Hi!
I like that "hyperbolic winging sort of way". And yes, they do look like errant boomerangs when they fly.
Globally, the barn swallows are estimated at 200 million. The 3 million from that one roost in south Africa will stay primarily intact on its way to Europe. Vagrants will occur, but not likely all the way to you in Mexico.
Barn swallows are found in most parts of the world. The African group migrates back and forth from Europe. The Asian group migrates back and forth from NE Asia to Micronesia/Indonesia, and those in the western hemisphere migrate back and forth from South America to Canada. All of them breed in their northern climes, and winter in their southern habitats.
The
Journey North is a good site to explore. While it is written with schoolchildren in mind, it is a good little primer about various migrating species. It is for the uninitiated of any age.
This is a
dynamic map of eBird.org barn swallow observations from around the world. Keep in mind that these numbers come from the citizenry when and as they are able or so inclined. So it is not comprehensive, but it is representative to a certain degree.
For a really in-depth and excellent treatise on barn swallows, please see the late AC Bent's
Life Histories of Familiar North American Birds. Bent was a dedicated ornithologist who persevered mightily to create this magnum opus. He was an outstanding observer, and had friends throughout this continent who were his eyes and ears and who shared avian anecdotes- most of which Bent chose to pass on to us. His entries are always a good read.
And thank you for the fine compliment regarding Melville. That is a first for me...