Or HaChaim on Genesis 8:9/אור החיים על בראשית ח׳:ט׳

1 וישלח ידו ויקחה ויבא אותה אל התבה. He extended his hand, took her (the dove) and brought her to him into the ark. The verse tells us that the dove was exhausted from its mission, not having found a place to land. Noach was worried that the dove did not have strength enough to enter the ark on her own and would fall into the water and drown.