The bird? Cave Swallow. Yep, cute lil’ bug-catchin’, culvert-lovin’ birds from south Texas had made their way to Maine. In fact, they were being sighted all over the northeast, mid-Atlantic and Great Lakes areas, as well as the southeast and all points in between. I even knew from their generalized patterns of occurrence when and where to go looking for them: that’s how I found them.
Throwing it into reverse
What’s up with that? It’s an example of a poorly understood phenomenon often called reverse migration or sometimes in this more specific case warm-weather migration. Birds fly in a direction more-or-less opposite their typical migration route. Often the reverse migrators are young (so-called hatch year or first-fall) birds. Other times the winds involved are very strong and might actually “push” or otherwise disorient migrant birds. Cloud cover, which obscures the stars and other navigational aids, might also be involved. But for the most part we don’t really know why birds reverse-migrate -- we only know that they do. All over the world, and sometimes predictably.
Dig those crazy Cave Swallows
As for why Cave Swallows wind up so far north and east of where they ought to be heading in fall (which is Mexico and the Caribbean), there’s more to the story of their unusual -- yet increasingly predictable -- pattern of vagrancy (a term ornithologists use to refer to birds that stray out of their typical range).
Cave Swallows get my vote for cutest North American swallow. They’re pudgy, colorful and incredibly agile and unpredictable in flight. Not an easy bird to keep your binoculars on! They look very much like the much more widespread Cliff Swallow but with paler, more buffy throat markings that contrast strongly with their dark caps. Their backs are more gray/brown (less bluish) than Cliff Swallow, and their little forehead markings are much darker and more extensive than on Cliff Swallow.
The southern half of Texas is the northern limit of the Cave Swallow’s breeding range, which spreads south down into Mexico and the Caribbean. There’s also a very small, localized summertime population in south Florida. They can stay year-round in the southern Rio Grande valley but many do migrate in fall.
In recent decades, more and more Cave Swallows from the Texas/Mexico population have been showing up in small numbers all over the northeast, particularly along the coast and Great Lakes, in late fall (and much less commonly in spring). In Cape May, NJ, for example (one of America’s top birding hotspots), the first Cave Swallow was seen in 1990. Since then they’ve become an annual, expected phenomenon anytime warm winds blow from the south after Halloween.
Home, home out of range
In fact, at this point it could be said that any swallow you’d see here in New England in November is now considered most likely to be a Cave Swallow! (Though be careful: Tree Swallows can migrate rather late sometimes.)
That’s a pretty quick and amazing change in distribution for a little bird that was unknown to this region just a few years ago.
The speculative reason for these occurrences is a rapid range and population expansion. Hatch year birds from many species are known to disperse more widely than adults. And the more young birds there are, the more can disperse and the wider their dispersal pattern can potentially be.
In the case of Cave Swallows, their population in Texas has been booming! This has been happening ever since they started using culverts as nesting spots. Development of the natural landscape has meant more and more culverts. By using them as nest sites, the birds aren’t limited to a comparatively few natural caves and overhangs. With this limiting factor removed, Cave Swallows have become super-abundant in many developed areas, such as along major highways.
When fall comes and the birds begin to disperse for the winter, a small percentage of the now quite large (and fast-growing) population of young birds moves off to seek its fortunes in a different direction. Buoyed by favorable winds they wander far. But eventually they find themselves smacked by a cold front and face a long journey home, at least some of it into the wind.
Good luck, little buddies! Wherever you roam I hope you find enough bugs to get you where you need to go.
Links
Here are some links to more info on Cave Swallows:
On the Cornell Lab’s All About Birds: view link
On Whatbird.com: view link
The Cave Swallow photo gallery on VIREO: view link
Peace and good birding,
Scott Cronenweth
Eons birding mentor
www.naturalpathwalks.com
posted by bamagran
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