Which of the following is true?

A haggard hawk is indeed a bird that has already fledged and is no longer a young eyas, which refers to a hawk that is still in its early life stage and has not yet learned to fly. Therefore, the statement about a haggard hawk being once an eyas is accurate since a hawk transitions from being an eyas to becoming haggard as it matures.

Additionally, considering a passage bird, which is also a hawk that has left the nest and is typically in its second year or older, indicates that it was at some point an eyas when it was fledging. Thus, it is also true that a passage bird was once an eyass before reaching maturity.

Since both statements about haggard hawks and passage birds have a direct link to their juvenile stage as eyasses, the combined option is correct, affirming that both statements A and C are true. This classification helps in understanding the stages of a hawk's development, which is essential knowledge for anyone involved in falconry.