Unlike mammals and fish, it is not mentioned in Tora any formal factors which woulld allow to distinct between kosher birds and non-kosher. Pentateuch only limits by specifying 24 species of birds which are prohibited for Jews to eat. (Vayikra, 11:13-19; Devarim, 14:12-18). What is more, over time Jews had forgotten what are these birds and as a result were forced to limit themselves with a small number of birds concerning which existed a stable kosher tradition.