Birds face a great many dangers in their daily lives, but humans have contributed significantly to certain kinds of bird mortality, one of those being deaths from impacts with automobiles. There are estimates that up to 80 million birds die every year in the US from collisions with cars (Erikson cited in Brown and Brown 2013). Among a population of cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, some researchers collected data on the number of road-killed birds over a thirty-year period (1982-2012) along a single stretch of road (Brown and Brown 2013). These birds live in colonies, and they build mud nests on vertical surfaces that are also sheltered by a horizontal surface. In the study area, many cliff swallow colonies were found under highway bridges, and other overpasses. They were also located in rectangular shaped concrete culverts under railway tracks and roads. The researchers believe that these colonies of birds were relatively new to the study area and to regular interactions with humans driving cars in the early 1980s, when the study began. Cliff swallows are often found sitting on a road, and they need to launch themselves quickly to avoid fatal collisions. Over the 30 years of the study, the number of road-killed birds in the study area declined significantly, despite the fact that the population of cliff swallows overall increased during the study period. Age does not seem to be a factor influencing the likelihood of an individual being killed by a car, and so automobile avoidance doesn’t seem to increase with exposure and learning by the young. There was no significant increase among the common scavengers that would have influenced the number of roadkill collected each year, and the traffic patterns in the area did not change much. The size of the cars driven by the average driver increased during this time period as the use of sport utility vehicles rose. Sport utility vehicles are probably more difficult to evade than smaller cars, so it is even more surprising that the number of roadkill declined. Those birds that were killed by cars had notably longer wings than the general population, but during the 30-year study period, the average length of wings in the general population actually decreased somewhat, while the average wing length of the birds killed increased somewhat. Swallows with somewhat shorter (and therefore more rounded) wings can take off and fly upward more vertically than those with longer wing lengths.
Questions for Example 2: What force of evolution is at work here? If you argue natural selection, be sure to discuss the three necessary and sufficient conditions in your answer. If it is natural selection, what mode of selection is it? Can you think of a different explanation for the patterns discovered in this study? If so, what else could be happening?