider not reliably distinguishable at this stage. Overall, it is safer to rely on a combination of the morphological features described above where possible.









There are also other recordings for the three species available on the xeno-canto website

Difficult Birds
Most corellas seen in the south-west can easily be identified by a combination of the features described above, aided by a knowledge of distribution. However, occasional birds turn up that can present more problems, especially around the Perth area where all three species are possible (though Western is rare, as previously stated).


Western Corella at Herdsman Lake, with Little Corella in the forground. Note particularly the longer upper mandible of the Western, and also the slightly darker, more extensive red lores (though beware overlap in this feature).

Long-billed Corellas have been observed hybridising with Galahs in the Perth area, so it is possible that Little and Long-billed Corellas also hybridise on occasion (this has been recorded on the east coast). If they occur, Little x Long-billed Corellas could be expected to be relatively similar to Westerns in appearance, as Westerns are intermediate between the other two species in many features (the exception being crest length).

Unusual corella photographed at Lake Joondalup. The bill is very long for a Little, but at the short end for a Western or Long-billed. The red on the lores is in the range of Western and Little, but the small red cut-throat is extensive for either species. This bird has some potential as a Long-billed x Little, or may be an odd-looking Western.

LCB is keen to investigate this further, and we would appreciate any photos of possible Western Corellas, or reports of hybrid breeding pair, from the Perth region if any readers have photos they are prepared to share with us.


Little (back right) with two Long-billed Corellas, Herdsman Lake. Compare upper mandible length and extent of red on throat and lores of the left-hand Long-bill with the Little.

4 comments:

  1. Very interesting article. Do you find that Western Corella crests, when erect, angle forward instead of backwards as observed in the other two species? The photos in this post also seem to suggest that.

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    1. Nah, I think that's just an artefact of the photo selection. When fully erect all three species angle forward, maybe less obvious on Long-billed as the crest is shorter. Double-checked this at Herdsman recently!

      One feature we didn't mention is head size: Long-billeds are much bigger-headed than Little - compare the two species in the last photo. Western seems to be bigger-headed than Little, but less noticeably so, and smaller-headed than Long-billed

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    2. Recently acquired a beautiful long billed, but know little of his past. How do I tell his age?

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    3. Not really sure, sorry. I'm not sure it would be possible, HANZAB indicates that ageing is difficult after juvenile plumage. 1st year immatures may apparently be ID'd by retained juvenile primaries but after that no plumage features are mentioned for ageing

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