I've noticed that most vertebrates have stumbled, independently, on live birth as a way of bringing their offspring into the world. Birds seem to be the exception. Why is this? Is it because of the hard-shelled egg that they lay? Is it because they don't have a uterus equipped to carry offspring for longer duration? You would think that such a development would be benificial to some flightless birds like penguins at least. Also (bonus question) Why aren't a lot of bird species active at night? (Owls being a notible exception) Is it because mammals have that particular niche occupied? Have any bird species developed a type of sonar the way bats and dolphins have?
PS thank you for the input on some of the other questions I've had, this sight rocks!