Final answer:
The second essential feature of an instinct is that it is unlearned, meaning it is a behavior that organisms are born with and do not require learning to exhibit.
Step-by-step explanation:
The second of the three essential features of an instinct is that it is unlearned. This characteristic is fundamental to instinctual behavior, as opposed to learned behavior, which occurs only after experience or practice. Birds building nests, infants suckling for nourishment, dogs shaking water off their fur, salmon swimming upstream to spawn, and spiders spinning webs are all examples of unlearned behaviors that occur naturally in organisms. These innate behaviors are genetically hardwired and do not require prior learning or experience to manifest in the appropriate context.
Instincts are more complex than simple reflexes and involve the movement of the organism as a whole, utilizing higher brain centers. They allow an organism to respond to stimuli, ensuring survival without the need for learning processes.