United States songbirds migrate with the wind, new study

This 2015 video from the USA is called Amazing Migration: Blackpoll Warbler.

From Wildlife Extra:

US songbirds are found to migrate with the wind

Millions of tiny songbirds, many weighing less than an ounce, migrate thousands of miles from North America to Central and South America each year. How they do it has been somewhat of a mystery, but now scientists have discovered how far they take advantage of prevailing wind patterns to save calories.

“Most of what we’ve known about migration routes comes from ducks and geese,” said Frank La Sorte, a Cornell Lab of Ornithology research associate and lead author of the paper in the Journal of Biogeography. “But terrestrial birds are much smaller and they aren’t reliant on the same kinds of habitats. There really isn’t a narrow migration path for them, and they aren’t necessarily in the same place in spring and fall.”

The findings from this study are important as they may help refine ideas about how and where to plan for conservation along migratory pathways.

“All these species migrate at night, at high altitudes, where we can’t see them,” La Sorte said. “But when the sun comes up in the morning they have to find somewhere to land. So any new knowledge about where they’re travelling is valuable to conservation planners.”

For years US scientists assumed songbirds followed the same well-defined “flyways” that ducks, geese, and shorebirds used to travel up and down the continent. There is one known wildfowl flyway along each coast, one up the Mississippi River valley, and one in the centre of the country. Those flyways were marked out from studies which compiled data from recoveries of birds with leg rings and records kept by hunters, but those methods don’t work for small songbirds that migrate at night.

The new work solved this problem with a fresh approach using crowd-sourced data submitted to the Cornell Lab’s eBird project between 2004 and 2011. The researchers analysed thousands of people’s sightings to develop, for each of 93 species, an aggregate picture of where a species is during spring and autumn migration. Although they weren’t tracking individual birds, collective sightings gave them an indication of how the species were migrating. They then used computer models to sort species with similar movement patterns into groups and compared migration routes with seasonal patterns of prevailing winds at night.

The study revealed that most US land birds fit into three main groups: a western one of 31 species, a central group of 17 species, and an eastern of 45 species. Examples of each group include the black-throated gray warbler, the clay-coloured sparrow, and the American redstart, respectively. The researchers noted that the flyways for these are much more spread out across the continent than those of wildfowl, and routes in the central and eastern groups overlap considerably.

The analysis also revealed that many more land birds than previously realised follow different routes through America in spring and autumn — particularly in the East, where many species cross the Gulf of Mexico in a single overnight flight.

Unlike wildfowl, which migrate north and south along the same relatively narrow routes, rather like lorries on a motorway, songbirds are more like passenger cars touring back roads. They are less tied to a single wetland habitat, so they can fan out. By shifting routes, birds take advantage of stronger tailwinds in spring and less severe headwinds in autumn. Tailwinds represent a huge advantage for small birds heading north to their breeding grounds, while finding weaker headwinds allows southbound birds to make the best of a bad situation.