What term describes a bird's relaxing behavior with feathers slowly erecting?

The term that describes a bird's relaxing behavior where its feathers slowly erect is known as "rouse." This behavior indicates that a bird is comfortable and at ease, often following a period of rest or when it is preparing for preening. During rousing, the bird fluffs up its feathers, which helps to realign them, lift dust or debris, and maintain body temperature by trapping air within the down feathers. It is also part of the natural grooming process that contributes to feather health and insulation.

In contrast, the other terms refer to different behaviors not associated with the relaxing state indicated by "rouse." For instance, "gorge" typically refers to the act of consuming food excessively. "Hack" is a term used to describe a method of training or releasing a bird, and "rangle" does not correspond to a recognized behavior in this context. Thus, "rouse" is the most fitting choice to describe the described behavior of a bird.