Neural control of daily and seasonal timing of songbird migration
Bird migration is one of most salient annual events in nature. It involves predictable seasonal movements between breeding and non-breeding habitats. Both circadian and circannual clocks are entrained by photoperiodic cues and time daily and seasonal changes in migratory physiology and behavior. This mini-review provides an update on daily and seasonal rhythms of migratory behavior, and examines the neuroendocrine and molecular pathways involved in the timing of migration in songbirds. Recent findings have identified key neural substrates, and suggest the involvement of multiple neuroendocrine regulatory systems in controlling seasonal states in migrants. We propose that four distinct neural substrates are involved in the timing of migration and include (1) pineal gland and suprachiasmatic nucleus (mSCN); (2) a cluster of hypothalamic nuclei, the mediobasal hypothalamus (MBH); (3) dorsomedial hypothalamic nucleus (DMH); and (4) tanycytes along ependymal layer of the 3rd ventricle (3V). Cluster N, a nucleus in the telencephalon involved in the integration of geomagnetic cues, likely maintains functional connectivity with brain regions involved in timing songbird migration. These nuclei form an interconnected network that coordinates daily timing (pineal gland/mSCN), annual photoperiodic response (MBH, 3V), energetic state (MBH, DMH, 3V), and magnetic compass information (i.e., cluster N) for migration in songbirds.