Missouri ‘radar rings’ may be huge roosts of purple martins

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SPRINGFIELD, MO – The National Weather Service office in Springfield, Missouri, has seen multiple recent bird roost rings on their radar. On Monday, they shared an GIF on Facebook showing what they guess to be purple martins taking flight in the morning.  

Roost rings are commonly seen late in the summer when communal bird species gather ahead of a fall migration. The rings take on an expanding doughnut or arc shape on the radar. It is more common to see them on clear and calm mornings when cool air settles near the ground causing an inversion, which allows the radar beam to bend downward and detect the birds close to the ground. Insect swarms, butterflies, and bats can also be detected in the same way.  

Purple Martins gather in large flocks in the late summer to rest before migrating to South America. Some of the roosts may contain hundreds of thousands of birds coming from a wide area. They can be found near large bodies of water or in trees and man-made structures like bridges in urban or suburban areas. The birds may use a particular roost for 8-12 weeks before migration and return yearly.

The birds typically spend their winters in the Amazon Basin. They return to the United States in February in the south and early spring in the east.

Nearly all Purple Martins now nest in birdhouses in the eastern United States. The National Audubon Society says that the species has declined in parts of the west and their numbers are shrinking in the east. The reasons are not well known, but competition with starlings for nest sites may be involved

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