As you know, birds do not have sexual organs because they would
interfere with flight. [In fact, this was the big breakthrough for the
Wright Brothers. They were watching birds one day, trying to figure
out how to get their crude machine to fly, when suddenly it dawned on
Wilbur. "Orville," he said, "all we have to do is remove the sexual
organs!" You should have seen their original design.] As a result,
birds are very, very difficult to arouse sexually. You almost never
see an aroused bird. So when they want to reproduce, birds fly up and
stand on telephone lines, where they monitor telephone conversations
with their feet. When they find a conversation in which people are
talking dirty, they grip the line very tightly until they are both
highly aroused, at which point the female gets pregnant.
-- Dave Barry, "Sex and the Single Amoeba: What Every Teen Should Know"