Migration in birds isn’t just an instinctive urge to fly south or north — it’s a finely tuned physiological response to internal and external cues that prepare a bird’s body for one of the most energetically demanding events of the year. Bird migration can be viewed through a “push–pull” framework where internal changes pull birds toward their seasonal grounds, while external, environmental conditions push individuals to begin their journeys.
A recent study of Mallards suggests that spring migration is more strongly influenced by internal “pull” cues, including hormonal and reproductive readiness changes triggered by longer days, while autumn migration is more influenced by external “push” factors like deteriorating weather. Those internal changes to a bird’s body can be significant. Birds gain weight and build fat reserves, some gaining as much as 50% of their body weight, in a process called hyperphagia. They sleep less and their internal organs may change, with some species making room for fat stores or increasing the size of their hearts for better endurance. Researchers are exploring how a better understanding these physiological changes may have human health implications.