Adam Clarke on Genesis 8:7

He sent forth a raven, which went forth to and fro - It is generally supposed that the raven flew off, and was seen no more, but this meaning the Hebrew text will not bear; ויצא יצוא ושוב vaiyetse yatso vashob, and it went forth, going forth and returning. From which it is evident that she did return, but was not taken into the ark. She made frequent excursions, and continued on the wing as long as she could, having picked up such aliment as she found floating on the waters; and then, to rest herself, regained the ark, where she might perch, though she was not admitted. Indeed this must be allowed, as it is impossible she could have continued twenty one days upon the wing, which she must have done had she not returned. But the text itself is sufficiently determinate.