Little and Long-Billed Corellas Learning to Use a New Food Source, the Seeds of Marri

136 AUSTRALIAN FIELD ORNITHOLOGY 2008, 25 , 136–139 Little and Long-billed Corellas Learning to Use a New Food Source, the Seeds of Marri ALLAN H. BURBIDGE Department of Environment and Conservation, P.O. Box 51, Wanneroo, Western Australia 6946 (Email: [email protected]) Summary . It has been shown that corellas Cacatua spp. are not capable of breaking open the larger, woody capsules of eucalypts to feed on the enclosed seeds. However, since being introduced to south-western Australia, both Long-billed C. tenuirostris and Little Corellas C. sanguinea have learnt to extract the large seeds of Marri Corymbia calophylla by modifying their feeding behaviour to tip out the seeds of the open capsules into the bill. Few birds eat eucalypt seeds, with some obvious exceptions being the Red- capped Parrot Purpureicephalus spurius and the black-cockatoos Calyptorhynchus spp. of south-western Australia (Woinarski et al . 1997). In particular, it is well known that these species feed on the seeds of Marri Corymbia calophylla , formerly Eucalyptus calophylla . (Both Eucalyptus and Corymbia species are hereinafter referred to as ‘eucalypts’, for ease of reference and because much of the literature on eucalypts refers to all of the previously recognised subgenera of Eucalyptus as ‘eucalypts’). Marri, which is common and widespread in south-western Australia, has hard, woody capsules (locally called ‘honky nuts’), measuring up to ~5 cm long and up to ~3.5 cm in diameter. Baudin’s Black-Cockatoo Calyptorhynchus baudinii extracts the seeds with its long upper mandible, causing little damage to the seed-capsules, Carnaby’s Black-Cockatoo C. latirostris chews at the rim, causing considerable damage to the seed capsule, and the Red-tailed Black-Cockatoo C. banksii chews at the base of the capsule or at the rim to extract the seeds (Higgins 1999; Johnstone & Kirkby 1999; Cooper 2000). Red-capped Parrots also extract the seeds, but they do it delicately with the long upper mandible, again causing little damage to the seed-capsules (Higgins 1999, p. 393). Cooper (2000) found that captive Western Corellas Cacatua pastinator were unsuccessful in their attempts to remove seeds from the capsules of Marri. Her observations suggested that corellas were unable to penetrate the tough woody casing that protects the seeds. They were unable either to exert as much power with their bills as the black-cockatoos, which were easily able to chew into Marri capsules, or to remove seeds with their upper mandibles as done by Baudin’s Black-Cockatoo and the Red-capped Parrot. She went on to suggest that corellas may be unable to utilise Marri as a food source in the field. However, Western Corellas have been recorded feeding on seeds of Marri in the field (Smith & Moore 1991), but they do so through a behavioural adaptation, rather than by using brute strength. The corella holds the capsule in one foot and probes it with its upper mandible, then raises the capsule and tips the contents into its mouth. Both the Long-billed Corella C. tenuirostris and Little Corella C. sanguinea have been introduced to the Perth area of Western Australia, from outside the range of Marri. The Long-billed Corella has not previously been recorded feeding on the seeds of Eucalyptus or Corymbia , although the Little Corella has been recorded feeding on seed from the very small capsules of River Red Gum E. camaldulensis in northern Australia (Higgins 1999). Given the ubiquity of both corellas and eucalypts, it is curious that there are not more records of corellas feeding on the seeds of eucalypts. VOL. 25 (3) SEPTEMBER 2008 Corellas Learning to Eat Marri Seeds 137 The 2007–08 season was one in which Marri trees had a bumper crop of capsules around Perth. In late December 2007 to late February 2008 I observed a flock of corellas in Wanneroo (31°44 aS, 115°48 aE), one of the northern suburbs of Perth. Maximum numbers were ~55 Little Corellas and four Long-billed Corellas. Most were present at the time of peak seed fall in early January. Visits by corellas to the Marris tapered off during February, as they switched to other food sources, and by March they were feeding elsewhere on Casuarina seeds, as they have done in the past (Burbidge 2006). On most mornings and evenings during the time of peak Marri seed fall, corellas came into a small dense stand of Marris that had a heavy crop of seed-capsules (up to 200 per cubic metre of canopy). Most observations were opportunistic, but on 6 January feeding in the afternoon started around 1700 h, and extended to about 1900 h (Western Standard Time). Most corellas flew into the trees and started feeding. Some individuals visually inspected the inside of a capsule first, before choosing one to bite off. Others simply bit off a capsule without