We don't keep records as to whether a woman who gives birth is a citizen or not. If you look at the link I provided to you, you will see that the migration rate is figured into the population clock, which is why it is included under the heading COMPONENT SETTINGS FOR MARCH 2007. If you notice as well, the international migrant (net) every 27 seconds is net, which indicates that it takes into account emmigration and immigration. The other components to the clock are births and deaths. Taken together with net migration, you wind up with a net gain of one person every 12 seconds.
Here is the technical description on how the clock works.
So I'm reading it that US citizens are repopulating themselves at the replacement rate (but just squeaking by), but when you factor in immigration, we are an increasingly smaller percentage of the overall population.
And you would be wrong. Anyone born in the US is a citizen. We also take in one million LEGAL immigrants a year, most of whom become citizens. The important fact is that most of the US population increase can be attributed to immigration, legal and illegal.
You are right about the components of population growth. and as countless demographers have shown, we are still numerically healthy as far as our fertility/replacement rate goes, compared to other parts of the world, particulary Europe and Russia. Steyn has shown definitively how Muslim birthrates all over the West ( and not just the West) can threaten the very foundations of our political and social systems. In the current climate, however, we are faced with a deep-seated perversity, which is echoed in different ways in other contexts: we can't talk about closing our borders without having "racism" thrown in our faces, because the open borders crowd don't think Islamic terrorism represents a threat at the borders, and we can't talk about the presence of tens of millions of illegals here as if it's ALSO not just another component of a bigger, "non-denominational" problem, called overpopulation. This is the tricky way that NOT JUST THE SOCIALISTS have of demagoguing the issue, and deflecting the REAL and LEGITIMATE CONCERNS away from the issues themselves and into irrelevant areas that are the only concerns (and ONLY the concerns) of special-interest groups, like the open borders crowd AND the Islamics intent on gathering political power as they contemplate how they will eventually take over.