Eight million American birds end up trafficked every year: a real massacre, but it seems that evolution is putting a piece in place. The mountain swallows (Petrochelidon pyrrhonota), small North-American bluish-brown birds, in fact nest under bridges and overpasses in particularly busy areas: for them death often has the appearance of an all-terrain vehicle launched at full speed.
The exaggerated nests of the weaving birds: the photos
Artists of the sky: the abstract figures drawn by flocks in flight
A group of researchers from the University of Tulsa in Oklahoma examined the remains of dead swallows for a clash with cars collected in Nebraska over the past 30 years, and compared them with live specimens captured for research (and then released). Since the 1980s, although the number of nests near the roads has increased, deaths of this type have drastically decreased. And this is not due to a reduction in traffic or greater attention from drivers.
The reason is strictly anatomical. The birds that died in a clash show longer wings than the specimens living today, provided on the contrary with shorter and more agile wings. Less bulky wings also mean a greater take-off and landing speed, and therefore less chance of ending up squashed against a windshield.
Other factors may have influenced this evolutionary feature, such as the fact that swallows with shorter wings are more adept at catching insects directly in flight. But if confirmed, this would be yet another example of how human activity influences animal evolution (see also the shrunken fish due to global warming and 7 animals that are changing size under our noses).