Humans usually have only one child at a time, so they will feed and nurture them very carefully. However, the situation of birds is different. Birds have to focus on giving birth in spring and summer when they have the most food, so many birds don't give birth to more than one offspring at a time. Among the babies hatched at the same time, how does the parent bird decide which chick to feed first? Is the feeding ordered or random?
Scientific research has found that birds raise the next generation out of instinct. When the parent bird returns to the nest with insects or other food, the hungry chicks will stretch their necks and eagerly make a cry for food. They open their big mouths or show particularly bright red or yellow around their mouths. These signals strongly stimulate the parent bird's feeding instinct. At this time, those chicks with particularly loud calls are usually more likely to attract the attention of their parents, so they can easily obtain food and grow stronger in the future.
The parent birds will not feed the nestlings that stay in the nest at the same time without opening their mouths or singing loudly enough. This can really be regarded as "barking children have milk". Conversely, those chicks who can get more food resources through loud calls are inherently healthier, while those chicks whose calls are not loud enough may be eliminated by nature or easier to be caught by predators in the future because they can't get enough nutrition in the case of insufficient food resources, which is also in line with the law of natural selection of the survival of the fittest.
In addition, some studies have shown that the throat structure of young birds is relatively special, and the swallowing action of young birds begins to slow down when they are about to eat. The parent bird sometimes casually puts food into a young bird's mouth, and then pays attention to observation. If the young bird does not swallow the food immediately, the parent bird will take the food out of the young bird's mouth and give it to another young bird. Young birds that swallow food immediately must be the most hungry.