The effect of “Black Whirlwind” starts a Chain. If an effect is chained, so that the Summoned “Blackwing” monster is no longer face-up on the field when the effect of “Black Whirlwind” resolves, you cannot add a monster to your hand.
“Black Whirlwind” looks at the ATK of your newly-Summoned monster as it is on the field. If a Field Spell Card is increasing the monster’s ATK, you can search your Deck for a “Blackwing” monster that has less ATK than the increased ATK value.
If you control multiple “Black Whirlwinds,” you can activate all of their effects. They form a single Chain, and you add 1 “Blackwing” to your hand as each one resolves.
If control of the Summoned “Blackwing” monster shifts during or before resolution of “Black Whirlwind,” you can still add a card to your hand because you can still see the ATK.
When resolving the effect of "Black Whirlwind", you add a "Blackwing" monster to your hand with less ATK than the ATK of the Normal Summoned monster while the Normal Summoned monster is on the field.
"Black Whirlwind" can only add to your hand a monster with less ATK than the ATK of the Normal Summoned monster. It cannot add a monster with equal ATK. When you Normal Summon "Blackwing - Gale the Whirlwind" and you do not have a monster with less than 1300 ATK in your Deck, then you cannot activate the effect of "Black Whirlwind".