My daughter is a very smart young woman. But love does strange things to people. She married this worthless bird-brain who has trouble adding up two and two and getting four.

他認為女兒是個聰敏能干的年青女人，但是被愛情沖昏頭腦嫁給這一無是處的女婿。他說女婿甚至連二加二等于四都弄不清。

這當然是他在夸張地描述女婿的蠢笨，可見他說女婿是bird-brain，意思是他是個沒頭腦、愚蠢的人。這就是bird-brain這個習慣用語的含義。