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

Well, earlier this week, six birders experienced exactly that, and their account, detailed in an absolutely , has the whole bird world abuzz. According to the report, early Monday morning the six birders arrived at the Tadoussac dunes, a popular birding destination in Quebec, Canada, hoping to see migrating warblers during their morning flight—a phenomenon that occurs when birds pushed off their typical migration pattern 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. Nine hours later, they ended up having a historic day. 

Here's just a taste of the team's final tally: 144,324 Bay-breasted Warblers, 108,243 Cape May Warblers, 72,162 Tennessee Warblers, 50,513 American Redstarts, 28,865 Blackburnian Warblers, and the list goes on and on. All 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 person who made the eBird report was Ian Davies, a project manager at eBird itself. So yeah, these folks knew what they were doing. 

"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, "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 through so low, they were flying between his legs. Unbelievable. 

What's even more wild is that the group was able to count and calculate estimates for so many birds. Davies detailed their methodology in the report: "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. The average birds/second was then used for that time period, until another rate estimate showed a different volume of movement."

While the southbound birds can be explained by the winds the night before, the weather doesn't explain why so many birds crossed this one exact spot. Davies and the rest of the crew are still birding around Canada—"I am out of the office until 4 June, chasing migration 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 for such an epic day. 

"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," Kaufman said in an email.  "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." 

And that is what makes this whole thing even more amazing: Not only were there birders at the dunes that morning, but they also belonged to an extremely small subset of people in this world that could identify and know how to count such staggering quantities of birds. "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."