Every spring, billions of birds launch journeys spanning thousands of miles — and it starts with light. As the photoperiod changes and days grow longer, specialized photoreceptors in birds' brains detect the shift in daylight. This triggers a cascade of hormonal changes that prepare the body for the enormous demands ahead: fattening, muscle growth, and zugunruhe, or migratory restlessness. These changes reflect a deeply embedded circannual rhythm that alters birds’ behavior and sends them skyward.
One hormone doing much of the heavy lifting is corticosterone. A research team experimentally elevated corticosterone levels in Snow Geese (Anser caerulescens atlanticus) during spring stopovers and found it led to increased foraging and earlier departure dates — direct evidence that this hormone helps translate seasonal cues into migratory action. Another research group looked at testosterone levels in Western Subalpine Warblers (Curruca iberiae) while at a stopover site. Their findings suggest that birds continue to change even during their migration journey, and that the conditions they encounter along the way impact their hormonal state and breeding readiness.