By Rex Graham
Nearly all of us have unintentionally killed birds while driving cars. Vehicles kill about 80 million birds annually in the U.S., and tens of millions more around the world and possibly more than one billion are killed due to all structures, pollution and pets provided by humans.
A new study published in Biology Letters reported that common European birds seem to have learned speed limits of their immediate surroundings and then time their fleeing flights, based on the local speed limit, in order to avoid being struck.
The report prompted dozens of stories in the popular press, and questions from ornithologists who study road-kill birds.
Pierre Legagneux, a behavioral ecologist at the University of Quebec in Rimouski, and colleague Simon Ducatez drove roads in western France and used a stop-watch to record individual fleeing bird’s flight-initiation distance (FID) in response to their oncoming passenger car. Speed limits of the roads they drove were 20 to 110 kilometers per hour.
“We found that FID increased with speed limit,” Legagneux wrote. “This suggests that birds adjust their flight distance to speed limit, which may reduce collision risks and decrease mortality maximizing the time allocated to foraging behaviours.”
He said the actual speed of his approaching vehicle didn’t matter to the birds. Only the local speed limit somehow entered into the birds’ location-adjusted, feed-or-flight response. The title of his paper, “European birds adjust their flight initiation distance to road speed limits,” was an attention-getter.
“We preferred to use a rather general title than saying that birds were habituated,” Legagneux said in an email.
Were the birds that fled Legagneux’s car habituating, adjusting, adapting, or learning? (Or, dare we suggest, evolving?)
“I suspect what it describes is a case of individuals in a given area just habituating to passing cars, and most cars travel at a given speed in a given area,” said Charles Brown, a biology professor at the University of Tulsa (Oklahoma) who studies road-killed Cliff Swallows.
“What would be really cool would be to show that the same individual bird adjusts its behavior when it moves from a school zone to an interstate, but the study had nothing on marked birds, nor did it track changes in the behavior of particular individuals,” Brown said. “That would really be necessary, because the study is implying some sort of facultative ‘adjustment’ of behavior in response to speed limits, which I am not sure has been shown here.”
Brown earlier reported in Science that Cliff Swallow colonies function as “information centers” in which hungry birds follow their well-fed neighbors to productive food sources.
A new sub-discipline of ecology, called road ecology, is focused on the effects of more and faster-moving vehicles on more roads penetrating more wildlife habitat. Rather than “adjusting” to the noise and mayhem as if humans have created a new habitat, scientists have coined the term “road-avoidance zones.”
The 14 distinguished scientists who co-authored “Road Ecology: Science and Solutions,” urge readers to do their own research. Instead of using a stop-watch, rely on your eyes and ears and stroll the side of any busy two-lane highway adjacent to a wooded area where you would expect birds.
You may quickly suspect that birds naturally distance themselves from the traffic. “So you move back into the wooded edge to look more closely,” the 14 authors wrote. “Many of the native forest birds seem to be missing – even for quite a distance into the forest.”
Road risks, rewards
Roads provide birds more than danger, noise and pollution. They offer localized bounties of road-kill animals, spilled grain, small bits of gravel for digestion and open areas between trees to catch insects and other prey.
“There is a paper in press in showing that crows on roads can distinguish between cars in the oncoming lane and the opposite lane,” Anders Pape Møller, director of research at the Laboratoire d’Ecologie, Systématique et Evolution in Paris, France, said in an email. “I have an unpublished study from this year with my Polish colleagues showing that birds in urban areas distinguish between oncoming humans walking directly towards them and humans on a path passing a focal bird. That is less the case in rural habitats.”
“We agree that birds can be habituated to average traffic in a given area, but another interpretation of our results could be that birds were selected with risk-prone individuals being killed by car collision,” Legagneux said in an emai. “Both interpretations are scientifically grounded and it is, for now, impossible to tease apart which one is the most likely.”
What would Darwin say?
Biology Letters publishes short, sound-bite-length studies. Darwin might call the publication Biology Memos. Is the invisible hand of evolution operating on highways?
Brown, the Tulsa biology professor, has been looking for that invisible hand during his studies Cliff Swallows for three decades. The insect-eaters nest on highway bridges, overpasses and in road culverts near the University of Nebraska’s Cedar Point Biological Station (map). The beaks of finches on the Galapagos can change over such brief periods of time under intense selective pressure.
Cedar Point Biological Station study site (east side of Ogallala Lake)
During 30 years of recording the characteristics of road-kill Cliff Swallows near Ogallala Lake in Nebraska, Brown recorded two major changes related to road-kill swallows:
- Fewer and fewer swallows were killed by vehicles.
- Swallows with longer wings were killed disproportionately more often than individuals with shorter wings. (Shorter wings give greater vehicle-avoiding maneuverability.)
Did the swallows learn, adjust, adapt, habituate or are they evolving?
“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,” said Brown. “If so, the frequency of road-kills should decline over time.”
