It was a warm late October evening — one of those days when summer doesn’t seem ready to leave — when I spotted the rarest bird that I’ve ever seen on Cape Cod.
I was driving home after a post-work visit to the Herring River. This route takes me along Chequessett Neck Road and past a small marsh that was still buggy into the fall. As I crossed it, my eyes locked onto a robin-size bird above me, flying into the air from a snag over the road. Its short tail and pointed wings immediately called to mind a flycatcher known as an eastern kingbird — but no, those are summer residents that have been gone for weeks now. And even in the golden evening light, the bird looked pretty yellow.
I yelled, “That’s a western kingbird!,” a species that typically lives only west of the Mississippi River. I screeched to a halt, parked along the side of the road, and ran back to the snag. There it was: a western kingbird, a yellow-bellied, gray-backed, black-tailed visitor from western North America.
I watched this bird for 20 minutes as it fed on airborne insects, the sun shining through its white outer tail feathers. I sent out messages to friends, alerting them. Then, as suddenly as it appeared, the kingbird flew off.
I was elated, as it was truly a rarity to see this bird on the Cape. Despite how far Cape Cod is from their usual range, western kingbirds are an approximately once-a-year visitor to the Outer Cape. Indeed, these kingbirds are one of many rare southern and western flycatchers that appear periodically on Cape Cod in the fall.
Flycatchers are a group of songbirds who are known for their “sallying” feeding style: a flycatcher perches atop a tree or bush, then flies into the air to snag an aerial bug before returning to its perch to eat its prey. Cape Cod is lucky to have a handful of breeding flycatchers including the forest-dwelling great crested flycatchers, the marsh-loving willow flycatchers, and the generalist eastern phoebes.
But except for phoebes, these birds all fly south by the end of September. Nearly simultaneously, a strange phenomenon occurs, where small numbers of flycatchers from western and southern North America appear in the north and east, mostly in October and November. And birders, obsessed as we are with rare sightings, are always on high alert for these uncommon visitors.
My western kingbird was far from the only rare flycatcher to turn up on the Cape this fall. A vermillion flycatcher, a tiny desert-dwelling bird from the southwest, was seen in Mashpee. A gray kingbird, a Caribbean species that usually just barely makes it to southern Florida, showed up at Fort Hill in Eastham. And a fork-tailed flycatcher, a tropical bird whose native range ends around Cancun, was found hanging out in a salt marsh in Chatham.
What is it that drives these birds, so adapted to places far from New England, to show up on the Cape with surprising frequency in the fall?
Perhaps the most common theory to explain vagrant birds — the term for birds showing up outside their usual range — is wind blowing them far from home. In some cases, this is certainly true, such as when hurricanes bring seabirds inland. But wind is perhaps too simple an explanation for these regular vagrants or else a good wind current from the southwest would theoretically bring hundreds of western kingbirds to the northeast.
Another popular yet controversial theory is that of the “reverse migrant.” This theory posits that some birds are born with internal compasses that are flipped 180 degrees — a fork-tailed flycatcher that shows up in Chatham, for instance, meant to fly to Argentina, but its compass was facing the wrong direction.
According to this theory, vagrants are broken birds, lost and confused as to why they didn’t get where they wanted to go. It breaks your heart, really. For regular vagrants, there is probably a kernel of truth to this, but there is likely a less tragic explanation at play.
One of the few studies of vagrancy in North American flycatchers, from 2019, examined ash-throated flycatchers, another southwestern flycatcher that in the last decade has shown up a bit less than annually on Cape Cod. The study looked at what factors correlated with large numbers of these birds showing up in the east.
It found that weather played a role, with winds out of the southwest pushing more birds east, but that was a secondary driver. The main cause, the authors found, was population increase — not simply the number of birds but how many young birds were born that year. This finding led the authors to suggest that most vagrants are birds who live with lots of competitors, which forces them to search farther away for territory.
But this search isn’t necessarily futile. Vagrants, the authors later wrote in an editorial for the journal Frontiers in Evolution and Ecology, are “the outer fringe of a growing population; vagrancy is the mechanism by which growing populations can colonize newly available habitats.”
The authors don’t say that these birds aren’t blown by strong winds or led astray by a wonky internal compass. But they suggest that these regular vagrants like flycatchers are vagrants for a reason: they are reluctant-yet-brave explorers seeking out new territory. Some could end up finding nothing and continuing south, and some may die in their travels. But a lucky few may find a new suitable habitat and return the following spring to settle down. A handful of generations later and that species goes from vagrant to regular visitor.
Birds’ ranges are changing all the time. I’ve written about how common species like northern cardinals, Carolina wrens, and tufted titmice were rare here 100 years ago. When they first appeared, they were vagrants. Now, they are an integral part of the Cape’s bird life.
The western kingbird I saw in Wellfleet is probably not the harbinger of a new species on the Outer Cape. But that doesn’t mean it’s necessarily some pitiable lost animal either. Without vagrancy, these species would be stuck exactly where they are, never able to find new habitats to live in. In a rapidly changing world, vagrancy may be the key to many species’ survival.
