One of 4.7 billion birds migrating south from the continental United States this fall, a male Orchard Oriole was photographed by Paul Konrad. Radar technology is helping to put a more precise number on the birds migrating through the United States each fall and spring. Graphic by Cornell Lab Illustrator Jillian Ditner.

Using cloud computing and data from 143 weather radar stations across the continental United States, Cornell Lab of Ornithology researchers can now estimate how many birds migrate through the United States. Their findings are published in the journal Nature Ecology & Evolution. “We’ve discovered that each autumn, an average of 4 billion birds migrate south from Canada into the United States. At the same time, another 4.7 billion birds leave the United States over the southern border, migrating to the tropics,” explained lead author Adriaan Dokter, a postdoctoral fellow at the Cornell Lab.

“In the spring, 3.5 billion birds migrate back into the United States from points south, and 2.6 billion birds return to Canada across the northern United States border,” Dokter added. In other words, fewer birds return to their nesting ranges after going through fall migration and spending months in their wintering ranges. But the researchers were surprised to find that the migrants arriving across the United States southern border had an average return rate of 76 percent during the five years of the study, from 2013 to 2017, and the birds wintering in the United States only had an average return rate of 64 percent.

“Contrary to popular thought, birds wintering in the tropics survive the winter better than birds wintering in the United States,” said Andrew Farnsworth, co-author of the study and leader of the Cornell Lab’s aero-ecology program. “That’s despite the fact that tropical wintering birds migrate three to four times farther than the birds staying in the United States.”

To estimate these numbers, the researchers developed complicated algorithms to measure differences in biomass picked up by weather radar – in this case, the total mass of organisms in a given area, minus insects and weather. Migrants crossing the northern border – such as many sparrows, American Robins and Dark-eyed Juncos – have shorter migrations from nesting ranges in Canada to wintering ranges in the United States.

Measurements from the southern border captured data on migrants that nest in the United States and spend their winters in places such as Central or South America, including most warblers, orioles and tanagers. One explanation for the higher mortality among birds wintering in the United States may be the number of hazards they face.

“All birds need suitable habitats with enough resources to get them through the winter,” explained Ken Rosenberg, co-author and conservation scientist at the Cornell Lab. “Birds wintering in the United States may have more habitat disturbances and more buildings to crash into, and they might not be adapted for that.”

Another reason for the disparity in migration return rates between short and long-distance migrants may have to do with nesting strategies. Birds wintering in the United States have high reproduction rates to offset higher mortality. Tropical wintering species have fewer offspring, but more adults survive through the winter and reproduce the following spring, despite their longer migrations. But it’s a strategy that may backfire without conservation efforts in the tropics.

“Longer distance migrants seem to be gambling on having high survival in the tropics, and they are therefore more sensitive to what happens to their wintering ranges,” said Dokter. “Even a small decrease in survival due to changes in their tropical habitats might cause a precipitous decline.”

For more information, see https://mailchi.mp/cornell/news-release-researchers-use-weather-radar-to-find-more-than-4-billion-birds-are-migrating-above-the-us-this-fall-1313129?e=f0b505020c