>At 14:00 16/10/97 +1000, John Leonard wrote:
>>Tricky question here:
>>
>>Why do certain noctural birds migrate south in summer (I'm thinking of
>>White-throated Nightjars and Little Bitterns, there may be other examples)?
>>
>>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!
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.
....Jim
----------------------
James D. Hengeveld Biology Dept.
Bloomington, Indiana 47405 Indiana University
812: 855-5353
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