You may not be conscious of it on a daily basis, but the daylight hours have been steadily increasing since the winter solstice on Dec. 21. It’s happening here in Arizona, too. At our Arizona winter home, we often sit at our back wall in the late afternoon to watch birds as they emerge from the desert brush for their evening feed.

Over the past several weeks, we have gratefully appreciated the increasing duration of the sun on our backs. Back home in Minnesota, despite often frigid temperatures, you are probably sensing the longer and brighter days, too, with the sun higher in the sky.

In biological speak, the length of daylight hours is known as photoperiod. The pattern of change in photoperiod over the course of the year is the most predictable of all environmental stimuli. Because of this, photoperiod has been one of the most important evolutionary forces determining how animals respond to environmental changes throughout the year.

The length of photoperiod is registered in the tiny pineal gland at the base of the brain. As days lengthen, the pineal sends signals that trigger critical physiological and behavioral responses in animals.

In migratory birds, for example, these signals make the birds hungrier, so they begin eating more food and storing the fat that will be crucial for fueling their migration north.

At the same time, other internal changes, also caused by increasing photoperiod, take place. Levels of sex hormones increase and the birds gradually begin to show familiar signs of courtship, like displaying and singing.

Here in Arizona, where environmental conditions favorable for nesting arrive much earlier that in Minnesota, we are already hearing much more song from species like thrashers and house finches. Species like gnatcatchers, mourning doves and verdins (cute chickadee-sized birds in the desert) are already pairing up.

Temperatures also play an important secondary role. In Minnesota, though photoperiod changes are already being felt by the birds, they’re not apt to “act out” until warmer temperatures free up feeding time for more amorous behaviors. After all, food is critical for survival, and cold temperatures demand more food, reducing time for other activities.

When temperatures warm up there’s time for other things. Expect your chickadees to start singing their fee-bee song on warmer days in February, maybe even earlier depending on the weather. Nuthatches and other species should also respond similarly. Listen for woodpeckers drumming on trees, their substitute for singing.

Don’t you start feeling better when the sun rises higher in the sky? Have no doubt that photoperiod is a fundamental influence on a wide variety of behaviors throughout the animal kingdom.

Marshall Howe is a retired biologist with the U.S. Fish and Wildlife Service. He specialized in bird population studies. Howe has been a Park Rapids resident since 2010.