Following the Genesis flood, as the ark was resting on Mount Ararat, Noah released a raven and dove from the ark at different times. The goal in sending these birds was to determine if the flood waters had abated enough for Noah and his family to exit the ark.
In Genesis 8:6-7 we read, "At the end of forty days Noah opened the window of the ark that he had made and sent forth a raven. It went to and fro until the waters were dried up from the earth." The "40 days" here is after the tops of the mountains were visible (verse 5), over seven months after the flood began. A raven was released and apparently never returned. No reason is provided regarding why a raven was selected rather than another bird. However, raven can eat carrion and would feed off dead animals in the water. A dove, on the other hand, would return to its point of origin if no land was found.
Noah sent a dove in Genesis 8:8-9: "Then he sent forth a dove from him, to see if the waters had subsided from the face of the ground. But the dove found no place to set her foot, and she returned to him to the ark, for the waters were still on the face of the whole earth. So he put out his hand and took her and brought her into the ark with him." The dove returned with no indication that a place had been found to alight.
A week later, in Genesis 8:10-11, Noah sent the dove again: "He waited another seven days, and again he sent forth the dove out of the ark. And the dove came back to him in the evening, and behold, in her mouth was a freshly plucked olive leaf. So Noah knew that the waters had subsided from the earth." Things had begun to grow once again; the earth was becoming more habitable.
Another week passed. Then, in Genesis 8:12, Noah sends out the dove one more time: "Then he waited another seven days and sent forth the dove, and she did not return to him anymore." The dove had no need to return to the ark, since it had found a home on land. The ark could soon be emptied, and humanity could begin to establish itself again in the world.
From Genesis 7:11 to 8:14, we know that the flood lasted a total of one year and ten days. The raven and the dove were released over a period of 21 days after the mountaintops became visible (Genesis 8:10-12). The raven served as a first attempt to discover dry land, and the dove became Noah's way of determining when to leave the ark.
Culled from : http://www.gotquestions.org/Noah-raven-dove.html