A penalty try is awarded in cases where a try would have been scored but wasn’t solely due to an act of foul play.
A possible 8 point try is awarded in cases where an act of foul play occurs against a try scorer while he is in the act of scoring - in other words, while he is grounding the football.
Bird was hit high before he was considered in the act of scoring, and he scored the try. In this situation, a penalty try cannot be awarded because he scored a try, and a possible 8 point try cannot be awarded because the act of foul play occurred prior to the act of scoring.
Hope that helps.