gary

Hi Geoff,

I think further causal discussion on this forum still cannot satisfy your curiosity.
May I suggest 2 references for you:
1.) Handbook of the Bird of the World Vol. 4: Foreword on species concepts and species limits in ornithology by Jürgen H. Haffer.
2.) Speciation in Birds by Trevor Price

I think some serious birdwatchers have the first and the HKU library collection has the second.

Gary