For decades now, puzzled birdwatchers in the southern tier of states have noticed that in some years, birds from Canada's boreal forests turn up for unexpected winter sunshine vacations in vast numbers.
These quintessential "snowbirds" journey hundreds or even thousands of miles from their usual winter range. Seed-eating species like pine siskins, cedar waxwings and evening grosbeaks are particularly likely to break out from the snowy north and turn up in the southwestern states.
For the longest time, the reasons for these unpredictable floods of feathered migrants kept researchers guessing - until just recently, when a combined team of ornithologists and atmospheric scientists made a breakthrough using crowd-sourced data.
As part of Project FeederWatch - a citizen science initiative run by Cornell's Lab of Ornithology- volunteers have systematically recorded more than two million bird sightings from November through April since 1989. That makes it possible to track these boreal bird irruptions on a continent-wide scale.
It turns out that teeter-tottering climate patterns are the culprit: shifts in rainfall and temperature drive boom-and-bust cycles of seed production in northern forests. But there may be a lag before the birds move, because the seed crop often depends on conditions in the two or three previous years.
The birds show how climate can act like a huge magnet that pushes and pulls massive bird "superflights" - and brings surprise and pleasure to the myriad birdwatchers waiting down south.
You too can contribute to new discoveries about birds by signing up online at feederwatch.org.
Earth Notes is produced by KNAU and the Sustainable Communities Program at Northern Arizona University.