What triggers the migration of robins?
I don't know about the specific cues that American robins use for migration. This species is also both a short range (e.g. between states or to lower altitudes) and long range migrant (e.g. Florida &amp; Mexico), so the cues that they use can probably differ between overwintering populations. There are also year-round populations of robins in the US, but probably not many (if any) in Massachusetts. There are however at least two papers that show that American robins are arriving earlier and earlier, at least in Wisconsin and Colorado (Inouye et al, 2000 and Jones et al, 2012). The studies find that arrivals dates have shifted ~13 days earlier between 1990 and 2010 in Wisconsin and 14 days earlier between 1981 and 2000 in Colorado. The study from Colorado also shows that the first date of bare ground has not changed, so snow melt has not changed over the time period. As a consequence, the interval between arrival and bare ground has grown by 18 days. This could indicate a phenological mismatch between spring arrival and the resources used by robins, and your observations seems to agree with this pattern.

This are at least some comments and a couple of papers that could be useful for your question. Sorry I cannot say more about the actual cues used for their migration.