Overall I approve the thoughts you brought into this card, especially that you try keeping it down, but thats the problem with Magic:
Cards that spam +1/+1 Counters are everywhere in Magic, so your card wouldn't be played at all. Instead people would use Village Cannibals etc.
Creatures that basically become invincible by the mass of counters of them is a common thing in Magic.
Don't be afraid of your cards getting out of control; As long as they are bound to triggers, they require strategic dexterity and are totally balanced.
You should really remove the instant-sacrifice or add a manacost; It's totally broken. The only weakness of the crow is its low toughness, but whenever you try to shock it, it just sacs itself by shift of priority.