Can Darwinian science explain the uncanny fact that a crow both looks and sounds ugly whereas as a prettier bird makes a prettier song? What possible purpose could such an aesthetic unity serve and why would humans be able to recognize it?

The first question is whether there's a fact to be explained. Do "pretty" birds typically have "pretty" songs? And do "ugly" birds typically have "ugly" songs? I'm no expert, but I'm betting not. Peacocks are usually considered attractive; their songs not so much. Swans are (conventionally, at least) beautiful; their honking (at least to my ears) isn't. Many people like the cooing of pigeons. But pigeons aren't usually seen as "pretty." I'm sure a real bird aficianado could multiply examples.

Of course, there are also questions about whether "pretty" and "ugly" are objective notions. That's a big question, but you can no doubt see that it's relevant. But leave that aside. If your speculation were correct, it would be an interesting fact. What might explain it is something that it's very hard to say in the abstract. We'd need a lot more detail, but in any case there wouldn't be much reason to expect philosophers to come up with the best answer.

Read another response by Allen Stairs
Read another response about Beauty, Biology