Birds Evolve Shorter Wings To Escape Traffic Crush Cars and trucks kill some 80 million birds a year on U.S. roads, a source of death that may now be a powerful force of natural selection. Charles Brown, a behavioral ecologist at the University of Tulsa, says cliff swallows caught for research have shorter wings than their kin killed on roads--possibly because shorter wings bestow better maneuverability in traffic.

Birds Evolve Shorter Wings To Escape Traffic Crush

Birds Evolve Shorter Wings To Escape Traffic Crush

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Cars and trucks kill some 80 million birds a year on U.S. roads, a source of death that may now be a powerful force of natural selection. Charles Brown, a behavioral ecologist at the University of Tulsa, says cliff swallows caught for research have shorter wings than their kin killed on roads—possibly because shorter wings bestow better maneuverability in traffic.


This is SCIENCE FRIDAY. I'm John Dankosky. Some 30,000 people die on U.S. roads every year, but the highway isn't just dangerous for us humans. Scientists estimate that our cars and trucks kill 80 million birds a year. That's 80 million. That death toll is high enough, they say, that it's turning out to be a powerful force of natural selection. In fact, some birds may be evolving shorter wings to let them navigate traffic more safely. That's according to a paper out this week in the journal Current Biology.

My next guest is the lead author of that paper. Charles Brown is a behavioral ecologist at the University of Tulsa in Oklahoma. He's also a professor in the Department of Biological Sciences there. Welcome to SCIENCE FRIDAY, Dr. Brown.

CHARLES BROWN: Hello, John. Good to be here.

DANKOSKY: So tell us about the birds you're studying.

BROWN: Well, I've been working on cliff swallows for about 30 years, and my major interest is why these animals are social. They live in large colonies, and I've been interested in the cost and benefits of group living. But in the context of doing this work, we were frequently driving around from colony to colony, and in the process, we would frequently see dead birds. These cliff swallows now nest largely on bridges and highway culverts, so they're frequently exposed to traffic.

And we would frequently find dead birds. I would usually stop for the ones that that we would find, pick them up, see if they had bands on their legs. And the ones that were - that hadn't been run over or weren't rotted, I would save them, and we would prepare them as museum specimens. So it was a sort of fortuitous discovery. We started out doing this, we didn't really have a purpose for these road kills. But over time, we began to notice that we were seeing fewer dead birds on the roads.

DANKOSKY: How many birds did you collect overall?

BROWN: Well, overall, we have about 200 - I think about 200 road kills that we were able to save over the years.


BROWN: And these, of course, were prepared, and we still have them. So we're able to take measurements on these dead ones.

DANKOSKY: So tell us what you found. What are the most surprising things you've found, as someone who studies these swallows for your career?

BROWN: Well, the most surprising thing was that the wing length on these road kill birds was longer than on a sample of birds that had died accidentally in mist nets. And we think that these net fatalities probably represent a sample of the population at large. So when we compared the road kills with the population at large, we found that the road kills had significantly longer wings. So we really have - we have two separate sort of indisputable patterns. We have one that the road kills have gone down in frequency over time.

We also have that the average wing length has gone down over time. Now, the link between those two results is not so clear. Probably they're each affecting the other in some way. But we do know that that the wing length in this population is going down over time.

DANKOSKY: So why do you think that they're evolving into having these shorter wings?

BROWN: Well, there are several possibilities. One is highway road mortality. I think that's having an effect on the morphology of wings. Another could be changes in the insects that these animals feed on. They feed on flying insects, and land use in the study area is changing quite a bit. Many of our prairies and grasslands are being converted into cultivated corn, and that, of course, is going to change the insect community, so some of the wing length changes may be in response to changing food.

