Cockatoos - Who's Chews?

Can you imagine gnawing into a box of Weetbix made of solid wood, using only your mouth, and finding just four small biscuits inside the bottom, then repeating this 150 times to get your daily nutrition? Welcome to a Black Cockatoo's daily routine.

When you are out bushwalking, you might have discovered dozens of ‘honkey nuts’ (the fruit of our native Marri trees, Corymbia calophylla) dropped on the floor, sometimes all over a track or firebreak. This ‘mess’ (which is actually an important part of bushland ecology) was likely left by one of the three species of Black Cockatoo unique to Western Australia’s south-west. But if the birds aren’t there, how can you tell which species is responsible?

Foraged Marri fruit dropped by Carnaby's and Forest Red-tailed Black Cockatoos.

When you pick up a ‘honkey’, the first thing you will notice is multiple chew-marks, where the cockatoo’s beak has damaged the fruit’s skin. As shown in the above image, these marks might be fresh, with the flesh still being green, or quite old, where the flesh has faded to a brown or grey colour. Such information is useful in establishing how recently the birds have been there, and how frequently different species visit that particular food source. As well as damaging the rim and neck of the fruit (i.e. the top), all Black Cockatoos leave lower mandible (beak) impressions at the base or bowl of the fruit (near the stalk), because their gape (the size of their open mouth) is large, unlike that of smaller parrots.

Now, have a closer look at the fruit. If you are holding a Black Cockatoo-foraged ‘honkey’, it will fall into one of the following three categories:

‘The Surgeon’ – Baudin's Black Cockatoo (Calyptorhynchus baudinii), one of the white-tailed species, has a long upper mandible specially adapted to probing into Marri fruit and extracting the seeds, leaving little damage (Picture 1B, below). The rim and neck are sometimes slightly frayed, with obvious puncture marks in the centre where the seed chamber has been perforated and the seeds expertly hooked out. Chew marks, however, never extend deeply into the rim. The base of the nut has numerous small ‘V’ shaped marks (from the corners of the lower mandible), or narrow, ‘square U-shaped’ marks (~5mm across), from the whole lower mandible’s impression.

‘The General’ – Carnaby’s Black Cockatoo (C. latirostris), the second white-tailed species, has a shorter upper mandible which causes much more damage to the rim and neck of the honkey nut (Picture 2A, below). Chew marks often cut into, or shave the skin from, the rim. The distinguishing feature of this species is the narrow, ‘square U-shaped’ lower mandible marks, virtually identical to those of the Baudin’s.

‘The Butcher’ – Forest Red-tailed Black Cockatoo (C. banksii naso), has a short and very robust beak which tears at and often completely removes the top half of the honkey nut, exposing the internal seed chambers. Several shallow dish-shaped marks (~10mm across) from the lower mandible’s impression are visible on the base (Picture 2B, below).

Click to enlarge any of the below images. Note: arrows point to low mandible impressions.

Picture 1: An unmarked honkey nut (A) and one foraged by Baudin's Black Cockatoo (B).

Picture 2: Carnaby's Black Cockatoo (A) and Forest Red-tailed Black Cockatoo (B) foraging debris.

Lately I've been building nest-boxes for a variety of birds, including cockatoos, and (as though the wildlife wants to be involved!), all three species of Black Cockatoo have visited to forage in a low Marri tree just above my workshop. This has provided some great opportunities for photographing the birds' behaviour and helps shed light on the different techniques used to extract precious seeds. 

I watched one female Forest Red-tail snip off quite a thick branch containing a bundle of honkey nuts, and 'hoist' it slowly up, using her bill to pull and her foot to grasp. She then held the branch with one toe of her right foot, then proceeded to pluck off one nut at a time to be held in her left foot for 'processing' (all cockatoos are 'left handed'!), all the while scanning the scene with her beady brown eyes to watch for predators.

The damage this 'Butcher' species causes to each honkey nut is evident in the above picture. By contrast, the 'Surgeon' leaves very little damage: below is a female Baudin's Cockatoo using her dexterous tongue and thin upper mandible to 'tweezer' out a single seed from a honkey nut. You can view more images of this species on my Flickr photostream here.

Do cockatoos visit your backyard or local reserve, and if so, which species are they? Next time you are on a bushwalk, remember it can be just as exciting to play ‘bush detective’ and work out who's who, without actually seeing the culprit! You can find further reference material about chewed Marri nuts on the WA Museum's 'Cockatoo Care' website.