"Bird-brained" is a term that means someone is forgetful, foolish, or lacks intelligence. There are several synonyms available for this term, such as "scatterbrained," "airheaded," "ditsy," "spacey," or even "absent-minded." All of these words describe an individual who is not operating at full mental capacity or has shown some level of inefficiency in decision-making. Similarly, "dumb," "chickenheaded," or "dim-witted" can also be used as synonyms for "bird-brained." However, it's essential to note that these words are often used casually, and people should try to avoid using them as it can come off as insensitive or offensive to those with certain disabilities.