Annual migration: Strategy for twice-a-year long-distance travel in obligate latitudinal nocturnal avian migrants

Latitudinal avian migrants show comprehensive changes at multiple levels in response to the prevailing environmental conditions, for example, photoperiod, temperature and food availability. These changes aid in decisions when birds begin their migratory flights. Twice-a-year, changes in identifiable, distinct behavioral and physiological phenotypes favor the generation and overall flux of energy required for the nocturnal migratory flight. For example, the accumulated fat stores via free fatty acids oxidation in the liver and protein-mediated transport supply energy required by the ‘working’ (flight) muscles while birds are in migration flight. However, it is still poorly understood how latitudinal migratory species prepare differentially for two similar seasonal travels; southwards in autumn to escape from the harsh winter condition at breeding grounds, and northwards in spring to reproduce. In this brief article, we aim to provide insights into seasonal plasticities that allow an obligate latitudinal migrant to accomplish annual journeys between nearly fixed destinations, based mainly on research in our laboratories on Palaearctic-Indian migratory buntings over the last four decades.