What triggers animal migration?

The answer

Animal migration is primarily triggered by photoperiod, the changing length of daylight hours, which activates hormonal cascades that prepare the body for travel. In birds, lengthening spring days stimulate the hypothalamus to release hormones that increase appetite, promote fat deposition (fuel for migration), and trigger migratory restlessness called Zugunruhe. Temperature, food availability, and weather patterns serve as secondary or fine-tuning cues. In some species, genetic programming determines the general timing and direction of migration; hand-raised birds that have never seen the night sky still orient correctly. Wildebeest in the Serengeti follow rainfall patterns, whales track water temperature gradients, and some fish respond to river flow changes. Climate change is increasingly disrupting these ancient cue systems, causing mismatches between migration timing and food availability.