Why is the lookout post on the top of a ships mast called the crows nest?

The lookout post was atop the mast just as a birds nest might be at the top of a tall tree. From that vantage point the bird could see any potential threats that were approaching. The same is true with the lookout in the crows nest. They could see any other ships or reefs or anything that could cause a problem well before anyone on deck might see it.