Research

Biomechanics

Flight performance of Turkey Vultures

University of North Carolina

Chapel Hill, NC

Morphological Evolution and

stable isotope ecology

Ecomorphology of Cinclodes ovenbirds

University of Wyoming

Laramie, WY

Click the project titles above for more information, or simply keep scrolling!

I am interested in the mechanics, physiology, and ecological consequences of animal movement, and in the interplay between ecology and animal performance.  I enjoy projects that draw from multiple disciplines (morphology, biomechanics, stable isotope ecology, etc.), and feel strongly that such multifaceted datasets are powerful tools with which to approach these questions.  I have a somewhat wandering focus, but my work has generally centered on form-function relationships and niche evolution in birds. 

Most recently, I have been 3-D tracking free-flying turkey vultures (Cathartes aura) across an elevation gradient to investigate how the birds compensate for lower density air at high elevation, and how they respond to variable wind conditions.  This work is still progressing, but the early cliff note seems to be that birds flying at high elevation maintain higher airspeeds than their low-elevation counterparts.

I have also studied morphological evolution in a lineage of South American birds called Cinclodes, and showed that despite being physiologically and ecologically diverse, the species of Cinclodes are morphologically quite similar to each other.  I also used stable isotope analysis to investigate niche evolution in Cinclodes, and found the species that inhabit broader ranges also consumed more diverse resources.  This result provides support for the Resource Breadth Hypothesis, and may be the first application of stable isotope analysis to address a macroecological question.

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! 

Why Vultures?

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. 

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 is in review!

Ecomorphology of Cinclodes ovenbirds

Cinclodes is a rapidly-evolving and ecologically diverse group of South American songbirds that may represent an unusual case of adaptive radiation, a phenomenon that is typically associated with organisms that inhabit island chains, like Darwin's finches. The Cinclodes case is unusual in that they span a vast range on a continent, rather than living on islands. The classic studies of adaptive radiation have focused on changes in morphology (body shape) among species, and the functions that each unique morphology confers to its bearer. So, following tradition, we analyzed the evolutionary patterns of morphological diversity among 13 Cinclodes species.

Isotopic niches support the resource breadth hypothesis

A primary focus of ecological research is to understand what limits the geographic distributions of species.  The distribution of resources that species depend upon is likely a limiting factor.  Indeed, it’s intuitive that species that occupy large geographic ranges (or broad geographic niches) might be exposed to, and take advantage of, a broader array of food sources than other (or broad dietary niches), more geographically restricted species (e.g.: narrow geographic niches).  This is a central tenet of the resource breadth hypothesis.  However, this is not necessarily the case.  A broadly distributed species could specialize on one particular food source that also happens to be widely distributed.  Or, conversely, a species with a narrow geographic range could sample from many or all of the resources available to it.  We tested the resource breadth hypothesis in a small group of South American ovenbirds (genus Cinclodes). We analyzed the stable isotopes in feathers of 12 species of Cinclodes from the collections of several natural history museums to assess the breadths their geographic niches (hydrogen and oxygen isotopes) and dietary niches (carbon and nitrogen isotopes).  We also considered relationships between the species’ niches and their body shape.

We found that the geographic niches and the dietary niches among Cinclodes were positively related.  That is, that the Cinclodes species with broad distributions also had the most diverse diets.  Our finding provides support for the resource breadth hypothesis.  We also found that the Cinclodes with the broadest geographic niches, which are known to be seasonal migrants, had more pointed wingtips.  This difference in wing shape may help to reduce the energy expenditure of long-distance migratory flight in these species.

Our study not only lends credence to idea that resource availability and geographic range are linked, supporting the resource breadth hypothesis, it highlights the usefulness of stable isotope analysis as a tool in the exploration of ecological niches. We have also illustrated the importance of scientific collections in ecological studies.

Adaptive radiation of Cinclodes

The Cinclodes lineage includes species that differ dramatically in their ecology. For example, habitats in which Cinclodes are found range from open grassland of the high Andes to the marine intertidal zone, and two species of this genus (C. nigrofumosus and C. taczanowski) are arguably the most marine-adapted of all passerine birds. Others appear to conduct seasonal migrations. Further, the genus contains ecological generalists as well as specialists.  Cinclodes not only experienced remarkably high rates of divergence, but that rapid evolution was accompanied by remarkable phenotypic (both morphological and physiological) and ecological differentiation.

Further thoughts

Adaptive radiation is traditionally thought of as an accumulation of body-shape differences among a rapidly-diversifying lineage. The Cinclodes case demonstrates that morphology is only part of the story, though. Carlos, Pablo and collaborators' elegantly demonstrated that some Cinclodes species have specialized kidneys that allow them to consume salt-heavy diets, which is rare among songbirds. Also, Cinclodes span a range of elevations from sea level to the highest reaches of the Andes mountains.  This implies that the high elevation species are adapted to the challenges of high-elevation life. Clearly, if we were so myopic as to only study morphology, we would have missed much of the rich story that is the Cinclodes radiation.