A monster flipped face-up by a card effect, such as "Book of Taiyou" or "Stop Defense", or as a result of battle is notconsidered a Flip Summon. Only flipping a Set monster manually is considered a Flip Summon.
So if you attack a face-down Stealth Bird, it will be flipped but not Flip Summoned, so its effect will not activate.
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A monster in face-down Attack position is unable to declare attacks. It can be flipped or Flip Summoned, both actions will switch it into face-up Attack position. It can also have its battle position change by the controlling player, switching it from face-down Attack position to face-down Defence position.
No monster can activiate its effect while face down unless it specifically says so; only face up.If you are referring to monsters who say their effects activate when Flip Summoned, that means you must flip the monster up manually during your Main Phase after it is set face down. A monster fliped face up from an attack does not count as a Flip Summon.
No. Monsters are only ever flipped if they are Flip Summoned, attacked while face-down, or when a card effect specifically says they are flipped. Monsters are never considered flipped if destroyed, or tributed while face-down.
Setting is similar, yet not identical to normal summoning. a normal summon is when you Summon a monster onto the field in face-up attack position, whereas a set is when you normal summon a monster in face-down defense position. When a monster is set, it is not yet considered a summoned monster.
You can manually change the battle position of a monster once per turn, (but not on the turn it was set or summoned from hand) If you flip a monster face down with Book of Moon or use the effect of Sonic Bird, then you are allowed to flip summon them in the same turn.