But probably - or clearly, we know that wing length is result changing in response weather events. Periodically, we have long - we have these periods of severe weather which will actually kill birds. They will starve to death during the bad weather. And we documented in 1996 that over a six-day period, the wing length and other body measurements on these birds changed quite dramatically. So we saw a very intense episodic selection event. And we know that during that circumstance wing length went down. So it may be a combination of factors that's affecting wing length evolution in this population. But I think it's quite likely that road mortality is one of the factors.

DANKOSKY: We're talking about rapid evolution in action with Charles Brown. He's a behavioral ecologist who studies these cliff swallows. If you want to join us, 1-800-989-TALK. That's 1-800-989-8255. So the theory is that these shorter wings help them maneuver more quickly, get around those speeding cars, maybe help them catch those flying insects. Would there be some downsize to having shorter wings? I mean, does it mean that they can't fly long distances, for instance?

BROWN: Well, there are actually maybe costs associated with this because some studies have shown that longer, more pointed wings are better for long-distance flight. And these birds certainly fly over long distances. They winter in Argentina, breed in North America, so they're - these are long distance migrants. So it's quite possible that there could be energetic costs associated with a shorter or a more rounded wing. It may just - it may simply be a trade off between certain costs and benefits.

DANKOSKY: As you talk about these migrations, I think it's important to mention the swallows flying back to Capistrano, this thing that we've been observing for years and years. I know you were out in California this week at the San Juan Capistrano Mission to wait for the swallows to come back. Did they come back? Are they coming back this year?

BROWN: Well, we're - we didn't have any this year. We haven't had any in several years. The numbers are decreasing in Southern California in general and at the mission in particular. And we think that this is a consequence of forestation. The landscape is becoming more forested and that, of course, disrupts these flying insects that the birds feed on, changes the community of flying insects. And as a result, the habitat in Southern California is becoming less and less suitable for this species. So, yeah, I mean, we're trying to restore them to the mission, but we haven't had much luck so far.

DANKOSKY: Are the overall populations of the swallows in decline?

BROWN: No. Overall, the population is going up dramatically throughout most of North America. In fact, Southern California is one of the few places where the species is showing a decrease. In other areas, such as the Western Great Plains where we study them, the numbers are going up. They're expanding their range into portions of eastern North America that they formerly did not occur. Mostly, again, using bridges and highway culverts and, therefore, coming into more and more contact with vehicles as they expand their range.

DANKOSKY: Why do these birds like these highway culverts so much? Why do they choose to live some place that's so inherently dangerous for them?

BROWN: Historically, they used cliffs, a vertical wall underneath a horizontal overhang on a cliff, but these artificial structures really present the same sort of place to nest but - and are much safer in the process. The birds are more successful there. The nests are less likely to be washed away by wind or waves. So consequently, the birds are just doing better on these artificial structures. The nests last from year to year, and the birds place a great premium on taking over an old nest at the start of the nesting season. So for that reason, these bridges and highway culverts are probably attracting birds year after year.

DANKOSKY: Let's go to Eli(ph), who's calling from Atlanta, Georgia. Hi there, Eli. You're on SCIENCE FRIDAY.

ELI: Hi. My question is - I was wondering if you'd studied populations of these birds who don't live near highways. So as - are there changes in their physical attributes as well and are they similar?

BROWN: Well, we would certainly predict that. But, no, we've not done any studies on cliff swallows using these natural populations. It's getting harder and harder to find them now because of this conversion of the birds to the bridges. But, no, that would be a logical next step, is to identify populations that have not been exposed to vehicles and - with wing morphology in those guys.

DANKOSKY: You said before you study swallow societies. They sound like pretty fascinating birds. I mean, what are some things about them that might surprise our listeners?

BROWN: They're incredibly interesting. Cliff swallow society is a bit of a paradox because these animals show this intense desire to always be near others of their own kind, yet in the process of being together, there is just continual conflict among individuals. For instance, the birds that nest near each other in a nesting colony, they are constantly trying to do things to each other. They get in each other's nests. They still nesting material. They throw out their neighbor's eggs. They will try to lay eggs in their neighbor's nest. Males will try to copulate with the female living next door. They're really mean little animals.


