We mark the seasons as honking geese head south and as robins return north. Every autumn we marvel at their numbers going south, and every spring we delight that they have come back.
But until now scientists have never been able to put a number on exactly how many birds migrate across North America.
The bird experts at the Cornell Lab of Ornithology now have done that, using data from 143 weather radar stations across North America from 2013-2017. Their findings were published Monday in the Journal Nature Ecology and Evolution.
They counted an average of 4 billion birds heading south out of Canada coming into the U.S. in the fall, and 4.7 billion birds in the U.S. that head south to Mexico and beyond. Yes, that's billion with a B.
In spring they counted an average of 3.5 billion birds coming back into the U.S. from points south and 2.6 billion birds that head north out of the U.S. back into Canada.
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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.
The data can now be compared with future years to watch for trends and help foretell emerging problem areas.
The numbers suggest that birds wintering in tropical areas are doing better at surviving winter than those spending cold months in the U.S. Tropical migrators - such as warblers, orioles, and tanagers - had a 76 percent return rate compared to just 64 percent of birds who wintered in the U.S. - such as most sparrows, American robins and dark-eyed juncos.
Migrants crossing the northern border have shorter migrations from breeding grounds in Canada to wintering grounds in the U.S.
"Contrary to popular thought, birds wintering in the tropics survive the winter better than birds wintering in the U.S.," said Andrew Farnsworth, a co-author of the study and leader of the Cornell Lab's aeroecology program. "That's despite the fact that tropical wintering birds migrate three to four times farther than the birds staying in the U.S."
One explanation for the higher mortality among birds wintering in the U.S. may be the number of hazards they face.
"All birds need suitable habitats with enough resources to get them through the winter," notes Ken Rosenberg, a co-author and conservation scientist at the Cornell Lab. "Birds wintering in the U.S. may have more habitat disturbances and more buildings to crash into, and they might not be adapted for that."
Breeding results also are different for the tropical migrators. Birds wintering in the U.S. 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 scientists said that species "strategy" may backfire without conservation efforts in the tropics to make sure that adult survival continues to remain high.
