This site suggests that Marcion's version of The Gospel of Luke - a version which includes no birth narrative - was written before the version of Luke that includes the birth narrative, and that the birth narrative was probably written in response to Marcion. Others, however, say that Marcion removed the parts of Luke that he didn't agree with.
Was Marcion's gospel a shortening of a version of Luke that included the birth narrative? Or was the birth narrative added in response to Marcion's beliefs?
It is very important to keep in mind that we have no manuscripts of Macion's gospel and no translations of it. Furthermore, we have no extant neutral or pro-Marcion commentaries. The two commentaries we have, by Tertullian and Epiphanius, have a strongly anti-Marcion agenda, furthermore they disagree with each other at some key points. This makes saying anything certain about Marcion's gospel very difficult.
Your question really contains several separate questions:
Was Marcion's gospel older and closer to the historical Jesus than all the canonical gospels?
The answer to this is a resounding no. All scholars think that Mark predates Marcion's gospel and that Marcion's gospel was dependent on Mark. Furthermore, all scholars agree that Matthew also predates Marcion. Note that Matthew has a birth narrative, so birth narratives are indisputably older than Marcion. (Of course, Mark does not have a birth narrative, so gospels without birth narratives also indisputably predate Marcion.)
Was the birth narrative a later addition to Luke?
Many critical scholars think it's plausible and perhaps likely that the birth narrative is a later addition (possibly by the same author). The evidence is mostly circumstantial: the location of the genealogy is difficult to explain without this theory, Luke 3:1 reads like the beginning of a book, and many scholars think it makes more sense that the gospel that Marcion received (before he did whatever editing he did) also did not have the first two chapters. The evidence is far from overwhelming, and many scholars disagree, but it is a common opinion.
Was the birth narrative written in response to Marcion?
The vast majority of critical scholars think that Luke was written around 80 CE, and that Marcion's prime was much later (130 CE or later). So even scholars who think that the birth narrative was a later addition, generally don't think that it was a response to Marcion. On the other hand, there's a minority of scholars who think that Luke-Acts was written much later and in response to Marcion. A major book advocating this position is "Marcion and Luke-Acts: A Defining Struggle." This is a small minority opinion, but not a fringe opinion.
(Full disclosure: personally I find the arguments for a late anti-Marcionite Luke-Acts pretty strong. But I'm not an expert, and most experts are strongly in favor of 80ish AD.)
Did Marcion heavily edit his gospel
Sadly we don't really know, since we don't have Marcion's gospel, and we don't know whether his source was canonical Luke or one of canonical Luke's sources. Many scholars think the most plausible situation is that before doing any editing, the gospel used in Marcion's region was somewhat different from canonical Luke, but that Marcion probably did some editing himself as well.Tweet