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.

他认为女儿是个聪敏能干的年青女人，但是被爱情冲昏头脑嫁给这一无是处的女婿。他说女婿甚至连二加二等于四都弄不清。

这当然是他在夸张地描述女婿的蠢笨，可见他说女婿是bird-brain，意思是他是个没头脑、愚蠢的人。这就是bird-brain这个习惯用语的含义。