The thrush lands on top of the spider, hoping its claws will provide a nice meal. The roll is 10 +2 (for charge) +1 (for higher ground) = 13. The spider dodges out of the way.
Round 5:
The spider tries to bite the bird. The roll is a natural 20 and the critical threat roll is 12. The thrush's momentum carries it into the fangs of the spider! The critical hit still only does 1 point of damage to the thrush, which still leaves it with 2 hp, but the bird succumbs to the poison (rolled a 5) and loses a point of strength!
The thrush is immobilized and helpless!
Round 6:
The spider performs a coup de grace on the poor bird. It takes another point of damage but makes its save and still lives! (Rolled a 14).
Round 7:
The spider tries again to finish off the thrush. This time the bird is put out of its misery. (Rolled a 5).
The spider is in the hall of fame!
Next up for NG: An aasimar (planetouched).