Ardea
Official journal of the Netherlands Ornithologists' Union

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Dawson A. (2002) Photoperiodic control of the annual cycle in birds and comparison with mammals. ARDEA 90 (3): 355-367
This review attempts to describe how birds perceive photoperiod and how this controls their physiology so that breeding and moult occur at the appropriate times of year, and to contrast this with the situation in mammals. The breeding seasons of birds tend to be shorter and more asymmetrical with respect to change in photoperiod than those of mammals. In mammals, the eyes, pineal gland and a circadian clock interact to produce a daily pattern of melatonin secretion that changes seasonally. This is used to time gonadal maturation and regression. In birds, the eyes and pineal are not involved. Instead, photoreceptors within the hypothalamus directly interact with gonadotrophin-releasing hormone (GnRH) neurones to regulate GnRH secretion. In most non-tropical birds, GnRH secretion increases as photoperiod increases leading to gonadal maturation. Later, long photoperiods inhibit GnRH synthesis, which results in gonadal regression and photorefractoriness. This dual role of long photoperiods, firstly increasing GnRH secretion and then inhibiting GnRH synthesis, leads to asymmetry of breeding seasons. Long photoperiods also cause prolactin secretion, and this may act to fine tune the timing of gonadal regression in advance of the decrease in GnRH synthesis. Prolactin may also induce the slight asymmetry in the breeding seasons of non-photorefractory species. In tropical and opportunistic birds, endogenous circannual rhythmicity may play a more pronounced role. Moult is also caused by long photoperiods, but in addition, there is a close physiological relationship with breeding, such that moult normally starts immediately after breeding ends. The nature of this relationship remains unclear


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