Moving around a city can be much more dangerous than large spaces, and this, it seems, is also true for migratory birds, especially if they have the habit of screaming the night.
A new study has analyzed the deaths of 70,000 songbirds who died after a collision with buildings in Cleveland and Chicago at night. The researchers' analysis covers a 40-year period and has revealed that species that communicate with each other by flight calls are the most exposed.
Researchers believe that this is directly related to how we illuminate our cities.
"Calls for night flights have probably evolved to facilitate collective decision-making among the birds during navigation," said evolution biologist Benjamin Winger of the University of Michigan, "but this same social behavior can now exacerbate vulnerability to widespread anthropogenic disturbance: artificial light from buildings. "
Many species of birds migrate at night, traveling long distances in the dark, using flight calls to communicate – to help with navigation and orientation, to keep the flock together and to to make group decisions.
Previous research has shown that artificial light is linked to an increase in the number of nocturnal migratory bird flight calls. Over the cities, birds are called three times more often than in the darkest rural areas.
The Chicago Field Museum has been collecting bird strike data since 1978, and this is what the researchers used to conduct their investigation. They also used more recent data collected by volunteers in both cities.
Cleveland and Chicago are of particular interest because they are both in a north-south flyway of migratory birds. In addition to a more general analysis of city-wide collisions, the team paid particular attention to the Chicago Lake Convention Center, a known danger to migratory birds.
They found that when the convention center had turned off the light, fewer birds clashed.
They also found that of the 93 species in the data, several species of Cree Sparrows, Warblers and Thrushes made up the majority of fatal collisions, leaving several thousand dead.
When they compared crash data with population data, these "super collider" birds were vastly overrepresented. But birds that do not make nocturnal flight calls were under-represented.
"We hypothesized that if flight appeals are important social information for decision making during nocturnal migration, individuals belonging to called species may attract each other when they are in danger." they are disoriented by artificial light, "the researchers wrote.
"This relationship can engender a vicious circle of increased mortality rates if disoriented individuals drive other individuals to migrate to artificial light sources."
Of course, it is difficult to say with certainty without directly observing the relationship between night calls and collisions between buildings – but this indicates a direction in which research can be conducted.
And it's a pretty strong proof of the detrimental effect of human technology on wildlife.
"Our paper provides some of the strongest published evidence regarding the effect of artificial light on bird collisions in urban areas," said Winger.
The search was published in Acts of the Royal Society B.