A social bird, the western bluebird (Sialia mexicanus) often feeds in large flocks during the non-breeding season. The summer diet of this small thrush consists mostly of insects, while during the winter western bluebirds subsist mostly on fruits and berries.
These western bluebirds were part of a huge flock feasting on the mistletoe berries in infected California black oaks (Quercus kelloggii) along the Klamath River not far west of I-5 (Siskiyou County CA).
Although the day was sunny, temperatures were frigid. The bluebirds all looked like little butterballs with their feathers fluffed in an attempt to stay warm.