Hawks do sometimes form large flocks. They’re usually taking advantage of rising thermal air currents, and you may see dozens (and, during migration, up to thousands) soaring together in the right conditions. These swirling, circling flocks are called “kettles.” By watching for these gatherings, the hawks can more easily find thermals, minimizing their need to flap on their long journeys.
But what you’re describing sounds like it might be Turkey Vultures rather than hawks. These birds very often fly in gatherings of the size you describe, sniffing out the dead animals they feast on.
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