No, abortion is not natural. For one, I'm not sure if killing off our young is something that humans did back before the rise of civilization. Even a couple million years ago, I'm not sure if it happened (I'm not saying it didn't, just that we don't know).
Our 4500 years of recorded history have shown that human have done it from the time of the ancient Egyptian. To say that you need to know the millinia of unrecorded history before human civilisation, of which we can have no way of knowing, in order to verify this 4500 years of history is just bogus. It's the kind of intellectual dishonesty I was trying to avoid. It's not reasonable to believe that human were more likely to not have practiced infanticide before they became civilised
when we have evidence of the practice through out recorded history (and the fact loads of animals pratice it).
Secondly, the method(s) by which we practice abortion are not natural. In all the cases you talked about above, animals are killing their young after they have been born. I don't know of any species that kill off their young while in the womb.
Because animals don't know how to (or so we assume). Some animals may have the ability to self-medicate with plants in their environment, though perhaps they wouldn't be conscious of the fact that they "self-medicate". I don't know of anyone who have studied if animals take plants that induce abortion so I'm willing to accept that they don't do it absence the evidence that they do.
However what you are saying is that killing a baby after it's born is natural, but induced abortion is not. It's correct semantically, because abortion is not infanticide, but you're just avoiding the point that it's an act of killing the young (or the potential to the specie) as a way to natural selection. The mother can't raise the young well, she kills it. Essentially, she's terminating her biological investment. Abortion is a more efficient method of cutting her loses than infanticide.
Of course, pro-lifers would disagree that there is anything different between an abortion and the killing of a born baby. They are both infanticide to them.
And finally, the reasons that humans practice abortion are unnatural. In all the cases you mentioned above, the reasons for the animals killing their young are driven by the need to pass along their genes. Mothers kill the weakest of their young so that the strongest have a better chance of surviving. A pack leader may kill the young of other pack members so that his/her own offspring have a better chance of survival. Human abortions have nothing to do with that. Our reasons are unnatural, and only due to our higher thought processes.
The reason I thought of looking at animal behaviours in relation to abortion is because there are quite a number of women who choose to have abortion after they have a few children. They often feel that they won't be able to raise their existing children well enough if they have more children. Women who have abortion think about their future, often in relation to finding a new mate who would help them raise the new babies and having the life they want. They want a resource rich environment in which to raise their future children, which means those children will have biological advantage to the ones she aborted had she had it and raised it in a poor environment. There are of course women who have abortion just because they never want to have children at all their whole life. Then again there are animal mothers who refuse or kill their young out of aggression. Aberrations like that will happen.
I don't think it's right to say that if we consciously act in a certain way, that that act is outside of nature. We construct a lot of rituals around acts like eating or copulating. But at the root there's a very instinctual primitive impulse behind it. We may construct a lot of human reasons around abortion (or infanticide like the Greeks practiced) but it doesn't mean there isn't an instinctual primitive impulse behind it.