Genesis 8:7

And he sent out a raven; and it went forth, going forth and returning, until the waters had dried from the earth.

וישלח את העורב, to find out if the atmosphere had dried out after the tops of the mountains had become visible. Noach wanted to know if the atmosphere in the meanwhile was such that the raven could tolerate it.

ויצא יצוא ושוב, this proved that the atmosphere was not yet dry enough for the raven to tolerate it for any extended period of time.

וישלח, why did Noach send out the raven? He had said to himself that the raven is a flesh eating bird and will most likely find remains of dead animals to feed on. [our author understands the word עורב as referring to what is better known as the “black raven,” a type of vulture (corvus cornix) which feeds on carcasses. Ed.] If the bird would return with such remains in its beak, Noach would know that the waters had receded somewhat. The raven returned without anything in its beak, so that Noach did not learn anything new about the state of the waters. The raven returned to its nest, leaving from time to time to see if the situation outside the ark enabled it to survive on the outside. It carried on in this fashion until the surface of the earth had dried out. In Bereshit Rabbah 33,5 the sages view the fact that the raven was chosen as the experiment by Noach in a different light. The blackness of that bird is paraphrased in Psalms 105,28 where the psalmist writes שלח חשך ויחשיך, “he sent darkness and it became dark.” [a figure of speech, meaning that if one pins one’s hopes on something black, one is likely to receive darkness in return, i.e. not achieve one’s objective. Ed.] Seeing that this mission of the raven had proved totally unproductive, after waiting seven days, Noach sent out a pigeon. A pigeon is known to have a well developed sense of imagination and direction, finding its way back after having flown over a route only once or twice. It knows that it was dispatched for a purpose, i.e. to receive some information in return for having been dispatched [we know of carrier pigeons in our time, but if there had been such already in the days of Noach is highly speculative. Ed.] Moreover, it is in the character of pigeons to be attached to their home base, so that they can be depended on to return to their nest. When we have been told in Shabbat 49 that the wings of the pigeon protect it, the meaning is that seeing that the pigeons are being raised as carrier pigeons, transporting messages attached to its wings, the fact that their owners need them alive is their insurance against being slaughtered prematurely. Noach, in sending out the pigeon, was therefore convinced that the bird would return to the ark even if there were places on earth in the meantime where it could make its nest. ולא..ותשב אליו אל התבה, the Torah describes the return of the pigeon both as a return to Noach, i.e. אליו, and as a return to the ark, i.e. אל התבה. Why was the return of the pigeon described in such detail? It is first described as a return to Noach, seeing that the fact that it did not bring back any bit of vegetation indicated that it had been unable to find any, i.e. had not performed its mission and brought Noach information. Secondly, it did not re-enter the ark as the raven had done. It remained perched on the roof of the ark. It was unhappy at not having brought back some kind of message to Noach. This is why Noach stretched out his hand and brought it back into the ark. The amount of time that elapsed between the dispatch of the raven, and the dispatch of the pigeon, was seven days. How did we arrive at this information? ויחל עוד שבעת ימים אחרים, the addition of the word אחרים, “others,” proves that there had already been a waiting period of seven days