If enought bits go through the outer section of the Fan then they won't have gone through the burner. Only the inner 2/3 of the air will go down the hot section and the rest bypasses the burner (hence the name high bypass engine) This outer air is moderately cool so they can recover a single feather and identify the bird from that.
I had a strike at EMA one christmas morning. The fire service said it was a Golden Plover but ATC said they thought it was a Turkey.
