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Budgerigar (Melopsittacus undulatus) reproductive development is affected by the photoperiod (Putman & Hinde, 1973), and nest-hole entry by the female is accelerated by oestrogen (Hutchison, 1971). In the canary, long photoperiods affect nest-building behaviour, not only by the classical hypothalamus\p=m-\pituitary\p=m-\ gonad route, but also by influencing the effectiveness of exogenous oestrogen in ovariectomized birds (Steel & Hinde, 1972). The present experiments were designed to assess whether the latter effect occurs also in the budgerigar. Ovariectomy was performed by suction and cautery under sodium pentobarbitone and ether anaesthesia. After the operation the birds were transferred singly to the experimental cages and kept on an 11 hr light:13 hr dark (11L : 13D) regimen for 2 days. The nest-box entrance was then opened and the birds tested for a further 7 days. Nest-box entry was assessed by the presence or absence of marks on a piece of smoked paper placed inside the box. Any birds entering during this period were rejected. The remaining birds were allocated randomly to the experimental groups. The birds were injected at approximately 10.00 hours thrice weekly beginning at Day 0, with 0\m=.\1 mg \g=b\-oestradiol-3benzoate (Sigma Chemical Co.) in 0\m=.\1 ml saline. The groups within each experiment were run concurrently. In Exps 1-4 the effectiveness ofoestrogen in producing nest-box entry by ovariectomized females kept under short (8L: 16D) or long (14L:10D) photoperiods was tested. In Exps 5 and 6 a short photoperiod and a skeleton photoperiod was used (6L:2D:2L: 14D and 6L:6D:2L: 10D) to give a total of 8 hr light. In Exps 1, 2 and 3, each cage was watched for 60 min/day between the 2nd and 4th hr after the initial 'lights on', and the period spent in the nest box was recorded. At the end ofExps 1 and 6 the birds were killed, and the birds with ovarian fragments greater than 0-1 % of body weight were rejected from the analysis. The birds in long photoperiods in Exps 1-4 showed a shorter median latency to enter the nest box, and a higher proportion of birds entering the nest box, than did those in the short photoperiods (Table 1 ). The latency difference was significant in Exps 1 and 4 (P<0-05, Mann-Whitney U test, two-tailed). In all experiments in which observations were made, birds in long photo¬ periods that entered the nest box spent a greater proportion of the available

AN OESTROGEN-MEDIATED EFFECT OF PHOTOPERIOD ON THE REPRODUCTIVE BEHAVIOUR OF THE BUDGERIGAR