birding-aus
Hi,
Some thoughts, prompted by Jenny Bradford's posting on Yellow-tailed Black
Cockatoos getting stuck into wood-boring grubs:
Yellow-tailed Black-Cockatoo (YTBC) is reasonably common in the area where I
live (St Andrews: mainly wet sclerophyll forest, about 50 km north-east of
Melbourne). I think they are pretty much resident here, but with very large
territories so we don't neccesarily see them every day. On average we
probably have them within earshot of the house about 150 days per year; they
are about in all seasons, tending to occur in biggish flocks (usually 20-30
birds) in winter, small family groups of 2-5 birds in summer. I often find
them hunting wood-boring insects or grubs, particularly in late autumn and
winter. It's a spectacular sight: they are very approachable when feeding,
so you can get great looks at them digging their bills into a large branch
or tree-trunk, and ripping out a strip of wood and bark about 30 cm long,
which they use as a stance while they dig further into the tree.
In this part of the world they most often hunt wood-boring insects or grubs
in Silver Wattles and Pomaderis sp., in trees 5-15 m high. They are at least
as destructive here as Jenny Bradford reports them to be in SE Qld, leaving
very large and distinctive scars on the trees they have fed on. Indeed I
know of at least five trees on my local patch that they have actually killed
- the trees didn't die immediately, but their trunks snapped at the
YTBC-scar in subsequent gales. The biggest tree I've seen this happen to was
a Silver Wattle, at least 15 m tall, which snapped at a YTBC scar only 5 m
above the ground.
Moving closer to the point of this message, I watched a YTBC extract several
grubs from that particular Silver Wattle at the time when the damage was
caused. About a year later, I found YTBC's successfully extracting grubs
from the remains of the same tree, exploiting both the fallen branches and
the still-standing stump. This was the third time I had seen YTBC's do this,
first getting grubs out of a living tree and later getting grubs out of a
tree they had killed: first time I saw it was at Coranderrk reserve near
Healesville (I forget what the tree was); second time was in our garden, the
unlucky tree being a Pomaderis. On several other occasions near St Andrews
I've seen YTBC's hunting grubs in Pomaderis or Silver Wattles which showed
clear signs of earlier YTBC damage, though I didn't actually see the initial
attacks.
The question that intrigues me is this. Do Yellow-tailed Black Cockatoos
actually farm wood-boring insects? I'm making the assumption here that a
dead tree, or one that has been severely wounded by a YTBC, is more likely
to contain edible grubs than a live healthy tree. Are there any
entomologists out there who can enlighten me on this point? I certainly have
the field impression that YTBC's feed on previously damaged trees more often
than would be predicted by chance. I can't prove whether it is the same
individual YTBC's that revisit previously damaged trees, but my hunch is
that this could well be the case. Feedback would be welcome, and if anyone
fancies studying the topic seriously I think they could have a lot of fun.
Danny Rogers
340 Nink's road
St Andrews
Victoria 3761
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