Buzzards gather around carrion, or rotting bodies. Therefore buzzards are almost always a symbol of death or decay in a novel, or the circle of life. They can also symbolize the danger of nature, or the tragedy of nature, as number 2 suggests.
The buzzards can be argued to represent Vardaman's struggle to accept the fact that his mother has died. He keeps vigil over the coffin and chases the buzzards away just as he chases away ideas of the reality of the situation he is in.
There is also a hint that the buzzards are symbols of doom and not only of death. If we take an interpretive leap we can say the family is on an odyssey in the story, plagued by forces of nature, by poverty, by the cunning of pharmacist's assistants...and in the ancient tragedy intepretation of this story-line, we can see the buzzards as the ancient world would have seen them: a bad omen.