Birds singing at night due to daytime noise pollution.

From the Chicago Sun-Times:

U.S. observers have noticed birds singing long after dark this summer.

In England, researchers found that the British robin was forced to sing at night because daytime traffic was drowning out his mating serenades.

The study measured noise levels and singing at 67 sites. Scientists found that birds sang only during the day at 49 sites, and day and night at 18 sites. Daytime noise levels at those 18 sites were significantly higher.