What is the original meaning of "birdbrained" and what does it mean here in the first para

What is the original meaning of "birdbrained" and what does it mean here in the first paragraph？

A．Naive; intelligent.

B．Simple-minded; volant.

C．Narrow-minded; navigational.

D．Stupid; adapted for flight.

请帮忙给出正确答案和分析，谢谢！