BROWN: And they're constantly, you know, doing these things to each other, yet it is very rare to find a solitary cliff swallow. They are always together. And our research shows that the bigger groups often have - there are very strong advantages to being in the larger groups.

DANKOSKY: They're pretty ill-tempered birds. Why are you fascinated by them? You seem to love these mean little guys.

BROWN: Well, I guess, I see all the parallels with humans. But I mean, for instance, one thing they do is they show this incredible play behavior. You'll get 50 or 60 on a wire, and they'll all crowd together, completely touching each other shoulder to shoulder, and then other birds will come in from above and try to knock some of them off the wire. And - I mean, it's clearly play, and the ones that are on the wire try to avoid getting knocked off. They'll even, sometimes, hang upside down in order to avoid surrendering their place. And this will go on for, you know, five or six minutes, and then their everybody just starts to spreads out and goes about, you know, resumes preening. So they're really fascinating. And the more you look at them as we've done, the more you learn about them and the more fascinating you really become.

DANKOSKY: I'm John Dankosky, and this is SCIENCE FRIDAY from NPR. Of course, one thing that you've said happens when these birds get together in these big, old colonies is some diseases and other things. They have bed bugs in their nests?

BROWN: They do have bed bugs. They're the principal parasite. And in fact, one of the major costs of living in colonies for these animals is this thing called a swallow bug, but it's actually a bed bug. It looks very similar to the human bed bug. And these bugs live in the colonies. The infestations can reach 2,600 per nest. So there may be hundreds of thousands of swallow bugs in a single cliff swallow colony. And when the infestations become severe, of course, reproductive success just, you know, drops and the birds began to desert their colonies. The babies that do survive are malnourished and in pretty bad shape. So that's one of the principal disadvantages of living in very large colonies.

DANKOSKY: Mm-hmm. Let's go quickly to Cody(ph), who's on Oswego, New York. Hi, Cody.


DANKOSKY: Go ahead. You're on the air. What's your question?

CODY: OK. I'm just wondering with a study like this, you understand now that they're growing smaller wing spans. How does - what do we do from here? How does this further our study and our knowledge of the subject?

BROWN: Well, we don't know that we're going to be able to do a whole lot more with cliff swallows because the road kills are just drying up in this particular study area. We just don't find many road kills anymore. I mean, we're down to only getting a couple of birds a year. So in this case, it's beginning to look like road mortality is becoming a bid of a non-factor. I think what you'd want to do, as the previous caller mentioned, you'd probably want to go to a place where they've been exposed to vehicles more recently or maybe not exposed at all and look at wing morphology there.

DANKOSKY: Are cliff swallows found every place on the planet? It seems like they're the ubiquitous bird.

BROWN: No, they're actually restricted to North America as a breeding bird. There are swallows very similar that are, essentially, ecological equivalents that are found in Africa, South America, Australia and Asia. So there are related species elsewhere, but our particular bird is a breeder here, only in North America.

DANKOSKY: Just very quickly, Doctor, do you expect these birds to change more as there are more and more people, faster cars. They're living in the same sorts of environment. Do you expect to see bigger changes in these swallows over time?

BROWN: Well, I think what this study shows is that animals can likely adapt to these human - these urban environments relatively quickly. Most of what we've seen in the Nebraska population has occurred over the last 30 years, and we know that because it was about 30 years ago that the birds began moving on to these bridges and culverts in large numbers in this particular area. So I think that these urban environments in many ways represent a laboratory for studying evolutionary change in many of these animal populations that are exposed to cars or to large buildings, towers, many of these sorts of urban challenges.

DANKOSKY: And the stuff that we're seemingly building all the time. Charles Brown is a behavioral ecologist at the University of Tulsa in Oklahoma. He's also a professor in the Department of Biological Sciences there. Thanks so much for joining me today.

BROWN: My pleasure.

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