Migratory birds, such as geese, migrate back and forth between their breeding grounds and wintering grounds every year for survival, and their migrations exhibit a strong temporal rhythm. Ancient Chinese people noticed the relationship between migratory birds' migration and the change of seasons long ago. "Several early warblers are competing for warm trees, and whose new swallows are pecking at spring mud" describes the scene of early spring, while "When will the wild geese arrive, and the autumn water in rivers and lakes will be abundant" foreshadows the arrival of autumn. In some remote rural areas, migratory birds play a guiding role in agricultural production. For example, the Miao people in the Hengduan Mountains decide when to transplant rice seedlings based on the time when the golden-bellied swallows migrate back. So, how do migratory birds know when to start migrating?
This may be closely related to another question: why do birds migrate? There is currently no standard unified answer to this question. There are factors related to food and temperature. For some birds, a place is usually not suitable for their year-round habitation. They need to breed in the north where food resources are abundant in summer to better raise their offspring, and winter in the south where the climate is relatively warm and food is more secure. However, researchers have found that the migration timing of some birds seems to be unrelated to changes in food and temperature. They start migrating long before changes in temperature and food resources occur. Therefore, some studies suggest that the length of daylight is also an important factor affecting migratory birds' migration. In spring, the length of daylight gradually increases in the Northern Hemisphere, and the level of melatonin in birds' bodies decreases while the level of sex hormones increases. The urge to reproduce prompts migratory birds to return to the north. Food, temperature, and daylight are external factors affecting bird migration. In fact, what really determines when birds start migrating is their internal biological clock, which has been highly sensitive to food, temperature, and daylight over the years through co-regulation.
Migration is a race against time, neither too early nor too late. Studies show that although geese with earlier egg-laying have a higher hatching success rate and can raise more offspring, premature migration is undoubtedly a risk for adult birds. What awaits them in the north may be a mild spring or still-icy landscapes. If migration is too late, the hatching time of geese's chicks will be later, and they will face the risk of food shortage. Choosing an appropriate time to start migration is particularly important for migratory birds.