The Physics of Birds

In this second installment of my series on the physics of native gardening, I’ll talk about the physics of birds. We get a lot of birds in our yard. Robins visit the lawn and our crabapple tree. Too many house sparrows come to our bird feeders; they’re invasive pests. We see lots of blue jays, those big bullies, as well as goldfinches, downy woodpeckers, and black-capped chickadees. Every fall we know that winter is approaching when the dark-eyed juncos come down to Michigan from Canada. Canadian geese fly overhead, but they never stop at our house.

Flight

I often see birds high in the sky, soaring through the air without flapping their wings. I suspect many are red-tailed hawks, but I’ve never gotten close enough to one to say for sure. How does soaring work? First, it requires a thermal updraft. The sun heats the earth and the earth heats the air next to it, resulting in a temperature gradient: the air near the ground is hotter than the cooler air high above. However, hot air is lighter and therefore tends to rise. This unstable situation results in thermal updrafts. Hot air at one location will rise, and then as it cools will sink at some nearby location. The hawk can glide in the uprising air, so it slowly sinks with respect to the air but rises with respect to the ground. Once high up, it can then glide anywhere while searching for food, until it is low enough that it must seek another updraft.

Life in Moving Fluids,
by Steven Vogel.

Most birds don’t soar but instead flap their wings to fly. This flapping is complicated enough that I’ll let Steven Vogel—my favorite expert on biological fluid dynamics—explain it. The following excerpt is from his wonderful book Life in Moving Fluids.

In birds, bats, and insects, flapping wings combine the functions that airplanes divide between fixed wings and propellers—in a sense, they’re closer to helicopters than to airplanes, and it’s all too easy to be misled by our habit of calling the propulsive appendages “wings” rather than “propeller blades.” But they aren’t quite like ordinary propellers either, since flapping wings produce both thrust and lift directly, rather than producing thrust directly and getting lift by diverting some of the thrust to pay for the drag of fixed, lift-producing wings. The composite function, as well as their reciprocating rather than rotational motion, mean that the motion of flapping wings is inevitably complex… The downstroke moves a wing forward as well as downward and produces mainly upward force but usually some rearward force as well. The upstroke goes backward as well as upward, producing mainly rearward force but often some upward force.

Scaling

Scaling,
by Knut Schmidt-Nielsen.

Each summer my wife puts out a feeder filled with sugar water, and near it we plant red cardinal flowers, to attract hummingbirds. The hummingbirds are tiny and are constantly eating. In Intermediate Physics for Medicine and Biology, Russ Hobbie and I explain how an animal’s metabolic rate scales with its size and mass. The heat produced from metabolism increases with the volume of the animal, but heat is lost by an animal at its surface. As you compare larger animals to smaller ones, the volume increases with size faster than the surface area does. This means that large animals have trouble getting rid of excess heat, while small animals find it difficult to stay warm. The tiny hummingbird is smaller than other birds, so it tends to cool down quickly (it has a large surface-to-volume ratio). To keep warm, it has to boost its metabolism, which means it must eat a lot. A high metabolic rate requires not only much food but also oxygen, which implies that the hummingbird’s heart must pump a lot of blood. The heart rate of a hummingbird can be upwards of 1000 beats per minute (a normal heart rate for a human is 60 to 100 bpm).

Scaling relationships like we just saw for the hummingbird are common in biology. If you want to learn more about this topic, I suggest Knut Schmidt-Nielsen’s fascinating book Scaling: Why is Animal Size so Important?

Drinking

My favorite bird is the mourning dove. We sometimes will have eight or more of these sweet, gentle birds around our bird feeder. I love their low-pitched coo… coo… coooooooooo song. They mate for life.

Doves are unique among birds in the way they drink. Most birds fill their bill with water and then gravity pulls it down to their stomach. Sometimes they tilt their head back to help the water flow. Mourning doves, on the other hand, suck water into their bill, like we suck water through a straw. Professor Gart Zweers, from the University of Leiden, took high-speed x-ray photos, and concluded that doves draw a partial vacuum which pulls the water up.

Singing

Bird songs are analyzed using plots of time and frequency. As discussed in Chapter 11 of Intermediate Physics for Medicine and Biology, you can resolve any function of time into its component frequencies: Fourier analysis. If you plot the instantaneous frequency versus time, you get a sonogram. The higher the frequency, the higher the pitch that we hear. The northern cardinal’s song starts on a high pitch (around 4 kilohertz, which is about the frequency of highest pitched note on a piano) and then slurs downward an octave (to 2 kilohertz) in about half a second.

Trevisan and Mindlin (Philosophical Transactions A, Volume 367, Pages 3239–3254, 2009) have modeled the bird’s vocal organ, called the syrinx. Their model might be familiar to physics students: it is Newton’s second law, force equals mass times acceleration. The important parameters that enter the model are the mass, stiffness, and a constant characterizing the dissipation or attenuation of the motion. The dissipation can be nonlinear, leading to all sorts of complex dynamics. The model predicts an oscillatory behavior (like that for a mass on a spring). Furthermore, the beak acts as a resonance tube (somewhat like an organ pipe).

We get majestic red cardinals visiting our birdfeeders all the time. Next time you hear a cardinal singing, think of all the physics going on.

Magnetoreception

Are Electromagnetic Fields
Making Me Ill?

Many birds make long migrations, and one wonders how they find their way. One method is magnetoreception: the sensing of magnetic fields. Most organisms cannot detect magnetic fields, but some birds can. Magnetoreception is possible because the birds have small particles of magnetite, called magnetosomes, which provide a magnetic moment that can interact with a magnetic field. I discussed magnetoreception in my book Are Electromagnetic Fields Making Me Ill?

In 1963, German zoologist Wolfgang Wiltschko placed European robins inside a chamber and turned on a magnetic field comparable in strength to the earth’s field. He did not expect a response, but to his surprise the birds oriented with the field… The robins proved adept at sensing magnetic signals during their annual migration.

Some researchers believe there are other mechanisms for magnetoreception besides magnetite particles. I wrote

A few animals, including the European robin, may take advantage of free radical reactions instead of magnetite for magnetoreception. Sonke Johnsen and Kenneth Lohmann [Physics Today, Volume 61, Pages 29–35, 2008], after reviewing the data, conclude that “the current evidence for the radical-pair hypothesis is maddeningly circumstantial…” The jury is still out on this issue.

To tell you the truth, I’m skeptical that free radical reactions are important.

Another animal that may detect the earth’s magnetic field and use it to navigate is the bee. Next week we will continue this series on the physics of native gardening by examining the physics of bees.

 Northern cardinal song

https://www.youtube.com/watch?v=e_b4PkcpDe0