Avian circannual systems: Persistence and sex differences

Birds show adaptation in their physiology and behavior to daily and seasonal periodicities in the environment. A circannual clock system gates seasonal events in birds to happen at the most appropriate time of the year, since a mistiming will delay the event until the arrival of the favorable time next year. The circannual clocks, the self-sustained endogenous system, are expressed under aperiodic conditions with a period close to 12 months. In stonechats, it has been shown that circannual rhythms in testicular and molt cycles persist for 12 years; this suggests that circannual clocks are functional throughout the life of an individual. Circannual rhythms are synchronized with both the photoperiodic and non-photoperiodic cues, and a synchronized circannual rhythm provides information on annual timing of the physiological event. The integration of rigid circannual timekeeping with the cyclic environment ensures phenotypic plasticity that is required for successful survival of a species in its habitat. There can be however sex differences in the circannual rhythm characteristics and in the relationship between circannual rhythms and external environment. In few cases that have been investigated, females appear playing a larger role in defining the breeding season in the year in relation to the environment. There is no evidence for the involvement of circadian clock in the generation of circannual rhythms. Therefore, future researches need to focus on finding molecular gears that possibly form the neuroendocrine loop and are translated into a seasonal event. Here, we briefly review limited information that is available on circannual rhythms and their relationship with the external environment from a few bird species, inhabiting tropical and temperate environments.

• Circannual rhythms are involved in temporal organization of the avian seasonal cycles.
• In synchronization with external cues, circannual rhythms provide precision in the seasonal timing.
• Circadian clocks may not be involved in generation of circannual rhythms.
• Circannual rhythms and photoperiodism are mutually inclusive phenomena.
• Circannual rhythms may have sex-dependent relationship with the environmental zeitgebers.