The loggers include an accelerometer, to record movement, and a light sensor. Given the duration of the day and night and the time of year, scientists can determine the north/south position. Noting when the sun is at the midday position gives an east/west location.

In 2014 and 2015, they recaptured 19 swifts carrying these data loggers and found that, as expected, the swifts were spending their winters in West Africa.

Three birds never rested, Dr. Hedenström said. Some did rest occasionally at night for brief periods, but the data loggers showed that all the birds stayed in the air more than 99 percent of the time when they weren’t nesting.

The data confirmed what had long been predicted, that the birds stayed aloft when they weren’t nesting. Roosts had never been found in Africa. And Dr. Hedenström said that over the course of the past 100 years, about 50,000 swifts were banded in Sweden. Only one banded bird was ever recovered south of the Sahara.

The recordings also showed that long ascents, observed during the summer, happen throughout the year. During these ascents, often at twilight, the birds climb up to one and a half miles.