1. Wallraff, H. G., Z. Tierpsychol., 16, 513 (1959).
2. Schmidt-Koenig, K., in Adv. Study Behavior (edit. by Lehrman, D.) 1, 217 (Academic Press, 1965).
3. Schmidt-Koenig, K., Naturwissenschaften, 51, 423 (1964).
4. Wallraff, H. G., Proc. XIV Int. Orn. Cong., 331 (1967).