Windows and cats are major sources of mortality for our birds but certainly aren’t the only major causes. Somewhere between 87 and 350 million birds are killed by vehicles each year in the U.S., although the estimates vary. See this study.
Barn owls seem particularly marked for doom. Now threatened or endangered in a number of states where they were once common, barn owls accounted for a large proportion of roadkill deaths. One survey estimated that as many as 1,500 barn owls a year died along a 150-mile stretch of interstate highway in Idaho – a death rate high enough to wipe out the local population.
You would think that with increasing traffic, there would be more roadkills of birds, but there is a non-linear relationship according to one research paper. It found that more birds are killed as the traffic on a particular road increases but at a certain traffic density the birds start to avoid the roads and roadkills decline. There’s an interesting paper on roadkills in Norway which found that roadkills decreased with increasing speed limit.
A 2013 paper reported on a thirty year study of Cliff Swallows killed by vehicles in Nebraska. Colonies nesting on bridges and overpasses flew over highways and the occasional bird was killed by passing traffic. In the 1980s the kills averaged 20 birds per year but over the past five years an average of five birds were killed annually, even though the colony doubled in size and vehicular traffic stayed steady. Researchers Charles and Mary Brown wondered what was happening, so they took measurements of the deceased birds and compared them to live birds in the colony. The live birds had a 4.2 in (105 mm) long wing while the road-killed birds’ wings were 4.5 in (112 mm) long. Studies of swallow flights indicate that shorter wings are better for making quick turns and make the birds better at dodging traffic; natural selection (by vehicles) is weeding out the longer winged birds.
Researchers in France measured the distance from vehicles at which birds began flying to avoid being hit. One would think that the faster the vehicle is moving, the greater the distance at which the bird would initiate flight. Actually, the birds responded not to the speed of the vehicle, but to the posted speed limit! It appears that birds which regularly inhabit a stretch of roadway have learned the most common speed, which is somewhere around the posted limit, and react as if the vehicle was traveling at that pace, even if it were dawdling or speeding. Since most cars travel around the posted limit, this appears to be the best strategy to avoid collisions with oncoming traffic.
Can we reduce bird-vehicle collisions? Yes, there are some solutions. See some suggestions by the U.S. Fish and Wildlife Service.