I recall reading a study by Recher and Holmes which included that in
NSW... "birds were most abundant and species richness greatests during the
spring and summer. This coincided with the peak abundance of insects..."
So, I would conclude that the insectivorous nightjars
etc. are going where the food is located in most abundance. This
argument complements the ideas expressed by Peter and James.
Now, to confirm these ideas all we need to do <grin> is determine if the
nigthjar's nestling phase (or period when food is in most demand during
the breeding cycle) corresponds to the period of peak abundance of
insects. Similar arguments, of course, are used to explain the timing
and migration patterns of birds in the northern hemisphere.
Cheers, Jim
P.S. A special thanks to everyone who has sent me names of bird clubs
across Australia. As suggested, I'll compile a list and post it here
sometime in the near future.
On Thu, 16 Oct 1997, Jim Hengeveld wrote:
> >>
> >>Because in doing so these birds are moving into areas where the biological
> >>productivity is likely to be lower, and the nights shorter, at a time when,
> >>as they are breeding, they are in need of more food.
>
> >Dr. Peter Woodall responded:
> >
> >here's my 2c
> >
> >I think that its more complex than simply
> >tropics=plentiful food,
> >temperate = less productivity.
> >
> >What is the limiting factor for productivity - temperature or moisture,
> >I think its more often moisture .... unless you're in the ACT!
>
> > Reply from James D. Hengeveld
> If I might add another suggestion:
>
> While nights are shorter further south, the length of twilight at both
> sunrise and sunset is longer as you go further south. These time periods
> might be important for "nocturnal" species.
>
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