During the day, birds generally have a better eye sight than humans, but at dusk and dawn it is worse. That is when the birds are attracted to lit up areas, which in turn can cause accidents.
Just over a year ago, a passenger plane had to emergency land on the Hudson River in New York, after having collided with a flock of geese. The collision took out the planes engine.
In Sweden, nothing as serious has happened, but some 160 accidents where planes crash into birds are reported annually.