It’s pretty bizarre by any standard: a bird species of the southwestern U.S., Mexico, and the Greater Antilles appearing in numbers in late fall in the northeastern U.S. and southeastern Canada.

Since 2005, cave swallows have made almost annual appearances here in Maine (and across the northeastern U.S. and Canada), usually in November. Most swallows species are entirely dependent upon flying insects for food. That’s why they migrate south in the fall; they head  to warmer places where insects are abundant for food. Some swallows, like cliff swallows, barn swallows, and bank swallows, can migrate as far south as southern South America. Tree swallows have the ability to eat bayberries and other berries to survive during cold weather, and they subsequently overwinter in the southeastern U.S.—farther north then others of their kind. In fact, in recent years, a few tree swallows have been found on December Christmas Bird Counts in southern Maine.

Cave swallows, on the other hand, are not known to eat anything but insects. They don’t winter north of Texas or the Caribbean. Migrating north in the fall toward progressively colder weather is not a good survival strategy for birds dependent on flying insects.

Western birds coming east in the fall—that’s a regular occurrence. It’s why birders love to go to places like Monhegan Island in September and October. We search for the rare but regular birds that appear there from farther west. Such species include lark sparrow, dickcissel, yellow-headed blackbird, western kingbird, and others. Sightings of some of these birds are hard to track to any particular weather event. It seems like they are just part of a general segment of the population that gets off track during migration.

What’s interesting about cave swallows is that they sometimes show up in numbers, not just as single individual but as a small flock. That’s harder to explain as a single bird’s anomalous behavior and shows the likelihood of a weather-related factor. The arrival of cave swallows across the Northeast usually coincides with a number of days of westerly or southwesterly winds. Birders around the Great Lakes often begin spotting them a few days before they arrive in New England.

That was the case this past week when birders began seeing them along the shores of Lake Erie and Lake Ontario on November 5 and 6. By the 8 and 9, cave swallows were spotted in Quebec, New York, and Connecticut.

And on November 9 and 10, cave swallows were found here in Maine—in at least three places that we are aware of: Fort Foster, Kittery (four birds); Fortune’s Rocks, Biddeford (a single bird one); and at Schoodic Point (at least six!).

Even if you accept that weather is implicated in the arrival of cave swallows here in the Northeast, there is still the question as to why other swallows might not get caught up in these same weather conditions and end up here at the wrong time of year.

Right now there are still plenty of tree swallows, northern rough-winged swallows, and barn swallows in places like Texas and northern coastal Mexico where they are overlapping with cave swallows. Why is it that only cave swallows, and not these other species, end up getting swept northeastward in numbers in November? Cave swallows are one of the only swallows increasing rather than decreasing in numbers in recent years. Is their travels far afield tied to their just being more numerous? Is that enough to explain why they seem to be the only swallow species showing up at such an odd time of year?

There are even more cave swallow mysteries to unravel beyond this one. For example, populations nesting in Cuba seem to disappear in winter but no one knows where they go. A few cave swallows have been seen and photographed on the islands of Aruba and Bonaire off Venezuela, and there are a handful of records from northern Venezuela and Panama. Are these birds migrating from the Caribbean population or from the populations in Mexico or the southwest U.S.?

And here’s one more bizarre bit related to the recent cave swallows’ Maine visit: We just happened to go birding at Fortune’s Rocks the same day the cave swallow had been seen there! Unfortunately, no cave swallow for us. The find had not yet appeared on the GroupME rare bird report app or else the technology was slow to register it. We nonetheless enjoyed watching the sanderlings scampering about at the edge of the surf and peering at a few dozen dunlin perched with some gulls on the rocks along the shore. Later that night, back at home, we found out that a cave swallow had been seen not far from where we’d set up our scope!

If you see a swallow this week here in Maine, chances are pretty good it could be a cave swallow! In fact, eBird doesn’t show records for any other swallow species here in Maine so far this November. In other words, a swallow you see in Maine right now is most likely a species that’s rare in the state – how bizarre!

Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology and Vice President of Boreal Conservation for National Audubon. Dr. Wells is one of the nation's leading bird experts and conservation biologists. He is a coauthor of the seminal “Birds of Maine” book and author of the “Birder’s Conservation Handbook.” His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine, a nonprofit membership organization working statewide to protect the nature of Maine. Both are widely published natural history writers and are the authors of the popular books, “Maine’s Favorite Birds” (Tilbury House) and “Birds of Aruba, Bonaire, and Curaçao: A Site and Field Guide,” (Cornell University Press).