In the documentary, we can see such scenes: newly hatched ducklings can walk and swim with their parents; while the swallows under the eaves can't even open their eyes after hatching and won't leave the nest to move around until more than 10 days after birth. Both ducks and house swallows are birds, so why are there such huge differences between their offspring?
Newly hatched birds can be divided into two basic types according to their ability to forage, move around, and maintain body temperature: precocial birds and altricial birds. Precocial birds hatch with their eyes open, have downy feathers on their bodies, and can follow their parents to forage food within a few hours. Chickens and ducks are typical precocial birds. Most passerine birds, such as house swallows, are altricial birds. Newly hatched birds have their eyes closed, are completely naked, cannot maintain constant body temperature, and rely entirely on their parents for food.
Between these two types, there are also some transitional types, such as gull chicks, which hatch with sparse downy feathers and have some mobility, but generally stay near the nest, belonging to semi-precocial birds. Therefore, different birds have different developmental types, which is the result of long-term adaptation to the environment and natural selection of different species.