Bird migration is happening, it’s probably good that I don’t live on the Caribbean coast. Not only would it be a challenge for my northern, cold-bred self to become accustomed to constant high levels of humidity, but with so many birds migrating through, it would be even more of a challenge to get anything accomplished. How not to run outside and see a river of raptors? How not to stop looking at the thrushes, warblers, and mesmerizing flocks of Eastern Kingbirds. Which flock might host a rare Gray Kingbird? An even rarer Couch’s or Cassin’s? And the thousands of Hirundines? Forget about it!
Somewhere up there with the massive Purple Martin movements is a Sinaloa Martin. The stream of swallows might host a few lost and adventurous Violet-greens, maybe even an insanely vagrant Asian swift. Unlikely, but not entirely out of the question because with so many thousands of birds being funneled through Costa Rica, lost Asian birds following their instincts south would probably also pass this way.
Identifying some mid blowing mega like that would require an extreme degree of luck but with constant monitoring of migrating Hirundines, there would be a slim chance. Although we lack such focused counting of migrating swallows and swifts at the moment, at least there are a fair number of birders in the field. Always out there, always watching, and unlike pre-digital days, most have cameras!
I hope we will eventually have more extensive counting of migrating birds in Costa Rica but in the meantime, there are plenty of other birds to focus on. Today, we take a closer look at the Gray-rumped Swift (Chaetura cinereiventris).
The common Chaetura (small, stubby tailed swifts) of the Caribbean lowlands and foothills, the Gray-rumped Swift is easily seen at any number of sites. These are the small groups of swifts that fly over the canopy at the La Selva biological station, the small dark swifts with pale rumps that zip overhead, are then marked down, and then subsequently ignored.
I can’t blame fellow birders for paying less attention to swifts; I am likewise guilty of taking the bins down from looking at many of these birds. When the swifts are way up there (which they often are) and in poor light (ditto), and are not vocalizing (not nearly enough), you would be fooling yourself if you didn’t call many of them Cypseloides sp. or even Chaetura sp. (because Vaux’s can range into the lowlands and look way more like Chimneys than we might admit).
The good thing about Gray-rumpeds is that they fly low enough to see that they are darkish, nearly blackish swifts with delineated or well-marked pale rumps. I might be fooling myself (and with swifts that’s almost a given) but I also have this impression that their wings are shorter than Vaux’s. Yeah, probably fooling myself but they definitely have shorter wings than the Chimney Swifts that share their sky space.
The other cool thing about Gray-rumped Swifts in Central America is that they very likely represent a separate, distinct species. Yes, one more endemic for the land bridge between North and South America! Maybe not but molecular studies published in 2018 revealed a whopping 6.2% difference between the subspecies that occurs in Central America and at least three subspecies of Gray-rumped Swifts found in South America. See this link for figures of phylogenetic trees.
Although they look rather similar, such high molecular differences are seriously telling and regarding phenotypical differences, Chaeturas don’t exactly stand out from each other anyways. As for vocalizations, although similar, to my ears, there appear to be differences in structure and maybe pitch from South American birds.
I am guessing that more data are needed to make an adequate proposal to separate the phaeopygos subspecies of Gray-rumped Swift, especially because birds in South America might also represent additional species (?) but given that large difference in mDNA, it sure looks like Gray-rumped Swifts in Honduras, Nicaragua, Costa Rica, and western Panama represent a cryptic species. Even if they were still retained as a subspecies, this bird would still act as one more example of Central America acting as an evolutionary hotspot. Watch those swifts!
On October 13th, Josh Beck mentioned that the occidentalis subspecies of western Colombia and western Ecuador was not included in the phylogeny. Since occidentalis includes two disjunct populations, phylogenies including both of these would be needed to further elucidate any conclusions about the Gray-rumped Swifts in Central America.