Final answer:
The subject of this question is related to animal behavior, specifically the feeding behavior of birds.
Step-by-step explanation:
In biology, the subject of this question is animal behavior, specifically related to the behavior of birds seeking food for their young.
The passage describes a nest of four baby birds that are hungry and open their mouths, indicating that they are ready to be fed by their parent bird.
This behavior is common among birds, as parents must find and retrieve food, such as worms or insects, to feed their offspring.