While watching the dark-eyed or “Oregon” juncos (Junco hyemalis) pick through the cracked corn Leonard throws out for the birds, only when the ground is covered in deep snow, I began to muse about these little birds. They summer across Canada and then migrate throughout the United States during the winter. In addition, there are year-round populations in the western mountains and Appalachia. Around our home in Lookout CA the Oregon junco population is resident, or here all year.
A common generalization is that migrant birds have longer and more pointed wings than resident populations of the same species. Larger, more pointed wings are thought to be favored by migrants because these ecomorphological changes enable faster, more energy efficient flight. Studies to test this generalization using pine siskins, song sparrows, horned larks, European blackbirds and the little dark-eyed juncos I was watching, among other birds, fill the literature. The results of the research are not always conclusive.
Why might a larger, more pointed wing be advantageous to a bird traveling long distances? Four forces – weight, lift, drag, and thrust – need to be in balance to achieve flight. The downward force of the bird’s weight must be countered by lift while thrust counteracts the drag or friction resisting forward movement caused by the bird’s body.
Without getting into a technical physics discussion, lift is created when air flows over and under the surfaces of an airfoil, an asymmetrically curved structure that tapers to the posterior. An airplane wing is an airfoil as is a bird’s wing. One of the many factors increasing the potential for lift is the volume of air deflected by the airfoil, which is a function of wing area. Thus a larger wing would provide more lift, or upward force, for a bird.
Drag, the forces that oppose a bird’s forward movement through the air, are caused by the profile of the bird itself and the turbulence of air around the airfoil. As the air moves over the airfoil, air eddies form at the rear edge of the wing and at the tip of the wing. Because a pointed wingtip creates less turbulence than a rounded wingtip, a bird with a pointed wing experiences less drag and increased flight efficiency.
Do these little, cheery juncos outside my kitchen window have shorter wings with more rounded tips than their migratory brothers? I do not know – however it is interesting to think about as I sit in my warm house watching them voraciously devour the cracked corn scattered on the icy surface of the deep snow.