Abstract
The paper discusses how birds by changing their responses to the critical daylength can adjust the time for the initiation of the gonadal growth to more northern latitudes. Some remarks are also given on how changes in an individual's sensitivity towards non-photoperiodic factors, such as, for example the temperature, play an important role in adapting the gonadal growth rate to the flexibility of the local environment. Special attention has been given studies on swans and great tits.

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