1. Mayr, E. Evolution 10, 105–108 (1956).
2. Bumpus, H. C. Biol. Lect., Marine Biol. Lab. Woods Hole, 1–15 (1898).
3. Johnston, R. F., Niles, D. M. & Rowher, S. A. Evolution 26, 20–31 (1972).
4. Rising, J. D. in Productivity, Population Dynamics and Systematics of Granivorous Birds (eds Kendeigh, S. C. & Pinowski, J.) (Polish Scientific Publishers, Polish Academy of Sciences, Warszawa, 1972).
5. Baker, M. C. & Fox, S. F. Am. Nat. 112, 675–682 (1978).