> Here's a thought on the nighttime singing of White-crowned Sparrow and
other
> birds: After dark it's safer to fly around in unfamiliar territory (given
> that you're less likely to get nailed by a falcon or accipiter). It's
> spring, so I can just imagine female birds, after a day of foraging,
getting
> out-and-about checking out some of the males on territories farther
afield.
> As a male it would behoove you to make your presence known.

Most normally diurnal birds have excellent color vision and poor night
vision. (Not enough room in the eyes for enough rods and cones to give both
excellent color AND excellent low-light vision.) I would think it would be
dangerous for a normally diurnal bird to "stumble" around in the dark while
the owls--with their excellent night vision and hearing--are out and about.
Street lights and city lights reflecting off clouds can make a city pretty
bright at night, however.