When this question was first posted the first thought that came to mind was
that the reality was the other way round. That these more southerly nesting
species were in fact going north for the winter. This makes sense for an
insect eater because the resource of flying insects is seasonal in temperate
regions.
In the case of the White-throated Nightjar however, we have a partial
migrant. Northern nesting birds are sedentary whilst the more southerly
nesting birds migrate to New Guinea, ( according to Cuckoos, Nightbirds &
Kingfishers of Australia, ed. Strahan 1994 ). This fits with the common
picture of leap frog migration and is not limited to nocturnal species - the
answer ( despite the very erudite suggestion that twilight is longer in
temperate regions ) will be a more general answer.
Imagine that a species evolves in the tropical Australia, unexploited
resources will be seasonally available at its southern boundary offering the
additional resources that are required for breeding. However birds that take
advantage of that resource return to tightly held territories of non-migrant
conspecifics, those that overfly to take territories to the north have the
advantage.
Essentially, the assumptions that I'm making are :-
- the tendency to migrate is heritable
- that the path of migration can be developed by small changes
- natural selection will pass through the sieve any strategy that works.
In other words they do it because it worked for their parents.
Regards
Rob Berry.
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