Yep. Move aside, Blackpoll Warblers. There’s a new trans-Atlantic fall migrant in town. Okay, actually Connecticut Warblers have probably been doing this for thousands of years, but it’s pretty surprising because we didn’t know it until now.
In my Scientific Naturalist paper in Ecology (or download directly McKinnon_et_al-2017-Ecology), I show how 4 Connecticut Warblers flew from the coast of the US for two solid days (minimum of 48 hours) over water to land somewhere in the Greater Antillean islands, probably Hispaniola. To make it even more stunning, they rested for about a week, then again flew almost 800 km over open water to the Gulf of Venezuela. And they weren’t done yet! They continued south to winter somewhere deep in the Amazon basin. The data are a little blurry on the exact wintering sites, but somewhere deep in the green that is the Amazon forest.
In contrast, the ocean-crossing is as clear as day because, well, there are no trees over the ocean! So when we are using light-sensors to track migration, it’s really obvious when the birds are not experiencing any shading at all from trees or other vegetation. The light levels just steadily increase and then decrease as dusk falls. In contrast, when the birds are on land, it’s a mess of light-levels going up and down, even as low as zero when birds are in really shady spots (giving us dozens of artificial ‘sunsets’ that we have to filter out). This is partly to blame for our fuzzy delineation of the exact overwintering sites for these individuals. The Amazon forest understorey is a dark place!
Back to the fall migration of this species – how did we miss this amazing feat of migration?
I think there are a lot of reasons, actually. First of all, Connecticut Warblers, on a global scale, are not very abundant, so they are not a species commonly captured or seen by banders or birders. They nest in the southern-boreal aspen-transition habitats in Manitoba, Ontario, Quebec, and some parts of the northern Great Lakes states like Wisconsin and Michigan, but they are not common even within their breeding range. Not many people venture out into bogs in June in Manitoba either, and I know why!
Okay, so not a lot of overlap between people and Connecticut Warblers (a range map is in my previous post on these guys). I also think that Connecticuts probably get confused with their more southern-breeding cousins, the Mourning Warblers, or their western cousins, MacGillivray’s Warblers. This would be especially the case in the fall, where the young-of-the-year in those species might actually have a complete eye ring like a Connecticut Warbler (normally they only have semi-circles, as in MacGillivray’s, or none, like the Mourning).
So all of this means that coming across one of these guys and positively identifying it in the fall especially might be tricky.
But we had some hints that they might do something drastic over the ocean. Hurricane Emily brought down 75 individuals on BERMUDA one October. What would they be doing way the heck over the ocean?!? There is also a really neat video capture of a flock following cruise ship lights in the Lesser Antilles. Finally, the famous ornithologist Alexander Chapman commented on how darn FAT Connecticut Warblers were in the fall in the northeastern states… hmmm, suspicious, no?
Now that we have some tracking data, I can see additional reasons why we would miss this. First of all, if their major Caribbean stopover is on Hispaniola, it would be easy to miss. Hispaniola’s landscape is rugged, and humanitarian crises have resulted in few bird surveys there in recent decades. Unfortunately, it’s also lost a lot of forest recently- something like 90% of the original forest cover in Haiti is gone! Yikes.
Now that we know Connecticut Warblers travel through this area, hopefully we can get some more details on what type of habitat they might use or need for these Caribbean stopovers. Clearly their strategy relies on extraordinarily long migratory flights, which suggests they also need extraordinary amounts of fuel in advance of these flights as well as for recuperation after.
For Connecticut Warblers, I hope this study gives them some well-deserved time in the spotlight. They’ve been missing in action for too long, and I don’t want them to end up missing in action for good!
Full citation for the paper plus links to download a pdf and supplemental files:
McKinnon, EA, Artuso, C., and OP Love. 2017. The mystery of the missing warbler. Ecology.