Answer:
According to the biological concept of species, this statement confirms that the Big Bird lineage is a separate species.
Step-by-step explanation:
The biological species concept states that species are groups of organisms capable of interbreeding and producing fertile offspring. Ernst Mayr (1970) literally stated that "species are groups of interbreeding natural populations that are reproductively isolated from other such groups". This statement is consistent with the lack of gene flow and reproductive isolation. In consequence, the biological concept of species is supported by above described, i.e., Big Bird males breed only with Big Bird females.
You can read this book:
Mayr, E. (1970). Populations, species, and evolution: an abridgment of animal species and evolution (Vol. 19). Editorial: Harvard University Press.