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There are good days birding, and then there are those spectacularÌýdays when you see hundeds of thousands of migrating warblers.ÌýWait, you haven't had one of those?Ìý

Well, earlier this week, six birders experienced exactly that, and their account, detailed inÌýan absolutelyÌý, has the wholeÌýbird world abuzz. According to the report, early Monday morning the six birders arrived at theÌýTadoussac dunes,Ìýa popular birding destinationÌýin Quebec, Canada, hoping to seeÌýmigrating warblersÌýduring theirÌýmorning flight—a phenomenon that occurs when birds pushed off their typical migrationÌýpattern during the night reorient themselves at first light. With strong southwesterly winds the night before, the team was hopeful to see birds heading southward to correct their routes.ÌýNine hours later, they ended up having a historic day.Ìý

Here's just a taste ofÌýthe team's final tally: 144,324 Bay-breasted Warblers,Ìý108,243 Cape May Warblers, 72,162 Tennessee Warblers, 50,513 American Redstarts, 28,865 Blackburnian Warblers, and the list goes on and on.ÌýAll together, the observers estimate they saw 721,620 warblers, along with loads of other birds. And in case you have any doubts about the report's authenticity or count estimates, the group included several staffers from the Cornell Lab of Ornithology, and the personÌýwho made the eBird report was Ian Davies, a project manager at eBird itself.ÌýSo yeah, these folks knew what they wereÌýdoing.Ìý

"Today was the greatest birding day of my life," Davies wrote to start out his eBird report. "On our arrival (545a), it was raining. A few warblers passed here and there, and we got excited about groups of 5-10 birds. Shortly before 6:30a, there was a break in the showers, and things were never the same."

"For the next 9 hours," Davies continued,Ìý"we counted a nonstop flight of warblers, at times covering the entire visible sky from horizon to horizon. The volume of flight calls was so vast that it often faded into a constant background buzz. There were times where there were so many birds, so close, that naked eyes were better than binoculars to count and identify. Three species of warbler flew between my legs throughout the day [Tennessee Warbler, Magnolia Warbler, Myrtle Warbler]. For hours at a time, a single binocular scan would give you hundreds or low thousands of warblers below eye level."

Worth repeating: There were so many warblers, passing throughÌýso low, they were flying betweenÌýhis legs.ÌýUnbelievable.Ìý

What's even more wild is that the groupÌýwas able to count and calculateÌýestimates for so many birds. Davies detailed their methodologyÌýin the report:Ìý"Movement rate estimates were made by looking through binoculars at a flight line, and counting the number of individuals passing a vertical line in that field of view, per second. This was repeated multiple times for each bin view, and repeated throughout the sky so that all flight at that moment was accounted for.ÌýThe average birds/second was then used for that time period, until another rate estimate showed a different volume of movement."

While theÌýsouthbound birdsÌýcan be explained by the winds the night before, the weatherÌýdoesn't explain why soÌýmany birds crossed this one exact spot. Davies and the rest of the crew are still birding around Canada—"I am out of the officeÌýuntil 4 June,ÌýchasingÌýmigration in Quebec," his automatic email reply reads—but ̽»¨¾«Ñ¡ field editor and renowned bird expert Kenn Kaufman was able to provide more insight into the other factors that made forÌýsuch an epic day.Ìý

"This kind of thing undoubtedly happens many times every spring, over much of North America, but it's not noticeable because the birds are not strongly concentrated as they move back south," KaufmanÌýsaid in an email.ÌýÌý"The same kind of flight could have been happening 200 miles farther west in central Quebec, and no one would have noticed, because the birds would have been spread out all across the landscape. But birds displaced to the region north of the St. Lawrence River and east of Tadoussac would have started south [on their morning flight], then run into the edge of the St. Lawrence River, which is very wide here—5 to 20 miles wide. Instead of crossing, they would have turned and followed the edge of the river toward the south-southwest, so that massive numbers of them came funneling past the observers at the Tadoussac dunes."Ìý

And that is what makes this whole thing even more amazing: Not only were there birdersÌýat the dunes that morning, but they also belonged to an extremely small subset of peopleÌýin thisÌýworld that could identifyÌýand know how to count such staggering quantitiesÌýof birds.Ìý"One of the most notable things about 5/28 was the presence of extremely skilled observers who made an intense all-day effort to count the flight," Kaufman said. "These were tiny birds, zipping past very quickly, and the average good birder would have missed a high percentage of them."Ìý