In the late summer, after the breeding season has completed and the yearling chicks taken to wing, many species of migratory birds tend to gather together in large flocks in anticipation for their travels down south to warmer climates. These flocks, depending on the species, can number in the tens-of-thousands. Purple Martins and Red-Winged Blackbirds are examples of such species.
Come daybreak, these flocks take to the air at once to test their wing strength and to forage for insects to fatten up for their often exhausting migrations. When their numbers are great enough to be detected on Doppler Radar, the resulting signature is known as a roost ring. Most roost rings expand and then disperse as the individual birds in the flock radiate away from the roosting site.
Purple Martins, for example, will typically roost overnight in the same spot every night. The roost rings that they produce can be observed with repeatability in these recurring locations. In their habitat in eastern North America, purple martins gather in over 300 distinct roosting groups annually. Martins tend to return to specific locations every year to build or maintain their breeding sites. The largest recurring roosting flock at Lake Murray, South Carolina contains up to 700,000 individual birds.
Studying roost rings can yield valuable data for wildlife biologists and ornithologists about the migratory habits of birds. The use of radar to track flocks of birds started over large portions of the United States from 1957 onward as the National Weather Service installed the WSR-57 weather surveillance network. Radar does not give a very accurate count of the number of animals, due in part to varying atmospheric conditions and varying displacement of the birds. Modern radar systems can provide accurate data of the velocity and direction of flights of birds, which are combined with GPS and ground spotter observations.