• Alex Taylor

Where Has All The Roadkill Gone?

We've paved paradise and now cars are everywhere. For the cliff swallows that live alongside them, changes to their bodies have allowed them to survive better. But what will the consequences be of a life evolved to cope with humans?

Highway - driver of evolution

The cliff swallow is one well studied bird! A population in southwestern Nebraska, USA, has been the focus of study since 1982, almost as long as I’ve been alive. They winter in South America but migrate to North America to breed, in vast colonies of up to 12,000 adults. Usually they build their mud-based, conical nests into the side of cliffs (hence their name) but the abundance of our infrastructure means that they have taken to building them under bridges and overpasses.

The colony in Nebraska had done just that, their nests dangle precariously from a highway overpass. The scientists who study them have examined many aspects of their life – social interactions and parasites and disease, to name but a few – with the ultimate goal of discovering why colony sizes vary. They’ve counted birds and eggs, nests, and banded individuals every nesting season for 3 decades.

One other thing they did was to pick up roadkill. And over the years, they started to notice that they were picking up fewer and fewer dead birds. Their data backed up their hunch, in turned out that in 1984 and 1985, roadkill averaged around 20 birds per season, but for the most recent 5 years, that number had dropped to just 5 birds per season. What on earth was going on? Where had all the roadkill gone?

The decline in bird mortality was not because there were fewer birds, because the population rose over time from 10,000 nests when they first started studying them to over 25,000 nests 30 years later. Nor was it due to a decrease in traffic, because that remained steady. An increase in the number of scavengers eating the dead birds? Nope. They had not increased, in fact, many species had declined in number.

What they did notice was that the birds they were finding dead on the roads had longer wings than the living birds. In 2012, the average cliff swallow killed had a wingspan of 11.2 centimetres. By contrast, the average wingspan of the population was 10.6 centimetres. This latter figure had decreased over time from 11.1 centimetres in 1982. The overall population’s wingspan had shortened, whereas the birds killed by vehicles had lengthened.

The conclusion? In response to cars on the road, the cliff swallows had evolved shorter wings. Shorter wings enable the birds to dodge the traffic more easily as they enter and exit their nesting sites, to turn more quickly and to take off faster from the ground. The birds with shorter wings escape the traffic and survive another day, to breed and pass on their short wing genes – those with longer wings aren’t able to escape so easily, and don’t survive the collision.

It just goes to show that despite our dominance and heavy presence in the landscape, other species can adapt to us. They can evolve ways to survive, and over very short time frames. But wait. Let’s not get ahead of ourselves. Let’s consider the implications of this.

Longer wings benefit birds that migrate long distances…birds like cliff swallows. They spend a lot of time in flight, catch prey such as flying insects and long wings help with that. They reduce the effort needed to glide and manoeuvre in the air. With shorter wings, will the cliff swallows find migration harder? Will they struggle to find food? No one knows.

The advantages of long term study means that, if scientists keep studying this population, time will tell them the answers. The selection pressure of cars and roadway-related adaptation is likely to be affecting other species too. Perhaps more studies will use roadkill in future, and uncover other surprising changes in nature like this one.

#birds #unnaturalselection #evolution