Answer: Assimilation
Explanation: According to the theory of Piaget, assimilation is defined as addition of information to the already present schemas or knowledge due to new experience or data. Children tend to add-on new experienced information with the existing concept to modify the schema accordingly.
Alex has also represented this act by seeing a flying squirrel and remarking it as bird with tails because he might be familiar with bird flying and so he remarked the squirrel as a bird with extra features .Thus, the correct answer is assimilation