It sounds like you're wondering why we aren't as specialized as birds. Not all birds can mate yet all humans can. We just aren't that specialized because evolution just hasn't gone that way for us. There hasn't been a need for us to evolve that way. There are way more species of birds than there are of humans and they interact with way more creatures than humans do causing specialization. There are different species of humans. Just not as different as birds. If you came to this planet from another and studied humans, you'd see we have more than one species. We're more like dogs where any breed could mate with any breed in theory if not for the major size differences like the Chihuahua and Great Dane. We have Africans, Hispanics, Europeans, Indians, etc. So we're not just one species. If there was one species of human, we'd all be truly the exact same. There wouldn't be a such thing as an interracial relationship.