City lights may cause migrating birds to perish

Birds like sparrows that call cities home are facing increasing hardships, yet it isn’t only resident birds like them that are at risk in urban environments. So are migratory birds, say the authors of a new study published by The Royal Society.

The researchers pored through 40 years’ worth of data on the 70,000 passerine birds that crashed into buildings in Chicago and Cleveland, both of which cities are located along a north-south flyway for migratory birds. The scientists’ aim was to estimate the rates of such collisions during nocturnal avian migrations.

They found that species that rely on flight calls are far more likely to suffer such nighttime collusions than those that do not. The at-risk species include sparrows, warblers, and thrushes.

The likely reason for these birds’ plight is the way cities are lit up at night. These artificial lights may well disorientate birds that are used to navigating by the light of the moon and the stars. “This relationship may spawn a vicious cycle of increased mortality rates if disoriented individuals lead other migrating individuals to sources of artificial light,” the researchers note.

Many bird species travel long distances at night when they are shielded from the eyes of predators in the dark. They call out in fight with calls that help one another navigate and orient themselves. These calls also help keep migrating birds flock together and fly in formation.

“Nocturnal flight calls probably evolved to facilitate collective decision-making during navigation, but this same social behaviour may now exacerbate vulnerability to a widespread anthropogenic disturbance,” the scientists explain. “Our results also suggest that social behaviour during migration may reflect poorly understood differences in navigational mechanisms across lineages of birds.”

The evidence isn’t conclusive, but it does seem to indicate that light pollution has a strong negative impact on birds, just as it does on many forms of wildlife.