I guess the usefulness of Ancestry source citations may vary. A lot are not very helpful at taking you to the original source. Here's the Source Information that Ancestry gives for the birth record of Robert Burnam in Boston in 1647:
Ancestry.com. Massachusetts, Town and Vital Records, 1620-1988 [database on-line]. Provo, UT, USA: Ancestry.com Operations, Inc., 2011.
The Ancestry page says that the original data comes from:
Town and City Clerks of Massachusetts. Massachusetts Vital and Town Records. Provo, UT: Holbrook Research Institute (Jay and Delene Holbrook).
As far as I can tell, this "original data" is not actually very original but another database, which does not seem to be available online. Not very helpful.
If no image of the actual source was available on Ancestry, I would have reached a dead end with no easy way to find the source of the real original data (ie that used by the Holbrooks). Fortunately, Ancestry does give the image of the source of the real original data, which by going to the first image turns out to be:
A Report of the Record Commissioner Containing Boston Births, Baptisms, Marriages, and Deaths, 1630-1699. Boston: Municipal Printing Office, 1908. p. 25.
Which is freely available on archive.org. The best citation for the original data shown in the information found on Ancestry.com is therefore actually something like:
A Report of the Record Commissioner Containing Boston Births, Baptisms, Marriages, and Deaths, 1630-1699. Boston: Municipal Printing Office, 1908. p. 25. Link to page at archive.org.
Ancestry.com and familysearch.org databases are a nice easy way to initially find the information you are looking for, but the best source for the information is almost always several levels below that and requires digging.