Tyler Cowen's interview with ornithologist Richard Prim is interesting throughout. A sample:
COWEN: If you were trying to give us the most fundamental explanation of why sexual dimorphism is different in birds compared to mammals, what would that be?
PRUM: Well, that’s actually a really big question. [laughs]
COWEN: Of course, but the most fundamental factor — what is it?
PRUM: The most fundamental factor is that most birds don’t have a penis.
COWEN: Talk me through the equilibrium there.
. . . .
PRUM: Birds have a very rapid period of rapid development. That means that they grow up and leave the nest, and you need two parents to do that efficiently in most diets or most kinds of ecologies. That means the dad’s got to be at the nest.
We usually thought that you have social monogamy, at least two birds helping raise the young, because the young are so needy and they have to grow up quickly. But there’s another possibility, which is that they could evolve to be so needy and grow up quickly because they managed to get males at the nest.
One of the things that happened in the phylogeny of birds — you’ve got ostriches and their relatives, and you’ve got chickens and ducks, and then you’ve got the rest of birds, and that’s a bunch. That’s the vast majority of them, and in that lineage leading to the rest of birds, the penis evolved away, and the question is why. My own theory is that female birds preferred mates that did not have a penis.
One of the ancillary benefits of that, one of the correlated benefits of that is that they were no longer subject to sexual coercion or sexual violence. They could be coerced behaviorally, but they couldn’t be forcibly fertilized. That means that they have freedom of choice, and what do they do with their freedom of choice? They choose beauty. One of the reasons why birds are so beautiful is that males don’t have a penis. They have to be subject to choice in order to effect reproduction, and also they have to invest if females require it.