Symbolism, Imagery, Allegory
These scavenger birds make their first appearance right at the start of the novel, indifferently eye-balling Mr. Tench. He's a curiosity, but not yet carrion. There must be carrion around, however, as these birds wouldn't be alive if there was nothing dead to eat. Vultures are a sign of impending death, and Greene places them here for precisely that reason. When, in a "faint feeling of rebellion," Mr. Tench throws a piece of the road at the birds, he's protesting his own mortality and mortality of everything around him (1.1.1). He's not dead yet!
But the birds also signify life. After all, they are living things. They show that life continues after death—and that sometimes life continues because of death. Pretty meta, huh? Death doesn't always have the last word, even when being interviewed by Bill O'Reilly. This is a major theme in The Power and the Glory. We don't know if the priest reaches heaven or falls into hell, but we do see that the Church continues after he is gone. Who'd have thought a vulture could figuratively point towards the Catholic Church? Well, Graham Greene, apparently.