Tributing this card is a cost to activate its effect.
Q&A Rulings
Q: The effect of "D.D. Jet Iron" that Special Summons monsters from the Graveyard is activated. If "D.D. Crow" etc. is activated in a chain, and 1 of the 4 "D.D. Esper Star Sparrow" etc. is no longer in the Graveyard, can I Special Summon the remaining 3 with the effect of "D.D. Jet Iron"?
A: Even if one of the four monsters selected as the target of the effect is no longer in the Graveyard by the time the effect of "D.D. Jet Iron" resolves, the remaining monsters in the Graveyard can still be Special Summoned.[2]