What are those large flocks of birds that eat the fruit on my trees?

These are Bohemian Waxwings, northern wanderers who are seen frequently in Central Alberta beginning in December into spring. They can be found west and north of the capital region year round. As with all birds, they move in order to find food. Waxwings are fruit eaters, which is why they are visiting your yard, usually to eat Mountain Ash berries. They typically descend on your fruit trees in the dozens and can be seen and heard flying overhead in the hundreds or thousands. Their flight behaviour as flocks is much like the movement of schools of fish. The name "Bohemian" refers to the nomadic movements of winter flocks. Because of their gypsy-like migrations they do not maintain breeding territories. A similar but much less frequently seen waxwing in our region, is the Cedar Waxwing. The Bohemian Waxwing has red undertail feathers and white edges to the wing feathers and a grey-brown body, while the Cedar Waxwing has yellow undertail and a yellowish belly. The colourful berries in their diet provides the pigment in the waxy yellow and red enlargements of the shafts of wing feathers. The oldest recorded Bohemian Waxwing was at least 5 years, 10 months old.