Bird brain / bird-brained

Bird brain is US informal for a stupid person, a scatterbrain, a person who lacks the faculties of focus and attention, dates from the 1920s, from the allusion that birds have small brains. Bird-brained was used a little earlier, from the early 1900s, whereas 'bird-witted' has been in use from the early 1600s, but rarely used these days.