Only what I learned in China: EVERY bird is edible!
-Blast
Ummm ... yes and no and maybe.
Most birds are okay to eat. But a lot depends on what the birds eat and how toxic their food is and how much they eat. Even common birds can eat berries that would sicken a human. If you don't clean the bird properly this could be a problem.
Vultures have been reported to take in so much of the bacteria that cause food poisoning that you can safely eat them only after they are well cooked. Hyena have similar attached lore.
Albatrosses have been reported to sometimes eat so much toxic jellyfish that it isn't stomach contents or bacteria that get you but rather the flesh itself may become poisonous. Some old stories claim they are irredeemably poisonous and should never be eaten. A few more recent reports say that cleaning, soaking in several changes of sea water, and cooking will allow them to be eaten safely. They also claimed the bird tasted terrible.
Way back in the day I was taught that 'If it walks, swims, slithers, or flies you can eat it'. That isn't wrong in the sense that of the thousands of species only a few are problematic. Of those dangerous few it may be just one part or another you need to avoid. Like the liver of the polar bear which has so much vitamin-A that it can kill. Or fugu, blow fish, which is delicious if you avoid the parts which contain a strong nerve toxin.
It comes down to local knowledge of the dangers and how food is prepared to avoid any danger. The bit about polar bear liver is nothing but an interesting factoid if your in the lower forty-eight. But it might save your life if your surviving in Alaska. In the end survival is a local affair.
Rules of thumb about all birds being edible are pretty safe. The odds are heavily on your side. But if you camp, sail or fly over an area try to get local knowledge on what to eat and specifically how to prepare local game. Properly prepared most every animal, even most of the toxic ones, can be eaten. But even properly prepared there may be exceptions.