- In the third week of September, days start to become shorter than nights. Elk and antelope breeding seasons peak. Aspen leaves hang golden before the fall.
- In the third week of September, moose approach the apex of their rut. Bull trout head to spawning tributaries. Asters bloom where many other prairie wildflowers have long faded.
- In the third week of September, some ground squirrels and marmots are already into their long winter nap. Bears remain awake, gorging themselves, taking in as much as 20,000 calories a day.
It's the autumnal equinox, which some equate with oncoming winter and death. But for hunters, anglers, hikers and floaters, the third week of September is a time to anticipate, to delight in, to anxiously await. And it's all due to the sun, or rather, daylight length.
Scientists call it photoperiod. Because the earth tilts slightly on its axis, our planet's annual dash around the sun means daylight is longer than darkness north of the equator from the third week of March to the third week of September.
This seasonal shift touches everything on earth, from behavior to temperature, climate to reproduction, birth through death. Every school child has to be taught this. Every plant and animal inherently knows it.
There are short-day plants and long-day plants. No one knows why asters evolved to bloom in September and columbines in summer. It works and that's all that has to happen. In the third week of September at Montana's latitude, leaf color presents the most visible effect of changing day length. The decrease in daylight triggers plants to stop producing chlorophyll, a chemical agent that turns leaves green. It also masks other colors. When the green disappears, yellows and reds shine. Temperature and precipitation come into play, but day length is the driving force. Leaves change color even when it's warm.
In the kingdom of animals, birds provide dramatic examples of the effects of photoperiodism. Back in the spring, birds respond to lengthening daylight by putting on a layer of fat to use when migrating and breeding. That happens again in August in response to decreasing daylight, except the energy stores are used for migration, not reproduction. A neat trick that.
When sufficiently plump, birds in the northern hemisphere fly south, turning their spring navigational maps upside down. For those just hatched that has to happen even though they have never been to their wintering ground or experienced a migration.
For mammals, the third week of September represents a mixed bag: Some reach a peak of behavior, like the elk and antelope rut. Others are just beginning an activity, such as the moose rut. And a third group, like bears, seems unaffected by the equinox, though bears have their own fall behavior. Triggered by decreasing daylight, they enter a stage known as hyperphagia where they eat for 20 hours a day or more, anticipating winter hibernation.
Even underwater, daylight length reigns supreme. Fish in Montana are affected by photoperiod, though usually in combination with water temperature and water flow. All these animals have a built-in biological clock. So do us humans, but our dependence on alarm clocks, calendars, or all night truck stops has long muted nature's clock.