OLD LYME, CT A huge flock of Tree Swallows (Tachycineta bicolor) gathers in the sky above the Connecticut River at dusk. During fall migration roughly half a million swallows mass in the sky over Goose Island at sunset each evening before descending in a tornado-like vortex into the tall reeds on the island to roost for the night. Tree swallows exhibit this phenomenon in specific areas as a defense in numbers strategy against predators.