Washington, July 8 : A new study has indicated that some birds keep together at night on their migratory journeys, flying in dispersed flocks, even when they are 200 meters or more apart.
The study, done by researchers from the University of Illinois and the Illinois Natural History Survey in the US, is the first to confirm with statistical data what many ornithologists and observers had long suspected: Birds fly together in loose flocks during their nocturnal migration.
Researchers have spent decades trying to determine how birds migrate at night, when most bird migration occurs.
But nighttime tracking of tiny flying objects a quarter mile to a half mile up is no easy task.
According to Ronald Larkin, a professor of animal biology, who conducted the new study with Robert Szafoni, previous studies sometimes very strongly suggested that the birds were flying tens of meters apart and yet somehow keeping together. But the evidence for this was indirect and suggestive.
Even if it could be established that the birds were flying in groups, no one knew whether they were simply being swept along together passively or whether they were actively, intentionally, traveling together.
In the new analysis, the researchers took a fresh look at bird-flight data Larkin had collected in the 1970s and '80s using low-power-density tracking radar.
The radar directs microwaves in a narrow cone - a "pencil-beam" that can be pointed at virtually any target within range.
"If there is a bird target here, you can see it on the radar display as an echo," said Larkin. "You throw a switch and it locks onto the target, it tracks the target, and wherever the bird flies, the radar points at it," he added.
The radar kept track of a target's distance (from the radar), altitude and direction of travel over time. It also provided data used to calculate the frequency of a target's wing beats.
Once the radar operator had identified a flying object that might be a bird and began tracking its flight, he or she looked for other objects entering the radar's beam.
If another potential target appeared, the radar could follow it for a few seconds before switching back to the first.
By repeatedly switching back and forth between two targets, the operator could potentially detect the discrete flight details of two birds at a time.
After analyzing dozens of trials, the researchers determined that a significant proportion of the pairs of birds they had tracked were flying at the same altitude, at the same speed and in the same direction.
Some of these birds were quite far apart, more than 200 meters away from each other, and yet they were traveling together.