birding-aus
Interesting questions as to how young Koels learn to eat fruit, and how they
learn to migrate.
As to the fruit-eating, I guess that even if Koels looked after their own
young they would still have to feed them some insect food as they grew up,
as fruit has little or no protein. So you would probably find that amongst
all frugivorous birds there is a developmental threshold of insects (+/-
some fruit)->almost wholly fruit, at a point when they have done most of
their growing.
As to the migration, with some cuckoos, ie the Bronze-cuckoos the migration
isn't spectacular, and could just be a instinctive urge in the brain saying
'days are getting shorter/it's getting colder, therefore move north', but
the Koels' migration is a very definite thing, as for examle is the Oriental
Cuckoo's. Perhaps this could still be instinctive, I remember reading the
'standard work' on migration (probably superseded by now), by Thomas
Alerstam, and I seem to remember in that book that he notes that many
passerine spp and waders travel in distinct adult and juvenile flocks on
their migration, so their migratory urge must be instinctive (though
presumably the mortality of juveniles would be much greater).
On the other hand someone on birding-aus a couple of years ago asked the
question 'why do cuckoos keep on singing into late summer and autumn?', and
certainly Koels carry on, I'm not sure as to other spp, and perhaps this is
to attract juveniles away from their foster parents and back to their own
kind, to be sheparded on migration.
John Leonard
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
John Leonard (Dr),
PO Box 243,
Woden, ACT 2606,
Australia
"If they could tell you in advance exactly how much
shit you're going to have to take it wouldn't be so bad;
then you'd be prepared." Ralph Bridges
http://www.spirit.net.au/~jleonard
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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