If you’ve casually observed the activity around your backyard bird feeders, you’ve probably come to expect to see certain bird species at certain times of the year as they make their trip north in the spring and south as the temperatures drop. What you may not realize, though, is that the hummingbirds you see buzzing around in fall may not be the same ones you saw arriving in springtime — depending on where you live.
A recent study by researchers at the Cornell Laboratory of Ornithology and the University of Nebraska’s School of Biological Sciences found that two "flyways” (routes for migrating birds) shift seasonally. Scientists examined songbird species in the eastern and the western parts of the country and found that the birds did not take the same route back south as they had used to make the journey north in the springtime.
The study, published in the Proceedings of the Royal Society B, Western bird species opted for lower-elevation routes in the springtime, which coordinated with wetter weather and greener vegetation. In the fall, the areas they flew over were less green but the routes were somewhat more direct (who wants to dawdle on the way to Christmas vacation, after all?).
In the east, there was less variation in the north and south routes, but the birds did seem to maintain a similar tie to regions with the most green space.
It’s more likely that bird watchers in the west are able to see the same birds twice a year than those in the west, due to the variability in the eastern migration loops.
"Migration fronts in the east tend to be very broad and in the west more narrowly defined,” said Frank La Sorte, research associate at Cornell Lab of Ornithology. "Therefore, between spring and autumn migration, looped migration trajectories are not likely to result in substantial differences in community composition at a specific location in the east, but this is more likely to occur at a specific location in the west.”
So why is it western birds have less variability in their migration patterns, but seem more concerned about finding some snack food?
"The physical and natural environments differ quite dramatically between the eastern and western portions of the continent, and we believe these differences have promoted the different migration strategies documented in this study,” La Sorte said.
To identify these patterns, the scientists analyzed a staggering amount of publicly sourced data — 1.7 million bird checklists from eBird (seven years’ worth of observation of 57 species). Ebird is a website that allows birdwatchers to record and upload their observations about which species are in which location on a given date. It seems to have been used primarily by hobbyists — until recently.
"This is relatively new resource, and only during the past several years have researchers begun to take advantage of the rich spatial and temporal detail contained in the database,” La Sorte said.
This effect of moving with the vegetation changes in the region is referred to as following the "green wave,” which is accompanied by numerous insects. Energy storage is important for birds on their trips back and forth, which often span thousands of miles and are rife with dangers.
The findings are the first to discover a common pattern adopted across a wide array of species. They also help to complete the picture La Sorte and his associates began painting in 2013 when they first noticed that many species flew in looping routes, usually clockwise. At that point, they figured out that eastern birds were taking advantage of southern tailwinds in their route over the Gulf of Mexico in spring and selected a route with less severe headwinds in the fall.
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