Most daytime-active animals respond to daylengths of 12 hours or more by getting ready to reproduce: singing, courtship displays, nesting, territorial disputes, etc. There are hundreds of studies showing this works in a huge variety of species from bacteria to bugs to birds, etc. (Actually, some plants, too.)
But actually, it’s not only the absolute length of the day that is important, but the increasing or decreasing nature of the day length. Right now, we are gaining an additional 3 minutes of daylight each day, as the sun rises higher on its arc through the sky, a welcome change from the dreary days of December.
We won’t hit the magic number of 12 hours of light from sunrise to sunset until March 18 in Minnesota, but you know birds will be singing their heads off by then. So, is 12 really the magic number?
Factor in that beautiful light that we receive at dawn (morning civil twilight when the sun is 6 degrees below the horizon until sunrise) and at dusk (sunset through evening civil twilight when the sun is again 6 degrees below the horizon), and we will reach the 12 hours of daylight on February 26 in Minnesota.
So I propose an experiment. Look up the dates for 12 hours of light from sunrise to sunset vs 12 hours of light from dawn (morning civil twilight) to dusk (evening civil twilight) in your area and monitor how much singing you hear before or between or after those dates. What cues are birds really using to get them started on their “spring fling?”
I’ll check back with you in April.