Nest building was induced in ovariectomized photosensitive canaries with a standard dose of oestradiol (0·05 mg thrice weekly) at various times of year. On a range of daylengths between 6 and 18 hr light/day, building behaviour tended to be more intense if the birds were exposed to long photoperiods. A similar result was found in photorefractory birds, both intact and ovariectomized.
Differences in building were unlikely to have been due solely to the amount of time available to birds on different photoperiods as hourly building rates still revealed a difference between groups. A marked difference in the latency of the response to oestrogen was found in the refractory birds.
Ovariectomy was not complete in many birds but the evidence from such birds and from the few which had been totally ovariectomized, as well as from those in the refractory period when long days did not result in ovarian growth, showed that endogenous oestrogen secretion was negligible.
The effect of long photoperiods in augmenting the induction of nestbuilding by oestrogen could not be reproduced by injections of either ovine or avian LH. This and other evidence suggests that a direct effect of gonadotrophins is not involved.
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