Article in journal
Report number 128 from Kvismare Bird Observatory:

Observations of a ZZW female in a natural population - implications for avian sex determination.

Arlt D, Bensch S, Hansson B, Hasselquist D & Westerdahl H.

Published 2004 in Proceedings of the Royal Society, London, B (suppl) , volume 271, pp. S249-S251.