All three species of bluebirds share some similar migration behaviors. They all migrate during the day and many join up with resident flocks of bluebirds to find food, water and roost sites. Fall migration seems to be determined by the shortening of daylight rather than weather. Food is still plentiful and weather conditions are still pleasant when they begin to depart. Weather can influence migration, however birds may linger for longer periods at foraging sites when the weather is mild. When weather turns inclement, it may cause them to move south at a faster pace. Its important to note that not all bluebirds go south for the winter, as long as there is food to be found some stay all winter long.
Seeing so many bluebirds together is an awesome site to behold. I hope you too will experience this migrating habit of bluebirds flocking together.