Tuesday, February 17, 2015

Soar like an eagle or flail like a hummingbird

By Ben Davidson

Flying is a constant battle against gravity, and keeping your weight down is crucial. This is one of the main reasons we can’t seem to stay airborne, despite some of our best efforts

Birds have incredible adaptations that allow them to maintain physical function while minimizing their own weight. One of the most well-known, but often misinterpreted adaptations is their hollow bones. When thinking of a hollow bone a lot of people imagine something like this:

When birds have an incredible system of support structures that traverse their bones making them stronger and denser than ours!

Now I know what you’re thinking, “Hey, you just said their bones are lighter than ours, how can you say that, and then say they’re denser?” And you’re right, if we just look at the bone structure itself. But what makes the difference is what’s inside the bones. While our bones are all filled with marrow and thick fluids, many bird bones contain air sacks that help them breathe! We’ll talk more about this amazing feature in a minute, but for now we’ll focus on bones. So now you understand how bird bones may be stronger and denser than ours, but still reduce the overall weight of the bird. Let’s get into some functionality.

Joints in the wings have unique morphologies to make flight less difficult. One crucial adaptation is the wrist joint is curtailed, so that it can only move along a single plane, rather than circularly as ours do. This allows the birds to put the bulk of the air pressure on their skeletal structure, rather than their muscular structure. Another key adaptation is their keel, a huge breast bone that extends down from the sternum and attaches their ENORMOUS breast… muscles. Without the keel, their breast muscles would rip the birds’ chest cavities open when they tried to fly! Their breast muscles are so big that they can take up a quarter of a birds’ body mass! Imagine a 160 lb person, they would have 20 lb pecs… They would be more disproportionate than Barbie… or Arnold Schwarzenegger.

There are many different flying styles, and we ornithologists can identify genus and species based on flight pattern alone (ladies?!?). Soaring hawks, eagles, and vultures expend the least amount of energy, but require a large wingspan to be able to ride the thermal currents. On the other end of the scale, hummingbirds beat their wings as hard and as fast as they can, up to 80 times per second! It’s so fast that they look like a blur, unless you’ve had about 100 cups of coffee, in which case…

Despite their relatively inefficient flight patterns, hummingbirds have been found to fly about 40km/hr, or about 25 mph. Pretty incredible for an animal the size of your thumb!
Suarez and colleagues (1991) published a study looking at how hummingbirds use all of the energy they do. They found that O2 use in hummingbirds is about twice as efficient as mammals, so they investigated why this might be. They found that the mitochondria made up about 35% of fiber volume in the flight muscle cells, this is about as many mitochondria that can fit in a cell! Not only did they have way more mitochondria than mammals, but they had more capillaries too. The image below shows a cell of a humming bird where part a indicates capillaries with arrows and mitochondria with asterisks, and part b is magnified around one of the capillaries and shows the relative concentration of mitochondria compared to the size of the nucleus and capillary.

These incredible muscles take an incredible amount of fuel in the form of both oxygen and calories. Birds have a wide range of caloric needs, flightless birds require about the same number of calories as mammals, but a 5lb ferruginous hawk needs 86kCal per day, whereas a 5lb mammal needs about 55. Hummingbirds are in a league of their own, if humans had the metabolism of hummingbirds, we would need to consume about 155,000kCal in a day! Oxygen is needed for energy use, so with all the effort that birds put into flying, would our lung system get them enough oxygen? Not at all. They need so much oxygen that they have a constant flow of fresh air into their lungs all the time. So how do they do that? You remember those air sacks they have in their bones? Here’s where they come in: oxygen flows in through the trachea and fills the posterior air sacks and lungs first, as the first breath is expelled from the lungs and fills the anterior air sacks, the posterior air sacks contract, pushing fresh air across the lungs. Here is a link to an amazing gif illustrating the unidirectional airflow in bird lungs:

When watching hummingbirds, it may seem like they’re just flailing their wings and somehow staying airborne, but they have a very defined wingbeat motion that allows them to generate a powerful downward stroke, with high resistance and minimize resistance in the upward stroke. Below is a brief diagram depicting the wing motions, the first three pictures show a downstroke, and the fourth picture is the midpoint of an upstroke.

Conversely, eagles and hawks have elongated humorous, ulna, and radial bones that allow them to extend and lock their wings when soaring. They can still make minor shifts in their wing positions when soaring to catch and ride thermals. They still, however, expend huge amounts of energy when beating their wings to gain altitude, but they rotate their wings less dramatically than hummingbirds.

Bottom line: flying is hard; it takes a lot of energy and incredible adaptations! Whether it’s having a massive wingspan, that allows them to catch and ride thermals, flapping their wings just as fast as they can, or something in between, birds spend a lot of energy staying airborne. Their amazing ability to fly allows them to find new habitats and exploit ecological niches that would have otherwise been unavailable.


Suarez, R. K., J. R. B. Lighton, G. S. Brown, and O. Mathieu-Costello. 1991. Mitochondrial respiration in hummingbird flight muscles. Proceedings of the National Academy of Sciences of the United States of America 88(11): 4870-4873.

Gill, F. B. 1985. Hummingbird flight speeds. The Auk 102(1): 97-101.

Hedrick, T. L., B. W. Tobalske, I. G. Ros, D. R. Warrick, and A. A. Biewener. 2011. Morphological and kinematic basis of the hummingbird flight stroke: scaling of flight muscle transmission ratio. Proceedings of the Royal Society B DOI:10.1098/rspb.2011.2238.

Images and gifs from:

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