Bittersweet berries are one of the only foods that are readily available to birds in the late winter. For birds, food is more important than shelter. It is their number one priority for survival. Food fuels metabolism, giving birds the fat and energy they require to move and survive.
In winter, robins, for example, concentrate on berry bushes and winter vines simply because they are abundant. In spring and summer, one would most likely find a robin scouring the lawn for worms and insects. More simply, wintering birds go where the food is more plentiful.
The American bittersweet plant has showy orange berries that easily attract many varieties of birds. Because the bittersweet is a large plant, growing up to 30 feet tall, birds have a dual purpose for making the berries a common food source in winter. The plant becomes an easy target as it also provides shelter for them.
Bittersweet berries become more edible and tasty to birds after freezing and thawing several times. It can take the better part of the winter season to complete this process, which explains why birds gravitate to the berries more specifically in late winter. Despite the fact that birds do eat bittersweet berries in winter though, it is more often out of desperation that they resort to this food source after all other more nutritious high fat berries are gone.