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Effect of a skeleton photoperiod on the daylength-dependent response to oestrogen in canaries (Serinus canarius).

Previous studies have shown that ovariectomized canaries, injected with 50 mug oestradiol benzoate thrice weekly, show more oestrogen-induced nest-building on a long day than on a short day. In this study the same effect was obtained with castrated canaries. The influence of time available for building was assessed first by comparing castrated males on skeleton long days with birds on short days, the total number of light hours being equal. More birds in the skeleton long-day group placed material in the nest, and did so with a shorter latency, than birds in the short-day group. Thus the differences between short- and long-day birds are unlikely to be due entirely to differences in the time available. However, some of the long-day birds were no more active than the short-day ones suggesting that the skeleton long-day schedule does impose some constraint. In a third experiment photorefractory females (whose ovaries do not secret oestrogen in response to long days) kept on a long skeleton-day scored similarly on all measures of building as birds kept on a full 14 h day and significantly more than birds on a short day. The results also indicate that the birds are measuring daylength by means of a diurnal rhythm of photosensitivity, an effect comparable to that of longer photoperiods on gonadal growth.