Here in central Oklahoma a couple years ago I was in my backyard when dozens of robins flew in. I've never seen so many robins at one time. It was like a robin convention with orange breasts everywhere as the trees filled, they lined the fence, landed on the ground and on shrubs. I posted this on the Oklahoma Forum and learned that they change their diets and habits in winter. Rather than migrating, they form large flocks and travel together where the food is. In summer they are solitary and forage the ground for worms and insects but in winter they become vegetarian congregations in groups searching for berries. When one robin spots food, it signals to the others and they all come in to feast.

The only holly I have is the Texas native tree--Ilex vomitoria (Yaupon) which holds it berries all winter. Birds in general don't seem too interested in it but that year the robins feasted off the berries. They all left as quickly as they came all at once in a group.