The damage required is cumulative and non-repeatable from all opponent attacks that turn. If you take at least 7 damage, you get 1 Duck/Swan that turn and that's it. Due to Life's abilities to maximize creature potential (Empathic/Feral Bond, Mitosis/Mitosis^, Adrenaline/Epinephrine, and by extension Green/Life Nymph) and heal the damage taken (Empathic/Feral Bond, Heal/Improved Heal, Shard of Gratitude/SoG^) I think summoning any more than one free creature per turn would be too much and as such have not included scaling the damage to summon more birds.
Something I see now is a need to address when in the turn order this summon takes place. If the creature is summoned as soon as the threshold is crossed, this could be caused by an enemy spell, which leaves it open to being targeted. Additionally, if an enemy creature affected by Antimatter causes you to gain back health such that you haven't lost 7 total, that really shouldn't be counted.
My current thought is to adjust the wording so that you get the summon at the end of your opponent's turn and before yours in order to bypass both of these. Anyone else have an opinion on when in the turn this happens?