The instruction God gave to Israel regarding those birds which can be eaten and those which should not be eaten is found in Deuteronomy chapter 14: 

Deuteronomy 14:11-18

11  "All clean birds you may eat.
12  "But these you shall not eat: the eagle, the vulture, the buzzard,
13  "the red kite, the falcon, and the kite after their kinds;
14  "every raven after its kind;
15  "the ostrich, the short–eared owl, the seagull, and the hawk after their kinds;
16  "the little owl, the screech owl, the white owl,
17  "the jackdaw, the carrion vulture, the fisher owl,
18  "the stork, the heron after its kind, and the hoopoe and the bat.

(See also Leviticus chapter 11 for similar wording.)

Clean birds may be eaten (verses 11) -- but these are only indirectly defined by listing unclean birds that must not be eaten (verses 12-18).

From this list of unclean birds, six characteristics have been identified as separating the clean birds from the unclean birds:

• a clean bird has a craw or crop
• a clean bird has a gizzard with a double lining which can be easily separated;
• a clean bird is not a bird of prey
• a clean bird does not devour food while flying
• a clean bird's hind toe and middle front toe are both elongated
• when a clean bird stands on a perch, it spreads its toes so that three front ones are on one side of the perch and the hind toe on the opposite side.

All unclean birds lack at least one of these six characteristics. 

For information, read our booklet What Does the Bible Teach About Clean and Unclean Meats?