Concerning a recent/current die-off of Cave Swallows in New England:
Cave Swallows from the southwestern population pelodoma (the one we see here in autumn) are doing very, very well. The species' IUCN conservation category is "Least Concern," which is the 'best' category possible. The expansion of Cave Swallow breeding range has been explosive in Texas. For some of the statistics, check out this abstract.
There are multiple articles on the range and population expansion over the last few decades. A Google or SORA search will likely get you to some of those online. There may be other theories/reasons for the expansion, but the most prevalent is the species' adaptation to breeding on man-made structures, and the increase in number of those man-made structures.
What we are seeing here is apparently a product of this exploding range and population. This may be an over-simplification, but vagrancy can be a method of, and/or result of, expanding populations. For a great read, check out this article by Dick Veit. He has written more on the subject if you're interested.
Cave Swallows will likely continue to increase in our region as long as their breeding population continues to expand. They show up here during late fall, when they are generally swept up by a warm SW flow that often stretches all the way from south Texas into southern Canada ahead of strong cold fronts.
Some, or even many, of these Cave Swallows may survive their journey through our region, especially those that appear earlier in the fall and can escape before it gets too cold. The general route, as far as I know, is from the Texas area to the Great Lakes via a SW flow. Then, on the heels of a cold front that clears with NW winds, these birds appear along the Atlantic coast. The main reason why the birds are generally seen along the lake-shores and ocean-front, and only rarely in between, is thanks to the concentrating effects of the coastline, whether along large lakes or the ocean itself. Once the birds hit our coastline their general movement is back southward (or, as is the case with the east-west shoreline in Connecticut and Rhode Island, they move west towards NYC).
I'm not really sure what they do from there. Some research into late fall-winter sightings in the southeast and border states would probably shed some light. Perhaps someone has looked into this and can chime in? Do the surviving vagrant birds continue south and eventually reach their wintering grounds south of the border? Or do some return south only to ride the next bout of SW winds back up to the northeast to perish? How many of these birds survive the journey to pass on their genes to offspring, and how many individuals make the same journey each year? We are seeing "survival of the fittest" at its best here.
It would be helpful to age as many of them as possible. Pyle (1997) states that in first-year birds "flight feathers not in molt in Jul-Aug but molting Sep-Mar" while adults "flight feathers in molt Jul-Sep but not in Oct-Mar." Also, there no known field marks to separate first-years from adults once first-years have completed their molt, which is typically between Jan-Mar. So, in November, adults should have completed their flight feather molt (which includes all flight feathers) while immature birds should still have a combination of new and old flight feathers.
Most, if not all, of the good photos I have seen recently have depicted presumed first-year birds that show a variable number of old, brown outer primaries and inner secondaries contrasting with newer, much darker feathers. For examples, see recent photos by Margo G. & Steve G. in MA and by Steve Mirick in NH.
This Cave Swallow story is incredibly fascinating, and we still clearly have much to learn. Despite having seen many of these birds in the northeast over the past handful of years, I'm still amazed each time I see one.