• Most Americans will lose an hour of sleep this weekend, but gain an hour of evening light for months ahead, as Daylight Savings Time returns this weekend. The time change officially starts Sunday at 2 a.m. local time. AP

  • The black-capped chickadees begin singing its songs when the days are noticeably longer and brighter. AP file

For the Monitor
Published: 3/9/2019 3:15:53 PM

Today we launch Daylight Savings Time, when we spring our clocks forward to take advantage of more light during our waking hours. But clocks don’t make the days shorter or longer, the tilt of the earth in relationship to the sun does that.

As the earth revolves around the sun throughout the year, the relationship between the earth’s tilt and the sun shifts, giving us seasons. During winter, the northern latitudes are tilted away from the sun, creating shorter days. However, since the winter solstice in December, our region has gained an additional one to four minutes of light each day. The increase in light is incremental, but it makes a noticeable and important difference.

The change of light is extremely important to plants and animals. Many aspects of their life cycles are triggered by changing light or “photo period.” The relationship between the photo period and emergence of leaves and flowers, migration, mating, and molting of fur and feathers has evolved over time because the changing light is a more reliable indicator of seasons than varying temperatures.

For example, in the past five years, the temperature in the Concord area on March 10 has ranged from a low in the high teens to the mid-40s to a high temperature between the mid-30s and the mid-60s. In contrast during that same period of time, the daylight on March 10 has been consistently 11 hours and 38 to 40 minutes.

If plants responded only to temperature, a few 60 degree days in March could fool them into opening their buds. If those warm days were followed by below freezing temperatures, the buds would be killed. Instead, bud and flower growth is activated by a photoreceptor protein in the plant which detects the length of the night. As nights get shorter, these proteins prompt growth of the leaf and flower tissues in the buds.

Once the growth has begun, temperature may impact the rate at which buds open. That is why some years there may be a slightly earlier or later flush of apple blossoms or lilac flowers, but the variation from year to year is usually only by a few days rather than weeks, thanks to the photo period.

Shifting length of night and day also initiates the color change of snowshoe hare and weasel fur. This time of year, though the ground is still white with snow, brown fur will begin to replace the white pelt which has helped these animals blend in with their surroundings and keep warm in the winter. The molt of the hare takes about three months to complete so starting in March when the days are getting longer keeps them on schedule to be completely brown again in May.

Antler growth in male deer and moose is also influenced by photo period. Spring growth and autumn drop of antlers is caused by hormones that are triggered by day length.

These physical adaptations are not the only changes that we can observe as days lengthen. Step outside and listen. You may hear the sweet clear whistle of the Black-capped Chickadee singing its spring song. “Spring’s here” is one paraphrase of its tune. Other people confuse it with an Eastern Phoebe because they hear the song as “fee-bee.” However, the chickadee began singing its song earlier in February as the days began to be noticeably longer and brighter. We won’t hear the Phoebe’s more raspy and emphatic “fee-bee” for several more weeks when it returns from its southern winter home. The behavior of woodpeckers has also changed. Their spring sound is a percussive drumming, made by rapidly tapping their beak against a hollow tree or other resonating structure. All of these sounds function as territorial indicators and mate attraction because with spring also comes the season of reproduction.

Light stimulates the pituitary gland in animals to release hormones that affect reproduction. Seasonal migration, mating calls, feather color change such as the yellowing of goldfinches, gathering of nesting materials, are the physical and behavioral responses to shorter nights and longer days and are preparations for creating offspring.

So while you enjoy the increased light and the promise of spring, pay attention to ways that our wild neighbors are also responding to longer days. There is a lot going on even if it still feels like winter!




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