How Does Mistletoe Grow and Other Questions From Our Readers

I know birds spread mistletoe by excreting the seeds, but why do I see trees with no mistletoe next to trees that have been so thoroughly invaded by this parasitic plant that they almost look like evergreens?
Susan Shackelford
Louisville, Kentucky

Mistletoe does not typically favor one species of tree over another. However, older, larger trees are affected more than others because birds often prefer to perch higher up, and the seeds they excrete germinate in those branches. A strong, healthy tree can tolerate mistletoe, but weaker trees may ultimately succumb to it.
Greg Huse
arborist, Smithsonian Gardens

How do airplanes fly upside down if their wings are optimized for flying right side up?
Ronald Redlinski
Elma, New York

An airplane wing’s lift is due, in part, to a combination of the shape of the airfoil and its angle of attack to the airflow. If the wing is turned upside down, it will still produce lift due to the angle of attack, but the amount of lift will be reduced. So airplanes can still fly upside down, just not as well.
John Anderson
curator of aerodynamics, Air and Space Museum

Were the earliest forms of music likely to be vocal or percussive?
Daniel Magnolia
Arlington, Virginia

We don’t know; what we call music likely predates any material evidence that offers clues to its origins. To consider the question, though, it is equally important to know when human beings began to conceive of their organization of sound as something distinct enough from other behavior to give it a separate label. While most ethnographers would probably agree that all of the world’s cultures practice some form of organized sound akin to music, not all cultures conceive of what they do as “music” (or a linguistic equivalent).
Daniel Sheehy
director and curator, Smithsonian Folkways Recordings

Did the development of the wheel have anything to do with the migration of humans to the Western Hemisphere?
Beulah Woodfin
Albuquerque, New Mexico

Not at all. Migration to the Western Hemisphere was on foot and perhaps—in one or two controversial events—by boat. All of humanity walked everywhere until quite recently, a few thousand years ago or so.
JoAllyn Archambault
director, American Indian Program, Museum of Natural History

Do wild animals and birds require salt in their diets?
Dennis Michele
Fairview, North Carolina

All animals require salt, but that includes sodium, chloride and macro-elements (inorganic nutrients needed in relatively high quantities) and trace elements—not just “table salt” as we know it. Based on habitat, animals acquire those minerals from the food they consume, soil, naturally occurring mineral licks, and even, in the case of marine life, water.
Michael Maslanka
head, Department of Nutrition Science, National Zoo

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