These shall be detestable to you from among birds… (Leviticus 11:13)
Birds are different from animals and fish in that we are not given signs to determine which birds are kosher. Rather, we are given a lengthy list of birds not to eat. The list runs from verse 13 through verse 19 and it includes twenty different species, plus sub-species (for a total of 24 – see Talmud Chulin 63a). The catch is that most of us are not ornithologists and we’re not experts in Biblical Hebrew – let alone both of these things – so identifying a “peres,” a “duchifas” or a “bas-haya’anah” is a tricky proposition at best. As such, we only eat birds that have a tradition of being kosher.
There happen to be signs by which kosher birds can be identified. For starters, they are not scavengers. Additionally, kosher birds have a crop (part of the digestive system), a gizzard with a thin layer that can be peeled, and an extra toe. A kosher bird on a wire will have three toes in front and one in back. The eggs of kosher birds are identifiable in that one end is narrower than the other. (All of this information was useful in establishing the turkey as a kosher bird despite lack of a received tradition to that effect.)
Kosher birds include chickens, ducks, geese, doves, and quail, among others. Non-kosher birds include owls, pelicans, eagles, ostriches, vultures and more. (Bats are included among non-kosher birds rather than among non-kosher animals because the Torah categorizes animals into “land animals,” “sea creatures,” “creeping things” and “flying things,” etc. rather than as reptiles, mammals, amphibians, etc. A bat is a flying thing even though it is a mammal. Similarly, a mouse and a lizard are both creeping things, even though one is a mammal and the other is a reptile.)
The reason underlying this mitzvah is the same as the one underlying all forbidden foods: God warns us about eating things that would be spiritually harmful to us. Just as shellfish are bottom feeders, non-kosher birds are predators and scavengers, not worthy of emulation.
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in the third chapter of tractate Chulin, particularly on pages 61a-65a. It is codified in the Shulchan Aruch in Yoreh Deah 82. It is #174 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #94 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.