One must examine birds one intends to eat to ensure that they are of a kosher species. This is comparable to the way in which animals must be so inspected, as in Mitzvah #153.
The reason for this mitzvah is the same as for Mitzvah #153: Just as certain foods are bad for us physically, God knows which things are bad for us spiritually. He has therefore warned us not to eat these things and instructed us to inspect animals and birds to ensure that they are kosher.
The Ramban (Nachmanides) does not include examining animals or birds among the 613 mitzvos, considering this a necessary precaution to avoid eating non-kosher creatures rather than a separate commandment.
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractate Chulin on pages 61a-64b. It is codified in the Shulchan Aruch in Yoreh Deah 82. This mitzvah is #150 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.