The birds breed in North America but fly to Brazil in summer; understanding habits is urgent to save declining populations.
Manaus, Brazil—On a tiny islet in the middle of the Negro River, in the state of Amazonas, a team of scientists in two speedboats closely examines the sky. Known as Comaru Island, it is submerged, as it happens every year in March, and only the treetops appear above the surface.
A group of purple martins buzzes past, cutting through the heavy, humid air, and just above the speedboats and the island, a flock of little black dots begins to gather, like clouds of birds.
They then form a synchronized whirlwind. A few minutes later, however, they fall like a black hailstorm, embedding the trees, while their sound intensifies and occupies the entire early evening.
In a matter of minutes this spectacle ends, leaving the sky again motionless.
The island of only five hectares—almost the size of Morumbi Stadium—attracts a huge number of these swallows with their shimmering feathers. Because it is visited by approximately 250 thousand individuals from February to April, it is considered one of the largest refuges of the species ever discovered.