Bill,
Having worked in Lamington National Park for three years in the late 1980s,
what
you describe fits an occasionally seen habit of the Topknot Pigeon. From the
lookouts and ridges we sometimes saw a tight flock flying high above the
rainforest
and then plummet almost straight down with a great noise of rushing air - an
amazing
roaring sound just as you have described - like 'an aeroplane with its engines
off'. Usually they would level out just above the canopy and settle in a tree.
Sometimes though where the canopy was more open they would level out and
continue
the flight at great speed like missiles through the forest for some distance.
There
is a term for this and I cannot remember what it is - something like
'whiffling' ??
- the noise being made as the air spills from their wings in a steep descent.
Someone who knows something about aerodynamics can probably answer that. We
never
saw a bird of prey about as Hugo suggests - the impression we got was that it
appeared to occur where the birds were travelling high, probably to a new
feeding
area and upon seeing suitable habitat below, dropped straight down. However,
on a
couple of occasions, we saw a flock split up with each bird going in a different
direction but still straight down to the canopy. Maybe that was an occasion
when
there was a falcon or hawk about and splitting up was an escape manouvre. Have
just
had a very quick scan through Hanzab and surprisingly it seems as though no
mention
is made of this habit. The only other birds I have seen do this is cormorants
(singly).
Lloyd Nielsen,
Mt Molloy, Nth Qld.
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