What is the symbolic signinficence of crows?

As a child I remember my Aunt telling me this poem for the omens associated with the number of crows seen at a time.

One is for bad news, Two is for mirth.

Three is a wedding, Four is a birth.

Five is for riches, Six is a thief.

Seven is a journey, Eight is for grief.

Nine is a secret, Ten is for sorrow.

Eleven is for love, Twelve for joy tomorrow.

A single crow is interpreted by some as the foretelling of ill health.

For some tribes, the crow is a messenger.