The term 'BIRD, ONCE' can be solved by three crossword solutions: EGG (3 letters), CELTIC (6 letters), and CELT (4 letters). The solution EGG refers to the fact that birds lay eggs, which is a common trait among most species of birds. The solution CELTIC refers to the CELTIC Bird Goddess, a figure in CELTIC mythology who was associated with birds. Finally, the solution CELT refers to the extinct bird species, the Great Auk, which was once found in the North Atlantic.