In the flood story in Genesis, the two birds released by Noah, after the ark had became grounded on a mountain, become the focus of several verses. [Gen. 8:6-12]
Ravens are large black birds, and they are inquisitive, cunning, and
bold. They are solitary until they find a mate, after which they may
societies. Mating is probably for life. Pliny noted their ability to
speak. They feature in many myths; one says that England will fall, if
ever the ravens abandon the Tower of London. Ravens with clipped wings
are still kept there.
They were declared unfit to eat in Lev. 11:15.
It is reported that Arabs call the raven Abu Aajir, the Father of Omens. 
In Greek mythology, ravens serve as messengers for Helios and Apollo.
To the Egyptians, ravens represented destruction and malevolence. "To have a raven's knowledge" is an Irish proverb meaning to have a seer's powers.
They have also been connected symbolically to witches, evil and death. If corpses were left unburied, ravens could pick out the eyes.
Ravens are known to hunt cooperatively with wolves, lions, or
coyotes. Ravens will alert wolves to prey, wait for the kill, then
feed. Ravens rely on lions and wolves to tear open a carcase.
Wolves and ravens have been seen to engage in playful behaviour.
Gene Stratton-Porter wrote: 
Almost all Biblical mention of the raven was made in the shape of simile and metaphor, but these allusions served to drive home a point and make a thing well remembered, which was the reason they were used. They also grounded a feeling against the bird, just as similar things have prejudiced the unthinking against the owl and hawk. The raven is a curious bird, and at different epochs in the world has figured in much interesting history.
An interesting trait of ravens is their power of mimicry. Warner
Shedd & Trudy Nicholson noted that a scientist spent six years
teaching a raven to say "nevermore" as in Edgar Allen Poe's poem. They
...there are incredible reports of ravens performing amazing feats of mimicry. Scientist Bernd Heinrich has done some outstanding work with ravens over the past fifteen years and contributed enormously to our understanding of these fascinating birds. In his book Mind of the Raven, Heinrich cites reliable sources for such things as a raven imitating radio static, a motorcycle revved up, the sound of flushing urinals, and -- most astonishing, an imitation of a demolition expert saying "three, two, one," followed by a reasonable facsimile of a dynamite explosion.
The raven was sent out from the ark by Noah, when the waters of the flood were retreating from the earth, and it "went forth to and fro, until the waters were dried up from off the earth." [Gen. 8:7]
Later, Noah sent out a dove.
Also he sent forth a dove from him, to see if the waters were abated from off the face of the ground; But the dove found no rest for the sole of her foot, and she returned unto him into the ark, for the waters were on the face of the whole earth: then he put forth his hand, and took her, and pulled her in unto him into the ark.
The above verse mentions "rest," and other scriptures associate doves with "rest." Seven days later, Noah sent the dove out again:
And the dove came in to him in the evening; and, lo, in her mouth was an olive leaf pluckt off: so Noah knew that the waters were abated from off the earth.
A dove with an olive branch in its mouth is a well-known symbol of peace.
The third time Noah sent out the dove, it did not return to him. [Gen. 8:12] Probably, Noah expected that it would return to him.
In later scriptures ravens brought food for Elijah, to sustain him during the three and a half years of famine in Israel.
4 And it shall be, that thou shalt drink of the brook; and I have commanded the ravens to feed thee there.
5 So he went and did according unto the word of the LORD: for he went and dwelt by the brook Cherith, that is before Jordan.
6 And the ravens brought him bread and flesh in the morning, and bread and flesh in the evening; and he drank of the brook.
Doves are connected with "rest" by David, who longed for "the wings of a dove."
And I said, Oh that I had wings like a dove! for then would I fly away, and be at rest.
Species of the dove or pigeon family (Columbidae) are widely
distributed in the earth, including oceanic islands. Isaiah pictured
the saints as doves. Referring to the scripture below, Gene
Stratton-Porter suggested that Isaiah referred to semi-domesticated
pigeons, which were housed in cotes of clay with openings that appeared
like small windows, to which the birds flocked like clouds, as they
brought food for their young. 
Who are these that fly as a cloud, and as the doves to their windows?
Hosea referred to doves as "silly," and "without heart." [Hos. 7:11-13]
When Jesus was baptized by John the Baptist, he saw the holy Spirit descending on him "like a dove." [Mat. 3:16] Is this an allusion to the flood story? Perhaps the Spirit descending on Jesus like a dove is connected with the third release of the dove by Noah in the flood story.
Jesus told his disciples to be "harmless as doves." [Mat. 10:16]
Doves and pigeons produce "crop milk," which is a highly nutritious substance that they feed to their young.
Since the Holy Spirit is pictured by a dove, which contrasts with the raven in the flood story, perhaps the ravens represent other spirits.
The three and a half years of famine in the days of Elijah, when he was fed by the ravens, is typical of the entire church age. Elijah's ministry pictures God's Spirit in the church age, when there is no further revelation, or "rain." The Holy Spirit is present with the church, but often remains hidden, as Elijah hid from Ahab and Jezebel.
In the Bible, the ravens may represent spirits other than the Holy Spirit, or in other words, delusions. One very common delusion says unbelievers suffer unending infernal torment, which is inconsistent with the harmless nature of a dove.
Such delusions lead to spiritual blindness. There may be a spiritual application to the following proverb:
The eye that mocketh at his father, and despiseth to obey his mother, the ravens of the valley shall pick it out, and the young eagles shall eat it.
Isaiah said God chooses the delusions of men.
I also will choose their delusions, and will bring their fears upon them; because when I called, none did answer; when I spake, they did not hear: but they did evil before mine eyes, and chose that in which I delighted not.
In the days of Elijah, Jezebel hunted and killed the prophets of God. She supported 450 false prophets.
Elijah's prayer raised the son of the widow Sarepta back to life. [1 Kings 17:17-23; Luke 4:26] In this story, there is a type picturing the Holy Spirit, which abides with the church, having raised up Jesus from the grave. Malachi's prophecy about Elijah is fulfilled by the Spirit of God that is given to the church. [Mal. 4:5]
In John's prophecy in Rev. 11, the two witnesses have powers similar to those of Moses and Elijah; their ministry is for 1,260 days, which is equivalent to the three years and six months for which the heaven was shut in Elijah's time. Rain is symbolic of the word of God. [Isa. 55:10-11]
The 1,260 days and the time,
times and a half in Rev. 12 are symbolic of the entire church
age, in which there has been no further revelation; this is pictured by
heaven being shut.
The two witnesses can be identified with the Spirit of God, and the
scriptures; Jesus said these would "testify" of him, and one who
testifies is a "witness."
Like animals such as sheep, wolves, serpents, lions, and eagles, the
raven has a symbolic meaning in the Bible. They are a striking
contrast to the dove, a symbol of peace, and rest, and the Holy Spirit.
2. Birds of the Bible, by Gene Stratton-Porter.
New York, Eaton and
Mains. 1909. p. 406.
3. Owls Aren't Wise & Bats Aren't Blind: A
Favorite Fallacies about Wildlife, by Warner Shedd, Trudy Nicholson.
Random House of Canada, 2001. p. 135.