Mawson & Massam came to the same conclusion in their 1996 study  of hundreds of birds around Donnybrook, finding “it is not possible to sex immature or adult birds on plumage colour alone”. Their long-tem moult study suggested that sex differences lie more in the rate of acquisition of adult plumage, rather than the end-point. The only discriminatory feature they could discern was an underwing stripe (row of white spots in the inner primary webs), usually lost at the first moult by males but more gradually by females. But even this is not fully discriminatory: an immature-plumage bird with less than 10 spots is definitely male, an adult-plumage bird with more than seven spots is definitely female, but of inside these sex is indeterminate. The ‘duller’-type adult plumage clearly does not identify females, as some were confirmed by dissection to be males.
Some plumage features thought suggestive of adult females include:
- presence of a few green feathers or green fringing in the red crown, especially around the eyes & lores (black fringing in fresh male plumage)
- a mixture of red and yellow-green feathers in the undertail and vent (males mostly bright red only)
HANZAB  also adds:
- duller, slightly more maroon, tinge to crown
- broader and duller appearance to lores
- yellow of rump ‘wraps’ further around onto top of thighs & undertail
- subtly less blue in uppertail
- duller upperwing
- head structure narrower and more rounded when viewed from front
However the implied conclusion of Mawson & Massam is that many of these are simply immature characters lost more slowly by females, thus older females will eventually grow to resemble males very closely.
|A 'duller'-type adult Red-capped Parrot. This individual is probably a female, note the small green flecks in the red crown around the eyes, and 'wrapping' of the yellow rump into the undertail.|
The preferred food source of Red-caps is eucalyptus seeds, particularly the marri Corymbia (Eucalyptus) calophylla, and its long tapered bill is thought to be specially adapted for surgically extracting these seeds. A steady hail of spent green marri nuts is often the only indication of Red-caps feeding overhead. They also often feed under trees on the ground. Though they feed on a variety of seeds, grasses and fruit (they remain a declared pest in orchard districts ), the special adaptations of the bill may explain why their distribution appears to closely parallel that of the marri itself.
|A (probably male) Red-capped Parrot specimen in the WA Museum, showing its specialised long, tapered bill. Specimen courtesy WA Museum.|
|The remains of a Red-capped Parrot meal of Marri seeds. Note the delicate crescent marks at the top of the nuts but absence of damage to the rim - compare these to the remains left by the more destructive Black-Cockatoos.|
If seen well, Red-capped Parrots are easily identified. However they are fairly timid and easily flushed, so a very useful fieldmark is their contrasting yellow rump, compared to the less distinct light green rump of ‘Twenty-eight’ Ringnecks (Australian Ringneck race semitorquatus). It is also useful to learn their calls - a screeching ‘check..check’ alarm call (slightly harsher and higher-pitched than the equivalent call of the Twenty-eight), and a distinctive staccato four-note contact call, like old-fashioned telephone pips. Their flight is faster, more fluttering, and straighter than the Twenty-eight, with less exaggerated flap-glide undulation.
|A Red-capped Parrot - probably a female - shows off its prominent lime-yellow rump, a very useful field mark for these flighty birds.|
Where to find Red-capped parrot:
The Red-capped Parrot is a restricted endemic of the higher rainfall areas of the deep southwest, from around Lancelin in the north to Esperance and Condingup in the east. They appear to have declined north of Perth but remain generally common . They are commonest within the Darling Range, and in the Walpole-Albany region; also of course in orchard areas such as Donnybrook. They are likely to be seen by the roadside during any extended drive in the south-west. They can be found throughout the Perth metropolitan area but are uncommon and patchy within the suburbs, becoming more likely south of the Swan River or towards the hills. On the Perth coastal plain the southern Beeliar reserves (eg. Thomsons Lake) are good sites, though any sites in the Perth hills (eg. Bungendore or Victoria Dam) is probably a better bet if you are targeting this species close to Perth.
 eds. Higgins P.J. & Davies S.J.J.F (1999) Handbook of Australian, New Zealand and Antarctic Birds Volume 4: Parrots to Dollarbird. Oxford University Press, Melbourne.
 Mawson P. and Massam M. (1993). Red-capped parrot Purpureicephalus spurius.
Moult, age and sex determination. Emu 96: 240-244
Edward Lear's famous plates from Illustrations of the Family of the Psittacidae, or Parrots (1832), depicting females (right) as a much duller, dirtier version of the gaudy male. We now know it's not that easy, yet many modern field guides make the same mistake - simply describing the female as 'duller'. Plates from CC-BY uwdigitalcollections, via Wikimedia Commons.