any apparent initial inspection. Capsules were bitten off by nipping through the stalk while holding the capsule in one foot. The Corella then held the capsule in its bill, with the lower mandible on the outside of the lower rim, and tipped its head back, so that the capsule was vertically above it. It was assumed that the seeds and loose aborted ovules (which are frequent in this and other eucalypts) fell into the mouth at this point, although I was not able to observe closely enough to see if that was the case. After tilting the capsule back for a variable number of times, the capsule was dropped, at least sometimes by accident, sometimes quickly. Seeds appeared to be eaten immediately, without any attempt at husking them. Some corellas were feeding on the ground, where there were many fallen capsules. Here, most birds were foraging amongst the fallen capsules, presumably picking up seed that had fallen from the trees. However, some birds were also picking up capsules from the ground. These corellas picked up a capsule in the foot to examine it, then bit it and tipped it up (Plate 15, front cover). When the head was tilted well back, one foot was extended forwards, presumably to maintain balance. Both Long-billed Corellas and Little Corellas showed similar foraging behaviour. No birds were seen to probe the capsule first. It is not clear how the birds chose capsules. However, corellas foraging in trees spent some time looking around them, and then investigating individual capsules. Those capsules in which the valves had not begun to open were greener than those in which the valves were opening, and these capsules were greener than those that had already dropped their seed (Plate 18). The corellas could therefore have learnt to choose on the basis of colour. Having selected a capsule to investigate, some corellas peered at it from below, possibly to see whether the valves had started opening. There were so many capsules on the ground that it was difficult to determine the colour or degree of valve opening in discarded or used capsules. Capsules on the ground varied from ones with unopened valves to ones that had lost their seeds some time before. Therefore, I did not attempt to compare the proportion of different-coloured capsules on the ground with the proportion available on trees, to determine whether birds were preferentially selecting intermediate-coloured capsules, as I had no way of knowing how long they had been on the ground, and colour changes and capsule valves open as the capsule dries. More detailed field observations would be of interest. Although Cooper’s (2000) observations on captive corellas showed that they were unable to penetrate the tough woody Marri capsules, all three species of AUSTRALIAN 138 BURBIDGE FIELD ORNITHOLOGY Marri capsules at different stages of development: the two capsules on the left are still sealed, the middle capsules have valves that are just opened, and the two on the right are older capsules, from which seeds have dropped. Plate 18 Photo: Allan H. Burbidge corella have now been observed apparently feeding on the seeds of Marri. However, they do so by means of a behavioural adaptation, whereby they inspect the capsule to ensure that it is at an appropriate stage to release the seeds, and then they tip the seeds into their mouths. There are many examples of parrots and cockatoos exhibiting learning of new or improved foraging methods. Carnaby’s Black-Cockatoos have long been known to utilise new foods (Perry 1948), and plantations of introduced pines Pinus sp. are now a valuable food resource (Cooper et al . 2002). They can also learn quickly: for example, a few birds were recorded feeding on seeds of Liquidambar Liquidambar styraciflua for the first time in 2000, but the following year many individuals were doing so (Mawson 2001). Improvements in foraging techniques are also known: for example, adult Red-capped Parrots are much more skilled at extracting seeds from Marri capsules than juveniles are, implying learning (Forshaw 1969). Apparently, both Little and Long-billed Corellas have learnt to extract seeds of Marri since these corellas were introduced to south-western Australia. Although one of these species presumably learned from the other, as they frequently occur in mixed flocks, it is unlikely that they learned from the Western Corella, as that species was extirpated from the Perth area in the decades following European settlement VOL. 25 (3) SEPTEMBER 2008 Corellas Learning to Eat Marri Seeds 139 (Storr & Johnstone 1988). No other bird species has been recorded handling Marri capsules in this way, so presumably Little and Long-billed Corellas have learnt without example, or possibly by observing Red-capped Parrots and adapting those methods within the constraints of their own morphology.

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