Only usable before range is chosen.
Opposing minion takes 1R damage each round of combat during strike resolution. A vampire can only play one Carrion Crows each combat.
As above, but with 2R damage damage.
Clarifications and Rulings
Carrion Crows does "1R each round" to the opposing minion (card text). The "during strike resolution" just tells you when that 1R per round is resolved. (2R for superior). This is akin to how retainer-based damage is handled. [LSJ 19971211]