In 1898, Hermon Bumpus published a paper about change in the morphology in a population of House Sparrows that he proposed was an example of Darwinian Natural Selection in action. Some time after that, the Bumpus study was incorporated in teaching about evolution and is now found in textbooks and problem sets for intro biology in high schools and colleges around the world. This week, a new study was published about Sparrows in Nebraska, that in some ways resembles the study by Bumpus and may well be a better example of Natural Selection in birds.
House Sparrows (Passer domesticus) are introduced birds over most of their very large range. They are a commensal species, meaning that they are a wild animal that lives in close association with humans, not as a pet or as a farm animal but more like a parasite, to the extent that it is hard to find them in truely wild settings, and it is hard to find human settlements without them. That is why they are called “House” Sparrows and not “Wild” Sparrows, I suppose. There are few areas of the world where they have been introduced and not survived (Greenland and Cape Verde are examples of this bird’s local demise). House Sparrows were introduced to New York City in 1852 and later in other areas. Bumpus’s study was carried out in the vicinity of Providence, Rhode Island (Bumpus was at Brown University).
Bumpus looked at the effects of a nasty winter storm on survival of several sparrows. He collected 136 individuals found laying around after the storm, and carefully measured them. Some of the sparrows survived and some died, so he compared the data from the survivors with the non-survivors. It turned out that the survivors had a narrower range of key morphological variables than those that died. This was proposed as an example of what we now call “stabilizing selection” … variation is constantly introduced into populations, but every now and then “selective forces” culls the variation out. Bumpus’s study has been re-examined numerous times since then, including by one of the Masters of Natural Selection in Birds, Peter Grant (co-author of The Beak of the Finch: A Story of Evolution in Our Time).
Now, there is a new study that has significant advantages of the Bumpus study, though the latter will still be useful in teaching about evolution because of its limitations and the questions it raises. The new study is about Cliff Swallows (Petrochelidon pyrrhonota) in southwestern Nebraska.
As you know, a lot of birds are killed in the U.S. because of collisions with vehicles. About 80 million, according to the Cliff Swallow study by Charles Brown and Mary Bomberger Brown. Brown and Brown examined the possibility that birds in a population subject to this particular form of mortality would undergo Natural Selection, with some traits that prevented death through vehicular collision being selected for over time. The study is available here, and is a classic example of a well written, concise piece of work. From the abstract:
… we might expect natural selection to favor individuals that either learn to avoid cars or that have other traits making them less likely to collide with vehicles. If so, the frequency of road kill should decline over time. … During a 30-year study on social behavior and coloniality of cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, we found that the frequency of road-killed swallows declined sharply over the 30 years following the birds’ occupancy of roadside nesting sites and that birds killed on roads had longer wings than the population at large.
These birds habitually build their nests on structures associated with roads or railroad tracks in the study area. Over the course of 30 years, the researchers collected all the road-killed swallows they encountered while doing their other research. They know how much time they spent in the vicinity and how much road they examined, so they feel strongly that they can estimate the kill-off rate from these observations.
The research on these swallows involved mist netting, a technique for safely collecting wild birds for measurement or banding and release. But it isn’t totally safe … now and then a bird dies during the mist netting process. The researchers collected and measured these birds as well. The birds collected during mist netting resembled the birds that survived, according to their observations. Thus, there are two samples, “the population at large” (from the mist netting) and “road kills” from those collected dead on the road.
They found that the number of road kill declined steadily over the 30 year period, while the number collected from mist netting remained stable (or increased very slightly). The population of swallows actually increased over this time period, so the decline in road kills could not be explained that way. Also, they did not observe an increase in scavengers that might have taken the road killed birds before the researchers had a chance to collect them. The amount of traffic on the roads did not decline during this period, and in fact, increased. Also, there is no reason to believe that cars became safer (for the birds) during this time.
In comparing road kill to the population at large …
… Wing length of road-killed cliff swallows was significantly longer than in the population at large … Over time, wing lengths of cliff swallows killed on roads increasingly diverged from that of the population at large … Average wing length of the population as a whole exhibited a significant long-term decline during the years of the study, whereas the opposite pattern held for the birds killed on roads.
This suggests that road related mortality exerted a selective force on this population of birds, such that birds with longer wings were differentially killed off, and therefore, the average wing morphology of the population shifted.
The mechanism behind this change may have to do with take-off speed. These swallows sometimes sit on the road. When a car comes along they fly away. But sometimes the car gets them. Birds with shorter wings are thought to be better able to take off vertically, those with longer wings less so.
The researchers note that severe weather events and changes in the kinds of insects the birds eat could also have influenced wing morphology. Also, the reduction in road kills could be a matter of change over time in the birds’ behavior. If this is partly due to behavioral changes, it might be an example of “social learning” whereby birds learn from observing each other, something that is thought to happen in swallows. This could account for a spread of “knowledge” over time among individuals. Personally, I don’t like that explanation because I don’t think this would occur over a 30 year time scale, but rather, either not at all or very quickly.
Again, the original paper is here and it includes a nice set of graphs.
Several years ago I started to work in South Africa, either running academic tours or carrying out fieldwork in archaeology and paleontology. The academic tour sites and the research sites were far flung across the region, so I’ve regularly traveled between the vicinity of Johannesburg, Pretoria, Kruger Park, the Waterberg, Kimberly, the Kalahari, Cape Town and a couple of other locations. A typical season would have me on the road for about 10,000 kilometers (I once returned a rental car with exactly 10,000 kilometers used!) Meanwhile, in the US, I’ve traveled great distances as well. Over the last 10 years, I’ve traveled an average of between 1,000 and 3,000 kilometers annually out in the country side through diverse habitats (prairies, woodlands, etc.) in Minnesota.
During this 10 year period, I hit one bird in the US. A pigeon. It actually hit me, slamming into my windshield. In any given trip to South Africa, the vehicle I was in (I only drove myself some of the time) probably hit about one vertebrate animal about every 2,000 kilometers, usually a bird. (We are not counting insects.) There may be more birds and mammals per square kilometer in South Africa. It is a subtropical region, after all. But I strongly suspect that the relationships between people, roads, animals, and all of that is simply different there to the extent that the probability of finding a car-naive animal in South Africa is much higher than finding a car-naive animal in the United states. And by “finding” I mean intersecting with a moving vehicle.
And now, enough about birds for a moment, I have something else to tell you about. Well, this is actualy about birds too. If you are a denizen of the Birdish Blogosphere, you must know by now that Princeton University Press is just now releasing it’s new Crossley ID Guide for Raptors, and that there is a blog tour going on related to this. My contribution to the blog tour came out this morning, and if you go there and correctly identify which of several pictures are Gold Eagles as opposed to some other bird, you could possibly win (depending on random chance) two pounds of coffee guaranteed to have NOT passed through the gut of a small cat-like Asian carnivore. Probably. Please go visit: Golden Eagles and Free Coffee! and show off your bird identification prowess.
Brown, C., & Bomberger Brown, M. (2013). Where has all the road kill gone? Current Biology, 23 (6) DOI: 10.1016/j.cub.2013.02.023
Bumpus, Hermon C. 1898. Eleventh lecture. The elimination of the unfit as illustrated by the introduced sparrow, Passer domesticus. (A fourth contribution to the study of variation.) Biol. Lectures: Woods Hole Marine Biological Laboratory, 209–225.