Comment Anthropic principle (Score 0) 138
Anthropic principle can be applied here. If birds had teeth, they would evolve into mammal-eaters and pose great threat to any mamal that lives above surface. It is hard to imagine human evolution in such circumstances. Basically, if birds had teeth, there would be nobody intelligent to observe them. It is one of many unlikely random events in nature (including many constants physics) that is somehow biased towards our existence.