Brown stops short of saying that his population of Cliff Swallows have “evolved.” However, he has urged fellow researchers to consider the possibility that road mortality may “exert selection.”
An ornithologist familiar with Brown’s explanation of the benefit of longer wings to Cliff Swallows is Prof. Reuven Yosef, director of the International Birding and Research Center in Eilat, Israel. He is not yet convinced that the breeding ground is the place to look for a correlation between wing length and road-kill avoidance.
“We always assume that wing length is dictated by distance of migration. Hence, although modification for survival is important on the breeding grounds, what are the implications on migration? Or at the wintering grounds? What is the trade-off?” Yosef said in an email. “These papers can be discounted as ‘spurious correlations,’ but I would like to first understand the basic biology of the species prior to drawing any conclusions.”
What would the Red Queen say?
Many modern biologists talk about a macro-evolutionary concept called the “Red Queen hypothesis,” based on the Red Queen’s advice to Alice in “Alice in Wonderland” that “it takes all the running you can do to keep in the same place.”
Maybe the altered behavior of birds in France is evidence that they are “running to keep in the same place”: in other words, evolving to survive new “predators with tires.”
“Considering the Red Queen hypothesis and the fact that some species identify the car as a predator, I think the idea for article in Biology Letters makes sense, and yes I believe it is possible,” Clarissa Alves da Rosa, a scientist at the Laboratório de Ecologia e Conservação de Mamíferos in Lavras, Brazil, said in an email. “I did not see the methods used, but Biology Letters is respected in the scientific world of ecology.
Does the authors’ experience with crows, blackbirds and a few other common birds apply to thousands of species and 8 million miles of roads and highways?
“There are some birds that appear to cope quite well with the road environment,” said Ross Goldingay, a wildlife ecologist at Southern Cross University in Lismore, New South Wales, Australia. He studies wildlife road-kills in Australia, and says some researchers underestimate how well some animals can habituate to human disturbance.
Newly arriving Florida Scrub Jays to the brambles of Venus, Fla., between 1986 and 1995 is an example. A group of researchers led by Ronald Mumme, a biology professor at Allegheny College in Meadville, Pa., monitored road-kills during that 11-year period of color-banded jays along two-lane highway at Archbold Biological Station.
“Mortality was especially high for immigrants without previous experience living along the road,” Mumme and his collaborators wrote in a paper published in Conservation Biology.
Archbold Biological Station
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If individual birds need time to learn, adjust, adapt or habituate to the danger of roads, some species are better at it than others.
“Australian magpies were the most common bird road-kills in our study,” said Goldingay in an email. “But the crows and ravens, depending where you are geographically, actually patrol the road for road-kill food and seem to have a very good sense of vehicle speed, or possibly speed zone. We did get some crows as road-kills but the number was quite low compared to the number you see on the roadside.”
However, Goldingay says many bird species avoid roads altogether. “So roads may have an impact we don’t see,” he said.
Bird road-kill randomness
Many birds don’t avoid roads. Vehicles, rather than acting as predators to week out the weak, have been shown to be random eliminators of the weak and the strong.
A team of researchers studied road-killed Yellowhammers, Barn Swallows and Chaffinches in southeast Poland from 2002 through 2009. They concluded in a study published in 2011 in Biological Conservation that the death of each bird is a “random” event based on fitness measurements of birds.
The researchers, Malgorzata Bujoczek and Michal Ciach, both from the University of Agriculture in Kraków, Poland, and Reuven Yosef, from Ben Gurion University in Eliat, Israel, compared the birds killed by vehicles to those killed by raptors.
Relying on a technique called ptilochronology, they compared feather growth bars to gauge the overall health of individual birds. They found that birds killed by raptors were generally in significantly weaker condition than those killed by vehicles. Basically, road-kill birds were fit birds at the wrong place at the wrong time.
“Our study does not concur with previous studies, which concluded that weaker individuals are more vulnerable on the road,” Bujoczek wrote.
Barn Owl road-kills
Road-kill Barn Owls fit the same random pattern. They are being killed by vehicles at high frequencies on every continent where they are studied. A study published in 1012 in the Journal of Wildlife Management, reported that 812 Barn Owls were killed along a 248-kilometer section of Interstate 84 in southern Idaho over a two-year period. That’s 6 road-kill owls per kilometer annually.
“Our data suggest that Barn Owls in this region may not persist under this level of mortality,” said Than Boves, author of the owl study and a postdoctoral fellow at the University of Illinois.
The ‘uneasy embrace’
Boves, Bujoczek, Goldingay, the authors of “Road Ecology: Science and Solutions,” and many other scientists have concluded that roadway improvements, such as special underpasses and overpasses for wildlife are required to prevent or reduce road-kill birds. Such changes take time.
Humans have spread a 5-million-mile net of roads over the land that few animals can escape. “These two giants, the land and the net, lie intertwined in an uneasy embrace,” said the “Road Ecology” authors. “The call for new knowledge and skills is stronger than ever.”
I predict that such new knowledge will be reported soon in leading peer-review scientific publications.