Kenn Kaufman posted a comment on FaceBook yesterday that helped me realize that some of what I said here a few days ago was incorrect. Will copy and paste his comment below. My primary error was that I attributed much of the huge flight of migrants to the huge expanse of breeding habitat north of the site. As it turns out, the flight was going *south*. It most likely consisted of birds which had flown quickly north overnight, overshot their intended destinations, and were making a corrective southwards flight once the sun rose and helped them realize where they were. In addition to the direction, this was also indicated by the species composition, which was heavily slanted toward species that mostly breed south rather than north of the observation site: many times more Bay-breasted Warblers than Blackpolls, similarly more Swainson’s Thrushes than Gray-cheeked, etc.
A few details below (end of this comment) that may be of interest to friends intrigued by bird migration. - By now, every birder in this galaxy undoubtedly has heard about the monster flight of warblers and other migrants last Monday (May 28) at Tadoussac, Quebec. My colleague Andrew Del-Colle wrote an account for Audubon, and I was able to supply some background for how this happened.
Briefly, on the night of May 27, millions of warblers were migrating north over southern Quebec, as is normal for nights at this season. During the night, very strong southerly and southwesterly winds carried many of the birds far north of their intended destinations. In the morning, those birds that had overshot their targets started flying south again. (These are all nighttime migrants but they will reorient in the daytime as necessary.) As they headed south they reached the north shore of the St. Lawrence River, which is many miles wide in the region east of Tadoussac. Rather than cross the river, they turned and followed the edge of it south-southwest, so that vast numbers of them came funneling right past the observers at Tadoussac.
The New York Times also carried a story about the flight, and it was mostly well done except for one odd misconception: the idea that some of these birds "have been blown off course and are heading back to known food sources before continuing on." There's no basis for thinking that some of these birds, flying south, were going to then turn around and continue on toward the north. It's safe to assume that essentially every bird going south-southwest past Tadoussac had gone beyond its intended target and was purposefully headed back south toward its final destination.
To understand this, it helps to know that these migrants are very site-specific in summer. An adult warbler migrating north from the tropics isn't just headed to "the boreal forest" in a general sense; more likely, it's headed to the exact spot where it spent the previous summer. These birds often get wind-drifted during their nocturnal flights, but they'll correct for it during the day.
It's instructive to look at a map to see exactly where Tadoussac is located, look at the actual eBird checklist linked in the story, and then look at the breeding ranges of individual species. The observers estimated 144,000 Bay-breasted Warblers but fewer than 1000 Blackpoll Warblers (even though the Blackpoll undoubtedly has a higher total population). Why the difference? One likely factor is that the breeding range of Bay-breasted Warbler is mostly south of Tasoussac, while the breeding range of Blackpoll is mostly farther north. Even if both species were carried north by the wind during the night, some Bay-breasts had to backtrack, while most Blackpolls didn't. Practically all the species recorded in highest numbers during the day, including Magnolia, Cape May, Blackburnian, and Tennessee Warblers, have breeding ranges mostly south of Tadoussac, so they would have had to reposition back to the south after the night's major wind drift.
For another example, look at thrushes. The team had 425 Swainson's Thrushes and no Gray-cheeked Thrushes. The breeding range of Gray-cheeked Thrush is entirely north of Tadoussac, so it's not surprising that none were moving south there during the day, even if many flew overhead during the previous night.
EDIT: Samuel Denault, one of Quebec's top birders, informs me that Bay-breasted and Cape May Warblers have extended their breeding ranges beyond what was known historically. He says the recent Quebec Breeding Bird Atlas found large numbers of them (as well as Tennessee Warblers) east-northeast of Tadoussac, feeding on outbreaks of spruce budworms. So the situation may be even more complicated than what I tried to describe. My comments on Blackpoll Warbler and Gray-cheeked Thrush should still apply. No wonder we find migration so endlessly fascinating!"