Photoperiodic control of seasonality in birds

This review examines how birds use the annual cycle in photoperiod to ensur e that seasonal events-breeding, molt, and song production-happen at the ap propriate time of year. Differences in breeding strategies between birds an d mammals reflect basic differences in biology. Avian breeding seasons tend to be of shorter duration and more asymmetric with respect to changes in p hotoperiod. Breeding seasons can occur at the same time each year (predicta ble) or at different times (opportunistic), depending on the food resource. In all cases, there is evidence for involvement of photoperiodic control, nonphotoperiodic control, and endogenous circannual rhythmicity. In predict able breeders (most nontropical species), photoperiod is the predominant pr oximate factor. Increasing photoperiods of spring stimulate secretion of go nadotropin-releasing hormone (GnRH) and consequent gonadal maturation. Howe ver, breeding ends before the return of short photoperiods. This is the con sequence of a second effect of long photoperiods-the induction of photorefr actoriness. This dual role of long photoperiods is required to impart the a symmetry in breeding seasons. Typically, gonadal regression through photore fractoriness is associated with a massive decrease in hypothalamic GnRH, es sentially a reversal to a pre-pubertal condition. Although breeding seasons are primarily determined by photoperiodic control of GnRH neurons, prolact in may be important in determining the exact timing of gonadal regression. In tropical and opportunistic breeders, endogenous circannual rhythmicity m ay be more important. In such species, the reproductive system remains in a state of "readiness to breed" for a large part of the year, with nonphotic cues acting as proximate cues to time breeding. Circannual rhythmicity may result from a temporal sequence of different physiological states rather t han a molecular or cellular mechanism as in circadian rhythmicity. Avian ho mologues of mammalian clock genes Per2, Per3, Clock, bmal1, and MON have be en cloned. At the molecular level, avian circadian clocks appear to functio n in a similar manner to those of mammals. Photoperiodic time measurement i nvolves interaction between a circadian rhythm of photoinducibility and, un like mammals, deep brain photoreceptors. The exact location of these remain s unclear. Although the eyes and pineal generate a daily cycle in melatonin , this photoperiodic signal is not used to time seasonal breeding. Instead, photoperiodic responses appear to involve direct interaction between photo receptors and GnRH neurons. Thyroid hormones are required in some way for t his system to function. In addition to gonadal function, song production is also affected by photoperiod. Several of the nuclei involved in the song s ystem show seasonal changes in volume, greater in spring than in the fall. The increase in volume is, in part, due to an increase in cell number as a result of neurogenesis. There is no seasonal change in the birth of neurons but rather in their survival. Testosterone and melatonin appear to work an tagonistically in regulating volume.