Scientists use radar to count billions of migratory birds

But until now, scientists have never been able to quantify exactly how many birds migrate to North America.

Bird experts at the Cornell Lab of Ornithology have now done it, using data from 143 meteorological radar stations in North America from 2013 to 2017. Their findings were published Monday, Sept. 17 in the Journal Nature Ecology and Evolution.

The United States counted an average of 4 billion birds heading for southern Canada arriving in the United States at the end of the year. autumn and 4.7 billion birds in the United States that were heading to Mexico and beyond. Yes, it's the billion with a B.

In the spring, they counted an average of 3.5 billion birds returning to the United States from the south and 2.6 billion. birds heading north outside the United States.

Researchers have developed complex algorithms for measuring the biomass differences detected by meteorological radars – in this case, the total mass of organisms in a given area, minus insects and weather conditions.

Data can now be compared to future years to monitor trends and help anticipate emerging issues.

The numbers suggest that wintering birds in the tropics survive better in the winter than those who spend cold months in the United States. in the United States – like most sparrows, American robins and black-eyed juncos.

Migrants crossing the northern border have shorter migration from breeding areas in Canada to wintering areas in the United States.

"Contrary to popular belief, birds wintering in the tropics survive better in the winter than birds wintering in the United States," said Andrew Farnsworth, co-author of the study and head of the program. 39, aeroecology of the Cornell Lab. "Despite the fact that tropical wintering birds migrate three to four times more than birds staying in the United States".

An explanation of the higher mortality among wintering birds in the United States could be the number of hazards they face.

"All birds need suitable habitat with enough resources to get them through the winter," says Ken Rosenberg, co-author and conservation scientist at Cornell Lab. "Birds wintering in the United States may have more habitat disruption and more buildings crashing, and they may not be suitable for that."

Breeding results are also different for tropical migrants. Birds wintering in the United States have high reproductive rates to compensate for higher mortality. Tropical wintering species have fewer offspring, but more adults survive in the winter and breed the following spring, despite their longer migrations. But scientists have said the species "strategy" could turn around without conservation efforts in the tropics to ensure that adult survival remains high.