In order to make the impulse have the same effect on the sprite node regardless of it's state, you could try setting it's velocity
property to zero. Like so:
_bird.physicsBody.velocity = CGVectorMake(0,0);
[_bird.physicsBody applyImpulse:CGVectorMake(0, 15)];
This should make the 'bird' (Flappy Bird?) behave the same way in the any situation.