The Air Density Problem...
Let's begin this section with a relaxing visualization exercise. Imagine yourself at the top of a tall mountain. Here… I have a picture to help you. I’m sometimes not very imaginative, and I need this. What do you notice? Yes, it’s beautiful and inspiring and all of that. The air is also cool, and perhaps it’s a bit breezy. One thing that you might have left out, though, unless you’ve spent a lot of time high in the mountains, is that the air is thin, and if you aren’t used to it, you might be a bit out of breath right now. So much for relaxing!
Well, why not? They’re ubiquitous throughout much of North America, which exposes them to the type of elevation effects that we want to study. There’s no evidence that vultures that live at higher elevations have bigger wings than those that live at low elevations. And, while I’ve frequently been told that laziness is not a desirable quality, I’m not sure that I completely agree. I think that their laziness is one of the vultures’ most underappreciated attributes! By lazy here, what I really mean is that they probably aren’t likely to do any more work than they absolutely have to.
Furthermore, vultures are large and easy to spot in the air, and they have a really convenient habit of roosting in large groups, in the same place, day in and day out. So, you can go out to their roost site, and set up a small fleet of cameras to do some 3-D tracking. Then, right on cue, your study animals will come to you. It’s the easiest field work ever! Do you see how this, too, might play into my overall theme of laziness?
What about birds?
Alright, now let’s add another element to this picture: a bird. I know, you were all worried about not being able to breath when I took some of the air away, but our friend here has more than that single problem. He’s trying to fly through that air, and the reduced air density at high elevation is making that more difficult for him. We understand this phenomenon pretty well from an engineering perspective, as it turns out. Lift production at high altitude requires more speed, bigger wings, or more power. Flying animals can’t escape this physical challenge, either, which begs the question, how do they compensate for low density air? Some studies have shown that high elevation birds have larger wings than their low elevation counterparts. Others flap with greater stroke amplitudes; that is, they work harder. But what about lazy birds? To answer this question, let’s take a look at some of the masters: vultures.
So, I’ve already told you that the vultures living at high elevation don’t seem to have larger wings, and we have pretty good reason to assume that they are equally lazy everywhere. If those assumptions hold true, that means that they must be maintaining higher flight speeds somehow. The nice thing about that is that it’s a lot easier to look for differences in flight speed than it is to measure their wings or how much power they can generate. It bypasses all of the unpleasantness that might accompany handling birds that feed on dead, rotting things. I’m pretty OK with that.
I traveled to three vulture roost sites, where I set up my fleet of cameras, and collected video of the birds returning from foraging at the end of the day. From those videos, we assembled 3-D tracks for the birds, and from those tracks and data about the ambient weather conditions, we were able to extract airspeeds for the vultures.
As predicted, I found that the vultures' airspeeds increased as elevation increased. That is, birds flying in that low density air that took your breath away earlier were, indeed, flying faster. And, what’s more, we were able to determine that their sinking speed, that is, how rapidly they were losing altitude while gliding, didn’t differ across different air densities. This suggests that the increased airspeed is sufficient to offset the effect of low air density.
Now, that brings us to the next logical question: How do the birds maintain higher flight speeds without flapping more, or flapping harder, or gliding at a steeper angle? Well, as it turns out, there’s one perhaps beneficial aspect to the reduced air density that we've been talking about. The drag forces that the birds encounter are also reduced, and by more or less precisely the amount that would explain their increase in flight speed. So, basically, the challenge that we thought the birds might face is mitigated by the exact same physical phenomenon. What we still don’t know, however, is over what range of altitudes this is true. Vultures have been spotted by pilots flying in excess of 20,000 feet above sea level. So, perhaps the small range of elevation that we were able to sample geographically simply isn’t enough to really challenge the birds’ ability to maintain lift. We’re hoping, in the future, to be able to GPS track the birds to address this, along with other interesting questions about how lazy birds can really be.
Stay tuned for more details, the manuscript on this work in forthcoming!